Review of the Ozone National Ambient Air Quality Standards

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Final action.

SUMMARY:

Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and the national ambient air quality standards (NAAQS) for photochemical oxidants including ozone (O₃), the EPA is retaining the current standards, without revision.

DATES:

This final action is effective December 31, 2020.

ADDRESSES:

The EPA has established a docket for this action under Docket ID No. EPA-HQ-OAR-2018-0279. Incorporated into this docket is a separate docket established for the Integrated Science Assessment for this review (Docket ID No. EPA-HQ-ORD-2018-0274). All documents in these dockets are listed on the www.regulations.gov website. Although listed in the index, some information is not publicly available, e.g., Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, is not placed on the internet and will be publicly available only in hard copy form. With the exception of such material, publicly available docket materials are available electronically through https://www.regulations.gov/. Out of an abundance of caution for members of the public and our staff, the EPA Docket Center and Reading Room are closed to the public, with limited exceptions, to reduce the risk of transmitting COVID-19. Our Docket Center staff will continue to provide remote customer service via email, phone, and webform. For further information on EPA Docket Center services and the current status, please visit us online at https://www.epa.gov/dockets.

Availability of Information Related to This Action

A number of the documents that are relevant to this action are available through the EPA's website at https://www.epa.gov/naaqs/ozone-o3-air-quality-standards. These documents include the Integrated Review Plan for the Ozone National Ambient Air Quality Standards (IRP [U.S. EPA, 2019b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-planning-documents-current-review,, the Integrated Science Assessment for Ozone and Related Photochemical Oxidants (ISA [U.S. EPA, 2020a]), available at https://www.epa.gov/naaqs/ozone-o3-standards-integrated-science-assessments-current-review,, the Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards (PA [U.S. EPA, 2020b]), available at https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review. These and other related documents are also available for inspection and copying in the EPA docket identified above.

FOR FURTHER INFORMATION CONTACT:

Dr. Deirdre Murphy, Health and Environmental Impacts Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail Code C504-06, Research Triangle Park, NC 27711; telephone: (919) 541-0729; fax: (919) 541-0237; email: murphy.deirdre@epa.gov.

SUPPLEMENTARY INFORMATION:

Basis for Immediate Effective Date

In accordance with section 307(d)(1)(V), the Administrator has designated this action as being subject to the rulemaking procedures in section 307(d) of the Clean Air Act (CAA). Section 307(d)(1) of the CAA states that: “The provisions of section 553 through 557 * * * of Title 5 shall not, except as expressly provided in this subsection, apply to actions to which this subsection applies.” Thus, section 553(d) of the Administrative Procedure Act (APA), which requires publication of a substantive rule to be made “not less than 30 days before its effective date” subject to limited exceptions, does not apply to this action. In the alternative, the EPA concludes that it is consistent with APA section 553(d) to make this action effective December 31, 2020.

Section 553(d)(3) of the APA, 5 U.S.C. 553(d)(3), provides that final rules shall not become effective until 30 days after publication in the Federal Register “except . . . as otherwise provided by the agency for good cause found and published with the rule.” “In determining whether good cause exists, an agency should `balance the necessity for immediate implementation against principles of fundamental fairness which require that all affected persons be afforded a reasonable amount of time to prepare for the effective date of its ruling.” Omnipoint Corp. v. Fed. Commc'n Comm'n, 78 F.3d 620, 630 (D.C. Cir. 1996) (quoting United States v. Gavrilovic, 551 F.2d 1099, 1105 (8th Cir. 1977)). The purpose of this provision is to “give affected parties a reasonable time to adjust their behavior before the final rule takes effect.” Id.; see also Gavrilovic, 551 F.2d at 1104 (quoting legislative history).

The EPA is determining that in light of the nature of this action, good cause exists to make this final action effective immediately because the Agency seeks to provide regulatory certainty as soon as possible and the Administrator's decision to retain the current NAAQS does not change the status quo or impose new obligations on any person or entity. As a result, there is no need to provide parties additional time to adjust their behavior, and no person will be harmed by making the action immediately effective as opposed to delaying the effective date by 30 days. Accordingly, the EPA is making this action effective immediately upon publication.

Table of Contents

The following topics are discussed in this preamble:

Executive Summary

I. Background

A. Legislative Requirements

B. Related O₃ Control Programs

C. History of the Air Quality Criteria and O₃ Standards

D. Current Review of the Air Quality Criteria and O₃ Standards

E. Air Quality Information

II. Rationale for Decision on the Primary Standard

A. Introduction

1. Background on the Current Standard

2. Overview of Health Effects Information

3. Overview of Exposure and Risk Information

B. Conclusions on the Primary Standard

1. Basis for the Proposed Decision

2. Comments on the Proposed Decision

3. Administrator's Conclusions

C. Decision on the Primary Standard

III. Rationale for Decision on the Secondary Standard

A. Introduction

1. Background on the Current Standard

2. Overview of Welfare Effects Information

3. Overview of Air Quality and Exposure Information

B. Conclusions on the Secondary Standard

1. Basis for the Proposed Decision

2. Comments on the Proposed Decision

3. Administrator's Conclusions

C. Decision on the Secondary Standard

IV. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Executive Order 13771: Reducing Regulations and Controlling Regulatory Costs

C. Paperwork Reduction Act (PRA)

D. Regulatory Flexibility Act (RFA)

E. Unfunded Mandates Reform Act (UMRA)

F. Executive Order 13132: Federalism

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution or Use

J. National Technology Transfer and Advancement Act

K. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

L. Determination Under Section 307(d)

M. Congressional Review Act

V. References

Executive Summary

This document presents the Administrator's decisions in the current review of the primary (health-based) and secondary (welfare-based) O₃ NAAQS, to retain the current standards, without revision. In reaching these decisions, the Administrator has considered the currently available scientific evidence in the ISA, quantitative and policy analyses presented in the PA, advice from the Clean Air Scientific Advisory Committee (CASAC), and public comments on the proposed decision. This document provides background and summarizes the rationale for these decisions.

This review of the O₃ standards, required by the Clean Air Act (CAA) on a periodic basis, was initiated in 2018. In the last review, completed in 2015, the EPA significantly strengthened the primary and secondary O₃ standards by revising the level of both standards from 75 parts per billion (ppb) to 70 ppb and retaining their indicators (O₃), forms (annual fourth-highest daily maximum, averaged across three consecutive years) and averaging times (eight hours) (80 FR 65291, October 26, 2015). In subsequent litigation on the 2015 decisions, the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) upheld the 2015 primary standard but remanded the 2015 secondary standard to the EPA for further justification or reconsideration. The court's remand of the secondary standard has been considered in reaching the decision described in this document on this standard, and in associated conclusions and judgments, also described here. Accordingly, this decision incorporates the EPA's response to the judicial remand of the 2015 secondary standard.

In this review as in past reviews of the air quality criteria and NAAQS for O₃ and related photochemical oxidants, the health and welfare effects evidence evaluated in the ISA is focused on O₃. Ozone is the most prevalent photochemical oxidant in the atmosphere and the one for which there is a large body of scientific evidence on health and welfare effects. A component of smog, O₃ in ambient air is a mixture of mostly tropospheric O₃ and some stratospheric O₃. Tropospheric O₃ forms in the atmosphere when emissions of precursor pollutants, such as nitrogen oxides and volatile organic compounds (VOCs), interact with solar radiation. Such emissions result from man-made sources (e.g. motor vehicles and power plants) and natural sources (e.g. vegetation and wildfires). In addition, O₃ that is created naturally in the stratosphere also mixes with tropospheric O₃ near the tropopause, and, less frequently can mix nearer the earth's surface.

Based on the current health effects evidence and quantitative information, as well as consideration of CASAC advice and public comment, the Administrator concludes that the current primary standard is requisite to protect public health, including the health of at-risk populations, with an adequate margin of safety, and should be retained, without revision. This decision has been informed by key aspects of the health effects evidence newly available in this review, in conjunction with the full body of evidence critically evaluated in the ISA, that continues to support prior conclusions that short-term O₃ exposure causes and long-term O₃ exposure is likely to cause respiratory effects. The strongest evidence continues to come from studies of short- and long-term O₃ exposure and an array of respiratory health effects, including effects related to asthma exacerbation in people with asthma, particularly children with asthma. The clearest evidence comes from controlled human exposure studies, available at the time of the last review, of individuals exposed for 6.6 hours during quasi-continuous exercise, that report an array of respiratory responses including lung function decrements and respiratory symptoms. Epidemiologic studies additionally describe consistent, positive associations between O₃ exposures and hospital admissions and emergency department visits, particularly for asthma exacerbation in children. Populations and lifestages at risk include people with asthma, children, the elderly, and outdoor workers. The quantitative analyses of population exposure and risk, as well as policy considerations in the PA, summarized in this document and described in detail in the PA, also inform the decision on the primary standard. The general approach and methodology used for the exposure-based assessment is similar to that used in the last review, although a number of updates and improvements have been implemented. These include a more recent period (2015-2017) of ambient air monitoring data in which O₃ concentrations in the areas assessed are at or near the current standard, as well as improvements and updates to models, model inputs and underlying databases.

In its advice to the Administrator, the CASAC stated that the newly available health effects evidence does not differ substantially from that available in the last review when the standard was set. Part of CASAC concluded that the primary standard should be retained. Another part of CASAC expressed concern regarding the margin of safety provided by the current standard, pointing to comments from the 2014 CASAC, who while agreeing that the evidence supported a standard level of 70 ppb, additionally provided policy advice expressing support for a lower standard. In summary, the current evidence and quantitative analyses, advice from the CASAC and consideration of public comments have informed the Administrator's judgments in reaching his decision that the current primary standard of 70 ppb O₃, as the annual fourth-highest daily maximum 8-hour concentration averaged across three consecutive years, provides the requisite public health protection, with an adequate margin of safety.

Based on the current welfare effects evidence and quantitative information, as well as consideration of CASAC advice and public comment, the Administrator concludes that the current secondary standard is requisite to protect the public welfare from known or anticipated adverse effects of O₃ and related photochemical oxidants in ambient air, and should be retained, without revision. This decision has been informed by key aspects of the welfare effects evidence newly available in this review, in conjunction with the full body of evidence critically evaluated in the ISA, that supports, sharpens and expands somewhat on the conclusions reached in the last review. The currently available evidence describes an array of O₃ effects on vegetation and related ecosystem effects, as well as the role of O₃ in radiative forcing and subsequent climate-related effects. The ISA includes findings of causal or likely causal relationships for a number of such effects with O₃ in the ambient air. As in the last review, the strongest evidence, including quantitative characterizations of relationships between O₃ exposure and occurrence and magnitude of effects, is for vegetation effects. The scales of these effects range from the individual plant scale to the ecosystem scale, with potential for impacts on the public welfare.

While the welfare effects of O₃ vary widely with regard to the extent and level of detail of the available information that describes the exposure circumstances that may elicit them, such information is most advanced for plant growth-related effects. For example, the information on exposure metric and relationships for these effects with the cumulative, concentration-weighted exposure index, W126, is long-standing, having been first described in the 1997 review. Utilizing this information in reviewing the public welfare protection provided by the current secondary standard, reduced growth has been considered as proxy or surrogate for a broad array of related vegetation effects. Quantitative analyses of air quality and vegetation exposure, including in terms of the W126 index, as well as policy-relevant considerations discussed in the PA, have also informed the Administrator's decision on the secondary standard. These include analyses of air quality monitoring data in areas meeting the current standard across the U.S., as well as in Class I areas, updated and expanded from analyses conducted in the last review. Lastly, in its advice to the Administrator on the secondary standard, the full CASAC found the current evidence to support the current standard and concurred with the draft PA that it should be retained without revision. In summary, the current evidence and quantitative analyses, advice from the CASAC and consideration of public comments have informed the Administrator's judgments in reaching his decision that the current secondary standard of 70 ppb O₃, as the annual fourth-highest daily maximum 8-hour concentration averaged across three consecutive years, provides the requisite public welfare protection.

I. Background

A. Legislative Requirements

Two sections of the CAA govern the establishment and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the Administrator to identify and list certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator is to list those pollutants “emissions of which, in his judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare”; “the presence of which in the ambient air results from numerous or diverse mobile or stationary sources”; and for which he “plans to issue air quality criteria . . . .” (42 U.S.C. 7408(a)(1)). Air quality criteria are intended to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in the ambient air . . . .” (42 U.S.C. 7408(a)(2)).

Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for which air quality criteria are issued (42 U.S.C. 7409(a)). Section 109(b)(1) defines primary standards as ones “the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.” Under section 109(b)(2), a secondary standard must “specify a level of air quality the attainment and maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of [the] pollutant in the ambient air.”

In setting primary and secondary standards that are “requisite” to protect public health and welfare, respectively, as provided in section 109(b), the EPA's task is to establish standards that are neither more nor less stringent than necessary. In so doing, the EPA may not consider the costs of implementing the standards. See generally, Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards.” See American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). At the same time, courts have clarified the EPA may consider “relative proximity to peak background . . . concentrations” as a factor in deciding how to revise the NAAQS in the context of considering standard levels within the range of reasonable values supported by the air quality criteria and judgments of the Administrator. See American Trucking Ass'ns, v. EPA, 283 F.3d 355, 379 (D.C. Cir. 2002), hereafter referred to as “ATA III.”

The requirement that primary standards provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. It was also intended to provide a reasonable degree of protection against hazards that research has not yet identified. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1154 (D.C. Cir. 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Both kinds of uncertainties are components of the risk associated with pollution at levels below those at which human health effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary standards that include an adequate margin of safety, the Administrator is seeking not only to prevent pollution levels that have been demonstrated to be harmful but also to prevent lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. The CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels (see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351), but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

In addressing the requirement for an adequate margin of safety, the EPA considers such factors as the nature and severity of the health effects involved, the size of the sensitive population(s),³ and the kind and degree of uncertainties. The selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator's judgment. See Lead Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353.

Section 109(d)(1) of the Act requires periodic review and, if appropriate, revision of existing air quality criteria to reflect advances in scientific knowledge concerning the effects of the pollutant on public health and welfare. Under the same provision, the EPA is also to periodically review and, if appropriate, revise the NAAQS, based on the revised air quality criteria.

Section 109(d)(2) addresses the appointment and advisory functions of an independent scientific review committee. Section 109(d)(2)(A) requires the Administrator to appoint this committee, which is to be composed of “seven members including at least one member of the National Academy of Sciences, one physician, and one person representing State air pollution control agencies.” Section 109(d)(2)(B) provides that the independent scientific review committee “shall complete a review of the criteria . . . and the national primary and secondary ambient air quality standards . . . and shall recommend to the Administrator any new . . . standards and revisions of existing criteria and standards as may be appropriate . . .” Since the early 1980s, this independent review function has been performed by the CASAC of the EPA's Science Advisory Board. A number of other advisory functions are also identified for the committee by section 109(d)(2)(C), which reads:

Such committee shall also (i) advise the Administrator of areas in which additional knowledge is required to appraise the adequacy and basis of existing, new, or revised national ambient air quality standards, (ii) describe the research efforts necessary to provide the required information, (iii) advise the Administrator on the relative contribution to air pollution concentrations of natural as well as anthropogenic activity, and (iv) advise the Administrator of any adverse public health, welfare, social, economic, or energy effects which may result from various strategies for attainment and maintenance of such national ambient air quality standards.

As previously noted, the Supreme Court has held that section 109(b) “unambiguously bars cost considerations from the NAAQS-setting process,” in Whitman v. American Trucking Ass'ns, 531 U.S. 457, 471 (2001). Accordingly, while some of the issues listed in section 109(d)(2)(C) as those on which Congress has directed the CASAC to advise the Administrator, are ones that are relevant to the standard setting process, others are not. Issues that are not relevant to standard setting may be relevant to implementation of the NAAQS once they are established.

B. Related O ₃ Control Programs

States are primarily responsible for ensuring attainment and maintenance of ambient air quality standards once the EPA has established them. Under sections 110 and 171 through 185 of the CAA, and related provisions and regulations, states are to submit, for the EPA's approval, state implementation plans (SIPs) that provide for the attainment and maintenance of such standards through control programs directed to sources of the pollutants involved. The states, in conjunction with the EPA, also administer the prevention of significant deterioration of air quality program that covers these pollutants. See 42 U.S.C. 7470-7479. In addition, federal programs provide for nationwide reductions in emissions of O₃ precursors and other air pollutants under Title II of the Act, 42 U.S.C. 7521-7574, which involves controls for automobile, truck, bus, motorcycle, nonroad engine and equipment, and aircraft emissions; the new source performance standards under section 111 of the Act, 42 U.S.C. 7411; and the national emissions standards for hazardous air pollutants under section 112 of the Act, 42 U.S.C. 7412.

C. History of the Air Quality Criteria and Standards

Primary and secondary NAAQS were first established for photochemical oxidants in 1971 (36 FR 8186, April 30, 1971) based on the air quality criteria developed in 1970 (U.S. DHEW, 1970; 35 FR 4768, March 19, 1970). The EPA set both primary and secondary standards at 0.08 parts per million (ppm), as a 1-hour average of total photochemical oxidants, not to be exceeded more than one hour per year. Since that time, the EPA has reviewed the air quality criteria and standards a number of times, with the most recent review being completed in 2015.

The EPA initiated the first periodic review of the NAAQS for photochemical oxidants in 1977. Based on the 1978 air quality criteria document (AQCD [U.S. EPA, 1978]), the EPA proposed revisions to the original NAAQS in 1978 (43 FR 26962, June 22, 1978) and adopted revisions in 1979 (44 FR 8202, February 8, 1979). At that time, the EPA changed the indicator from photochemical oxidants to O₃, revised the level of the primary and secondary standards from 0.08 to 0.12 ppm and revised the form of both standards from a deterministic (i.e., not to be exceeded more than one hour per year) to a statistical form. With these changes, attainment of the standards was defined to occur when the average number of days per calendar year (across a 3-year period) with maximum hourly average O₃ concentration greater than 0.12 ppm equaled one or less (44 FR 8202, February 8, 1979; 43 FR 26962, June 22, 1978). Several petitioners challenged the 1979 decision. Among those, one claimed natural O₃ concentrations and other physical phenomena made the standard unattainable in the Houston area. The U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) rejected this argument, holding (as noted in section I.A above) that attainability and technological feasibility are not relevant considerations in the promulgation of the NAAQS (American Petroleum Institute v. Costle, 665 F.2d at 1185). The court also noted that the EPA need not tailor the NAAQS to fit each region or locale, pointing out that Congress was aware of the difficulty in meeting standards in some locations and had addressed it through various compliance-related provisions in the CAA (id. at 1184-86).

The next periodic reviews of the criteria and standards for O₃ and other photochemical oxidants began in 1982 and 1983, respectively (47 FR 11561, March 17, 1982; 48 FR 38009, August 22, 1983). As part of these reviews, the EPA published an AQCD, a Staff Paper, and a supplement to the AQCD (U.S. EPA, 1986; U.S. EPA, 1989; U.S. EPA, 1992). The schedule for completion of this review was governed by court order. In August of 1992, the EPA proposed to retain the existing primary and secondary standards (57 FR 35542, August 10, 1992). In March 1993, the EPA concluded this review by finalizing its proposed decision to retain the standards, without revision (58 FR 13008, March 9, 1993).

In the next review of the air quality criteria and standards for O₃ and other photochemical oxidants, for which the EPA had announced in August 1992 its intention to proceed rapidly, the EPA developed an AQCD and Staff Paper (57 FR 35542, August 10, 1992; U.S. EPA, 1996a; U.S. EPA, 1996b). Based on consideration of these assessments, the EPA proposed revisions to both the primary and secondary standards (61 FR 65716, December 13, 1996). The EPA completed this review in 1997 by revising both standards to 0.08 ppm, as the annual fourth-highest daily maximum 8-hour average concentration, averaged over three years (62 FR 38856, July 18, 1997).

In response to challenges to the EPA's 1997 decision revising the standards, the D.C. Circuit remanded the standards to the EPA, finding that section 109 of the CAA, as interpreted by the EPA, effected an unconstitutional delegation of legislative authority. See American Trucking Ass'ns v. EPA, 175 F.3d 1027, 1034-1040 (D.C. Cir. 1999). The court also directed that, in responding to the remand, the EPA should consider the potential beneficial health effects of O₃ pollution in shielding the public from the effects of solar ultraviolet (UV) radiation, as well as adverse health effects (id. at 1051-53). See American Trucking Ass'ns v. EPA, 195 F.3d 4, 10 (D.C. Cir. 1999) (granting panel rehearing in part but declining to review the ruling on consideration of the potential beneficial effects of O₃ pollution). After granting petitions for certiorari, the U.S. Supreme Court unanimously reversed the judgment of the D.C. Circuit on the constitutional issue, holding that section 109 of the CAA does not unconstitutionally delegate legislative power to the EPA. See Whitman v. American Trucking Ass'ns, 531 U.S. 457, 472-74 (2001). The Court remanded the case to the D.C. Circuit to consider challenges to the 1997 O₃ NAAQS that had not yet been addressed. On remand, the D.C. Circuit found the 1997 O₃ NAAQS to be “neither arbitrary nor capricious,” and so denied the remaining petitions for review. See ATA III, 283 F.3d at 379.

Coincident with the continued litigation of the other issues, the EPA responded to the court's 1999 remand to consider the potential beneficial health effects of O₃ pollution in shielding the public from effects of UV radiation (66 FR 57268, Nov. 14, 2001; 68 FR 614, January 6, 2003). In 2001, the EPA proposed to leave the 1997 primary standard unchanged (66 FR 57268, Nov. 14, 2001). After considering public comment on the proposed decision, the EPA published its final response to this remand in 2003, re-affirming the 8-hour primary standard set in 1997 (68 FR 614, January 6, 2003).

The EPA initiated the fourth periodic review of the air quality criteria and standards for O₃ and other photochemical oxidants with a call for information in September 2000 (65 FR 57810, September 26, 2000). In this review, the EPA developed an AQCD, Staff Paper and related technical support documents and proposed revisions to the primary and secondary standards (U.S. EPA, 2006; U.S. EPA, 2007; 72 FR 37818, July 11, 2007). The review was completed in March 2008 with revision of the levels of both the primary and secondary standards from 0.08 ppm to 0.075 ppm, and retention of the other elements of the prior standards (73 FR 16436, March 27, 2008). A number of petitioners filed suit challenging this decision.

In September 2009, the EPA announced its intention to reconsider the 2008 O₃ standards, and initiated a rulemaking to do so. At the EPA's request, the court held the consolidated cases in abeyance pending the EPA's reconsideration of the 2008 decision. In January 2010, the EPA issued a notice of proposed rulemaking to reconsider the 2008 final decision (75 FR 2938, January 19, 2010). Later that year, in view of the need for further consideration and the fact that the Agency's next periodic review of the O₃ NAAQS required under CAA section 109 had already begun (as announced in September 2008), the EPA consolidated the reconsideration with its statutorily required periodic review.

In light of the EPA's decision to consolidate the reconsideration with the review then ongoing, the D.C. Circuit proceeded with the litigation on the 2008 O₃ NAAQS decision. On July 23, 2013, the court upheld the EPA's 2008 primary standard, but remanded the 2008 secondary standard to the EPA. See Mississippi v. EPA, 744 F.3d 1334 (D.C. Cir. 2013). With respect to the secondary standard, the court held that the EPA's explanation for the setting of the secondary standard identical to the revised 8-hour primary standard was inadequate under the CAA because the EPA had not adequately explained how that standard provided the required public welfare protection.

At the time of the court's decision, the EPA had already completed significant portions of its next statutorily required periodic review of the O₃ NAAQS, which had been formally initiated in 2008, as summarized above. The documents developed for this review included the ISA, Risk and Exposure Assessments (REAs) for health and welfare, and PA (Frey, 2014a, Frey, 2014b, Frey, 2014c, U.S. EPA, 2013, U.S. EPA, 2014a, U.S. EPA, 2014b, U.S. EPA, 2014c). In late 2014, the EPA proposed to revise the 2008 primary and secondary standards (79 FR 75234, December 17, 2014). The EPA's final decision in this review established the now-current standards (80 FR 65292, October 26, 2015; 40 CFR 50.19). In this decision, based on consideration of the health effects evidence on respiratory effects of O₃ in at-risk populations, the EPA revised the primary standard from a level of 0.075 ppm to a level of 0.070 ppm, while retaining all other elements of the standard (80 FR 65292, October 26, 2015). The EPA's decision on the level for the standard was based on the weight of the scientific evidence and quantitative exposure/risk information. The level of the secondary standard was also revised from 0.075 ppm to 0.070 ppm based on the scientific evidence of O₃ effects on welfare, particularly the evidence of O₃ impacts on vegetation, and quantitative analyses available in the review. The other elements of the standard were retained. This decision on the secondary standard also incorporated the EPA's response to the D.C. Circuit's remand of the 2008 secondary standard in Mississippi v. EPA, 744 F.3d 1344 (D.C. Cir. 2013).

After publication of the final rule, a number of industry groups, environmental and health organizations, and certain states filed petitions for judicial review in the D.C. Circuit. The industry and state petitioners argued that the revised standards were too stringent, while the environmental and health petitioners argued that the revised standards were not stringent enough to protect public health and welfare as the Act requires. On August 23, 2019, the court issued an opinion that denied all the petitions for review with respect to the 2015 primary standard while also concluding that the EPA had not provided a sufficient rationale for aspects of its decision on the 2015 secondary standard and remanding that standard to the EPA. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). The court's decision on the secondary standard focused on challenges to particular aspects of EPA's decision. The court concluded that EPA's identification of particular benchmarks for evaluating the protection the standard provided against welfare effects associated with tree growth loss was reasonable and consistent with CASAC's advice. However, the court held that EPA had not adequately explained its decision to focus on a 3-year average for consideration of the cumulative exposure, in terms of W126, identified as providing requisite public welfare protection, or its decision to not identify a specific level of air quality related to visible foliar injury. The EPA's decision not to use a seasonal W126 index as the form and averaging time of the secondary standard was also challenged, but the court did not reach a decision on that issue, concluding that it lacked a basis to assess the EPA's rationale because the EPA had not yet fully explained its focus on a 3-year average W126 in its consideration of the standard. See Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019). Accordingly, the court remanded the secondary standard to EPA for further justification or reconsideration. The court's remand of the secondary standard has been considered in reaching the decision, and associated conclusions and judgments, described in section III.B.3 below.

In the August 2019 decision, the court additionally addressed arguments regarding considerations of background O₃ concentrations, and socioeconomic and energy impacts. With regard to the former, the court rejected the argument that the EPA was required to take background O₃ concentrations into account when setting the NAAQS, holding that the text of CAA section 109(b) precluded this interpretation because it would mean that if background O₃ levels in any part of the country exceeded the level of O₃ that is requisite to protect public health, the EPA would be obliged to set the standard at the higher nonprotective level (id. at 622-23). Thus, the court concluded that the EPA did not act unlawfully or arbitrarily or capriciously in setting the 2015 NAAQS without regard for background O₃ (id. at 624). Additionally, the court denied arguments that the EPA was required to consider adverse economic, social, and energy impacts in determining whether a revision of the NAAQS was “appropriate” under section 109(d)(1) of the CAA (id. at 621-22). The court reasoned that consideration of such impacts was precluded by Whitman's holding that the CAA “unambiguously bars cost considerations from the NAAQS-setting process” (531 U.S. at 471, summarized in section I.A above). Further, the court explained that section 109(d)(2)(C)'s requirement that CASAC advise the EPA “of any adverse public health, welfare, social, economic, or energy effects which may result from various strategies for attainment and maintenance” of revised NAAQS had no bearing on whether costs are to be considered in setting the NAAQS (Murray Energy Corp. v. EPA, 936 F.3d at 622). Rather, as described in Whitman and discussed further in section I.A above, most of that advice would be relevant to implementation but not standard setting (id.).

D. Current Review of the Air Quality Criteria and Standards

In May 2018, the Administrator directed his Assistant Administrators to initiate this current review of the O₃ NAAQS (Pruitt, 2018). In conveying this direction, the Administrator further directed the EPA staff to expedite the review, implementing an accelerated schedule aimed at completion of the review within the statutorily required period (Pruitt, 2018). Accordingly, the EPA took immediate steps to proceed with the review. In June 2018, the EPA announced the initiation of the periodic reviews of the air quality criteria for photochemical oxidants and of the O₃ NAAQS and issued a call for information in the Federal Register (83 FR 29785, June 26, 2018). Two types of information were called for: Information regarding significant new O₃ research to be considered for the ISA for the review, and policy-relevant issues for consideration in this NAAQS review. Based in part on the information received in response to the call for information, the EPA developed a draft IRP, which was made available for consultation with the CASAC and for public comment (83 FR 55163, November 2, 2018; 83 FR 55528, November 6, 2018). Comments from the CASAC (Cox, 2018) and the public were considered in preparing the final IRP (U.S. EPA, 2019b).

Under the plan outlined in the IRP and consistent with revisions to the process identified by the Administrator in his 2018 memo directing initiation of the review, the current review of the O₃ NAAQS has progressed on an accelerated schedule (Pruitt, 2018). The EPA has incorporated a number of efficiencies in various aspects of the review process, as summarized in the IRP, to support the accelerated schedule (Pruitt, 2018). As one example of such an efficiency, rather than produce separate documents for the PA and associated quantitative analyses, the human exposure and health risk analyses (that inform the decision on the primary standard) and the air quality and exposure analyses (that inform the decision on the secondary standard) are included as appendices in the PA, along with other technical appendices that inform these standards decisions. The draft PA (including these analyses as appendices) was reviewed by the CASAC and made available for public comment while the draft ISA was also being reviewed by the CASAC and was available for public comment (84 FR 50836, September 26, 2019; 84 FR 58711, November 1, 2019). The CASAC was assisted in its review by a pool of consultants with expertise in a number of fields (84 FR 38625, August 7, 2019). The approach employed by the CASAC in utilizing outside technical expertise represents an additional modification of the process from past reviews. Rather than join with some or all of the CASAC members in a CASAC review panel as has been common in other NAAQS reviews in the past, in this O₃ NAAQS review (and also in the recent CASAC review of the PA for the particulate matter NAAQS), the consultants comprised a pool of expertise that CASAC members drew on through the use of specific questions, posed in writing prior to the public meeting, regarding aspects of the documents being reviewed, obtaining subject matter expertise for their review in a focused, efficient and transparent manner.

The CASAC discussed its review of both the draft ISA and the draft PA over three days at a public meeting in December 2019 (84 FR 58713, November 1, 2019). The CASAC discussed its draft letters describing its advice and comments on the documents in a public teleconference in early February 2020 (85 FR 4656; January 27, 2020). The letters to the Administrator conveying the CASAC advice and comments on the draft PA and draft ISA were released later that month (Cox, 2020a; Cox, 2020b).

The letters from the CASAC and public comment on the draft ISA and draft PA informed completion of the final documents and further informed development of the Administrator's proposed and final decisions in this review. Comments from the CASAC on the draft ISA were considered by the EPA and led to a number of revisions in developing the final document. The CASAC review of the draft ISA and the EPA's consideration of CASAC comments are described in Appendix 10, section 10.4.5 of the final ISA. In his reply to the CASAC letter conveying its review, “Administrator Wheeler noted, `for those comments and recommendations that are more significant or cross-cutting and which were not fully addressed, the Agency will develop a plan to incorporate these changes into future O₃ ISAs as well as ISAs for other criteria pollutant reviews' ” (ISA, p. 10-28; Wheeler, 2020). The ISA was completed and made available to the public in April 2020 (85 FR 21849, April 20, 2020). Based on the rigorous scientific approach utilized in its development, summarized in Appendix 10 of the final ISA, the EPA considers the final ISA to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [O₃] in the ambient air, in varying quantities” as required by the CAA (42 U.S.C. 7408(a)(2)).

The CASAC comments additionally provided advice with regard to the primary and secondary standards, as well as a number of comments intended to improve the PA. These comments were considered in completing that document (85 FR 31182, May 22, 2020). The CASAC advice to the Administrator regarding the O₃ standards has also been described and considered in the PA, and in sections II and III below. The CASAC advice on the primary standard is summarized in II.B.2 below and its advice on the secondary standard is summarized in section III.B.1.b.

Materials upon which this proposed decision is based, including the documents described above, are available to the public in the docket for the review. As in prior NAAQS reviews, the EPA is basing its decision in this review on studies and related information included in the air quality criteria, which have undergone CASAC and public review. The studies assessed in the ISA and PA, and the integration of the scientific evidence presented in them, have undergone extensive critical review by the EPA, the CASAC, and the public. The rigor of that review makes these studies, and their integrative assessment, the most reliable source of scientific information on which to base decisions on the NAAQS, decisions that all parties recognize as of great import. Decisions on the NAAQS can have profound impacts on public health and welfare, and NAAQS decisions should be based on studies that have been rigorously assessed in an integrative manner not only by the EPA but also by the statutorily mandated independent scientific advisory committee, as well as the public review that accompanies this process. Some commenters have referred to and discussed individual scientific studies on the health effects of O₃ that were not included in the ISA (“ `new' studies”) and that have not gone through this comprehensive review process. In considering and responding to comments for which such “new” studies were cited in support, the EPA has provisionally considered the cited studies in the context of the findings of the ISA. The EPA's provisional consideration of these studies did not and could not provide the kind of in-depth critical review described above, but rather was focused on determining whether they warranted reopening the review of the air quality criteria to enable the EPA, the CASAC and the public to consider them further.

This approach, and the decision to rely on studies and related information included in the air quality criteria, which have undergone CASAC and public review, is consistent with the EPA's practice in prior NAAQS reviews and its interpretation of the requirements of the CAA. Since the 1970 amendments, the EPA has taken the view that NAAQS decisions are to be based on scientific studies and related information that have been assessed as a part of the pertinent air quality criteria, and the EPA has consistently followed this approach. This longstanding interpretation was strengthened by new legislative requirements enacted in 1977, which added section 109(d)(2) of the Act concerning CASAC review of air quality criteria. See 71 FR 61144, 61148 (October 17, 2006, final decision on review of NAAQS for particulate matter) for a detailed discussion of this issue and the EPA's past practice.

As discussed in the EPA's 1993 decision not to revise the O₃ NAAQS, “new” studies may sometimes be of such significance that it is appropriate to delay a decision in a NAAQS review and to supplement the pertinent air quality criteria so the studies can be taken into account (58 FR at 13013-13014, March 9, 1993). In the present case, the EPA's provisional consideration of “new” studies concludes that, taken in context, the “new” information and findings do not materially change any of the broad scientific conclusions regarding the health and welfare effects of O₃ in ambient air made in the air quality criteria. For this reason, reopening the air quality criteria review would not be warranted.

Accordingly, the EPA is basing the final decisions in this review on the studies and related information included in the O₃ air quality criteria that have undergone rigorous review by the EPA, the CASAC and the public. The EPA will consider these “new” studies for inclusion in the air quality criteria for the next O₃ NAAQS review, which the EPA expects to begin soon after the conclusion of this review and which will provide the opportunity to fully assess these studies through a more rigorous review process involving the EPA, the CASAC, and the public.

E. Air Quality Information

Ground level O₃ concentrations are a mix of mostly tropospheric O₃ and some stratospheric O₃. Tropospheric O₃ is formed due to chemical interactions involving solar radiation and precursor pollutants including VOCs and nitrogen oxides (NO_X). Methane (CH₄) and carbon monoxide (CO) are also important precursors, particularly at the regional to global scale. The precursor emissions leading to tropospheric O₃ formation can result from both man-made sources (e.g., motor vehicles and electric power generation) and natural sources (e.g., vegetation and wildfires). In addition, O₃ that is created naturally in the stratosphere also contributes to O₃ in the troposphere. The stratosphere routinely mixes with the troposphere high above the earth's surface and, less frequently, there are intrusions of stratospheric air that reach deep into the troposphere and even to the surface. Once formed, O₃ near the surface can be transported by winds before eventually being removed from the atmosphere via chemical reactions or deposition to surfaces. In sum, O₃ concentrations are influenced by complex interactions between precursor emissions, meteorological conditions, and topographical characteristics (PA, section 2.1; ISA, Appendix 1).

For compliance and other purposes, state and local environmental agencies operate O₃ monitors across the U.S. and submit the data to the EPA. At present, there are approximately 1,300 monitors across the U.S. reporting hourly O₃ averages during the times of the year when local O₃ pollution can be important (PA, section 2.3.1). Most of this monitoring is focused on urban areas where precursor emissions tend to be largest, as well as locations directly downwind of these areas. There are also over 100 routine monitoring sites in rural areas, including sites in the Clean Air Status and Trends Network (CASTNET) which is specifically focused on characterizing conditions in rural areas. Based on the monitoring data for the three year period from 2016 to 2018, the EPA identified 142 counties, in which together approximately 106 million Americans reside where O₃ design values were above 0.070 ppm, the level of the existing NAAQS (PA, section 2.4.1). Across these areas, the highest design values are typically observed in California, Texas, Denver, around Lake Michigan and along the Northeast Corridor, locations with some of the most densely populated areas in the country (e.g., PA, Figure 2-8).

From a temporal perspective, the highest O₃ concentrations tend to occur during the afternoon and within the warmer months of the year due to higher levels of solar radiation and other conducive meteorological conditions during these times. The exceptions to this general rule include (1) some rural sites where transport of O₃ from upwind urban areas can occasionally result in high nighttime levels of O₃, (2) high-elevation sites which can be episodically influenced by stratospheric intrusions in other months of the year, and (3) mountain basins in the western U.S. where large quantities of O₃ precursors emissions associated with oil and gas development can be trapped in a shallow inversion layer and form O₃ under clear, calm skies with snow cover during the colder months (PA, section 2.1; ISA, Appendix 1).

Monitoring data indicate long-term reductions in peak O₃ concentrations. For example, monitoring sites operating since 1980 indicate a 32% reduction in the national average annual fourth highest daily maximum 8-hour concentration from 1980 to 2018. (PA, Figure 2-10). This has been accompanied by appreciable reductions in peak 1-hour concentrations, as seen by reductions in annual second highest daily maximum 1-hour concentrations (PA, Figure 2-17).

Concentrations of O₃ in ambient air that result from natural and non-U.S. anthropogenic sources are collectively referred to as U.S. background O₃ (USB; PA, section 2.5). As in the last review, we generally characterize O₃ concentrations that would exist in the absence of U.S. anthropogenic emissions (as USB). Findings from air quality modeling analyses performed for this review to investigate patterns of USB in the U.S. are largely consistent with conclusions reached in the last review (PA, section 2.5.4). The current modeling analysis indicates spatial variation in USB O₃ concentrations that is related to geography, topography and proximity to international borders and is also influenced by seasonal variation, with long-range international anthropogenic transport contributions peaking in the spring while U.S. anthropogenic contributions tend to peak in summer. The West is predicted to have higher USB concentrations than the East, with higher contributions from natural and international anthropogenic sources that exert influences in western high-elevation and near-border areas. The modeling predicts that for both the West and the East, days with the highest 8-hour concentrations of O₃ generally occur in summer and are likely to have substantially greater concentrations due to U.S. anthropogenic sources. While the USB contributions to O₃ concentrations on days with the highest 8-hour concentrations are generally predicted to come largely from natural sources, the modeling also indicates that some areas near the Mexico border may receive appreciable contributions from a combination of natural and international anthropogenic sources on these days. In such locations, the modeling suggests the potential for relatively infrequent events with substantial background contributions where daily maximum 8-hour O₃ concentrations approach or exceed the level of the current NAAQS (i.e., 70 ppb). This contrasts with most monitor locations in the U.S. for which international contributions are predicted to be the lowest during the season with the most frequent occurrence of daily maximum 8-hour O₃ concentrations above 70 ppb. This is generally because, except for in near-border areas, larger international contributions are associated with long-distance transport and that is most efficient in the springtime (PA, section 2.5.4).

II. Rationale for Decision on the Primary Standard

This section presents the rationale for the Administrator's decision to retain the current primary O₃ standard. This rationale is based on the scientific information presented in the ISA, on human health effects associated with photochemical oxidants including O₃ and pertaining to the presence of these pollutants in ambient air. As summarized in section I.D above, the ISA was developed based on a thorough review of the latest scientific information generally published between January 2011 and March 2018, as well as more recent studies identified during peer review, submitted in response to the Call for Information, or public comments on the draft ISA, integrated with the information and conclusions from previous assessments (ISA, section IS.1.2 and Appendix 10, section 10.2). The Administrator's rationale also takes into account: (1) The PA evaluation of the policy-relevant information in the ISA and presentation of quantitative analyses of air quality, human exposure and health risks; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the ISA and PA at public meetings and in the CASAC's letters to the Administrator; and (3) public comments on the proposed decision.

Within this section, introductory and background information is presented in section II.A. Section II.A.1 summarizes the 2015 establishment of the existing standard, as background for this review. Section II.A.2 provides an overview of the currently available health effects evidence, and section II.A.3 provides an overview of the current exposure and risk information, drawing on the quantitative analyses presented in the PA. Section II.B summarizes the basis for the proposed decision (II.B.1), discusses public comments on the proposed decision (II.B.2), and presents the Administrator's considerations, conclusions and decision in this review of the primary standard (II.B.3). The decision on the current primary standard is summarized in section II.C.

A. Introduction

As in prior reviews, the general approach to reviewing the current primary standard is based, most fundamentally, on using the Agency's assessments of the current scientific evidence and associated quantitative analyses to inform the Administrator's judgment regarding a primary standard for photochemical oxidants that is requisite to protect the public health with an adequate margin of safety. The EPA's assessments are primarily documented in the ISA and PA, both of which have received CASAC review and public comment (84 FR 50836, September 26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging the gap between the scientific assessments of the ISA and the judgments required of the Administrator in his decisions on the current standard, the PA evaluates policy implications of the assessment of the current evidence in ISA and the quantitative exposure and risk analyses documented extensively in appendices of the PA. In evaluating the public health protection afforded by the current standard, the four basic elements of the NAAQS (indicator, averaging time, level, and form) are considered collectively.

The final decision on the adequacy of the current primary standard is a public health policy judgment to be made by the Administrator. In reaching conclusions on the standard, the decision draws on the scientific information and analyses about health effects, population exposure and risks, as well as judgments about how to consider the range and magnitude of uncertainties that are inherent in the scientific evidence and analyses. This approach is based on the recognition that the available health effects evidence generally reflects a continuum, consisting of levels at which scientists generally agree that health effects are likely to occur, through lower levels at which the likelihood and magnitude of the response become increasingly uncertain. This approach is consistent with the requirements of the NAAQS provisions of the Clean Air Act and with how the EPA and the courts have historically interpreted the Act (summarized in section I.A. above). These provisions require the Administrator to establish primary standards that, in the judgment of the Administrator, are requisite to protect public health with an adequate margin of safety. In so doing, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose. The Act does not require that primary standards be set at a zero-risk level, but rather at a level that avoids unacceptable risks to public health, including the health of sensitive groups.

1. Background on the Current Standard

As a result of the last O₃ NAAQS review, completed in 2015, the level of the primary standard was revised from 0.075 to 0.070 ppm, in conjunction with retaining the existing indicator, averaging time, and form. This revision, establishing the current standard, was based on the scientific evidence and quantitative exposure and risk analyses available at that time, as well as the Administrator's judgments regarding the available health effects evidence, the appropriate degree of public health protection for the revised standard, and the available exposure and risk information regarding the exposures and risk that may be allowed by such a standard (80 FR 65292, October 26, 2015). In establishing this standard, the Administrator considered the extensive body of evidence spanning several decades documenting the causal relationship between O₃ exposure and a broad range of respiratory effects (80 FR 65292, October 26, 2015; 2013 ISA, p. 1-14), that had been augmented by evidence available since the prior review was completed in 2008. Such effects range from small, reversible changes in pulmonary function and pulmonary inflammation (documented in controlled human exposure studies involving exposures ranging from 1 to 8 hours) to more serious health outcomes such as asthma-related emergency department visits and hospital admissions, which have been associated with ambient air concentrations of O₃ in epidemiologic studies (2013 ISA, section 6.2). The 2015 decision, which provided increased protection for at-risk populations, such as children and people with asthma, against an array of adverse health effects, drew upon the available scientific evidence assessed in the 2013 ISA, the exposure and risk information presented and assessed in the 2014 health REA (HREA), the consideration of that evidence and information in the 2014 PA, the advice and recommendations of the CASAC, and public comments on the proposed decision (79 FR 75234, December 17, 2014).

Across the different study types, the controlled human exposure studies, which were recognized to provide the most certain evidence indicating the occurrence of health effects in humans following specific O₃ exposures, additionally document the roles of ventilation rate and exposure duration, in addition to exposure concentration, in eliciting responses to O₃ exposure (80 FR 65343, October 26, 2015; 2014 PA, section 3.4). These aspects of the evidence were represented in exposure-based analyses developed to inform the NAAQS decision with estimates of exposure and risk associated with air quality conditions just meeting the then-existing standard, and also for air quality conditions just meeting potential alternative standards (U.S. EPA, 2014a, hereafter 2014 HREA). The exposure-based analyses given greatest weight in the Administrator's consideration of the HREA estimates involved comparison of estimates for study area populations of children of exposure at elevated exertion to exposure benchmark concentrations (exposures of concern). The benchmark concentrations (60, 70 and 80 ppb) were identified from controlled human exposure studies (conducted with generally healthy adults).

In weighing the health effects evidence and making judgments regarding the public health significance of the quantitative estimates of exposures and risks allowed by the then-existing standard and potential alternative standards considered, as well as judgments regarding margin of safety, the Administrator's 2015 decision considered the currently available information and commonly accepted guidelines or criteria within the public health community, including statements of the American Thoracic Society (ATS), an organization of respiratory disease specialists, advice from the CASAC, and public comments. In so doing, she recognized that the determination of what constitutes an adequate margin of safety is expressly left to the judgment of the EPA Administrator. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1161-62 (D.C. Cir 1980); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). In NAAQS reviews generally, evaluations of how particular primary standards address the requirement to provide an adequate margin of safety include consideration of such factors as the nature and severity of the health effects, the size of the sensitive population(s) at risk, and the kind and degree of the uncertainties present. Consistent with past practice and long-standing judicial precedent, the Administrator took the need for an adequate margin of safety into account as an integral part of her decision-making.

In the decisions regarding adequacy of protection provided by the then-existing primary standard and on alternatives for a new revised standard, primary consideration was given to the evidence of respiratory effects from controlled human exposure studies, including those newly available in the review, and for which the exposure concentrations were at the lower end of those studied (80 FR 65342-47 and 65362-66, October 26, 2015). This emphasis was consistent with comments on the strength of this evidence from the CASAC at that time (Frey, 2014b, p. 5). In placing weight on these studies, the Administrator at that time took note of the variety of respiratory effects reported from the studies of healthy adults engaged in quasi-continuous exercise within a 6.6-hour exposure to O₃ concentrations of 60 ppb and higher. The lowest exposure concentration in such studies for which a combination of statistically significant reduction in lung function and increase in respiratory symptoms was somewhat above 70 ppb, while reduced lung function and increased pulmonary inflammation were reported following such exposures to O₃ concentrations as low as 60 ppb. In considering these findings, the Administrator noted that the combination of O₃-induced lung function decrements and respiratory symptoms met ATS criteria for an adverse response, and noted CASAC comments, which included a caution regarding the potential for effects in some groups of people, such as people with asthma, at exposure concentrations below those affecting healthy subjects (Frey, 2014b, pp. 5-6; 80 FR 65343, October 26, 2015). With regard to the epidemiologic evidence, the Administrator noted the ISA finding that the pattern of effects observed across the range of exposures assessed in the controlled human exposure studies, increasing in severity at higher exposures, is coherent with (i.e., reasonably related to) the health outcomes reported to be associated with ambient air concentrations in epidemiologic studies. Additionally, while recognizing that most O₃ epidemiologic studies reported health outcome associations with O₃ concentrations in ambient air that violated the then-existing standard, the Administrator took note of a study that reported associations between short-term O₃ concentrations and asthma emergency department visits in children and adults in a U.S. location that would have met the then-existing standard over the entire 5-year study period (80 FR 65344, October 26, 2015; Mar and Koenig, 2009). Taken together, the Administrator concluded that the scientific evidence from controlled human exposure and epidemiologic studies called into question the adequacy of the public health protection provided by the 75 ppb standard that had been set in 2008.

In considering the exposure and risk information, the Administrator's 2015 decision gave particular attention to the exposure-based comparison-to-benchmarks analysis, focusing on the estimates of exposures of concern for children in 15 urban study areas for air quality conditions just meeting the then-current standard. Consistent with the finding that larger percentages of children than adults were estimated to experience exposures at or above benchmarks, the Administrator focused on the results for all children and for children with asthma, noting that the results for these two groups, in terms of percent of the population group, are virtually indistinguishable (2014 HREA, sections 5.3.2, 5.4.1.5 and section 5F-1). The Administrator placed the greatest weight on estimates of two or more days with occurrences of exposures at or above the benchmarks, in light of her increased concern about the potential for adverse responses with repeated occurrences of such exposures, noting that the types of effects shown to occur following exposures to O₃ concentrations from 60 ppb to 80 ppb, such as inflammation, if occurring repeatedly as a result of repeated exposure, could potentially result in more severe effects (80 FR 65343, 65345, October 26, 2015; 2013 ISA, section 6.2.3). The Administrator also considered estimates for single exposures at or above the higher benchmarks of 70 and 80 ppb (80 FR 65345, October 26, 2015). With regard to the 60 ppb benchmark, while the Administrator recognized the effects reported from controlled human exposure studies of 60 ppb to be less severe than those for higher O₃ concentrations, she also recognized there were limitations and uncertainties in the evidence base with regard to unstudied population groups. As a result, she judged it appropriate for the standard, in providing an adequate margin of safety, to provide some control of exposures at or above the 60 ppb benchmark (80 FR 65345-65346, October 26, 2015).

In considering public health implications of the exposure and risk information, the Administrator concluded that the exposures and risks projected to remain upon meeting the then-current (75 ppb) standard were reasonably judged important from a public health perspective. This conclusion was particularly based on her judgment that it is appropriate to set a standard that would be expected to eliminate, or almost eliminate, the occurrence of exposures, while at moderate exertion, at or above 70 and 80 ppb (80 FR 65346, October 26, 2015). In addition, given that in the air quality scenario for the existing standard, the average percent of children estimated to experience two or more days with exposures at or above the 60 ppb benchmark approached 10% in some urban study areas (on average across the analysis years), the Administrator concluded that the existing standard did not incorporate an adequate margin of safety against the potentially adverse effects that could occur following repeated exposures at or above 60 ppb (80 FR 65345-46, October 26, 2015). Thus, the exposure and risk estimates were judged to support a conclusion that the existing standard was not sufficiently protective and did not incorporate an adequate margin of safety. In consideration of all of the above, as well as the CASAC advice, which included the unanimous recommendation “that the Administrator revise the current primary ozone standard to protect public health” (Frey, 2014b, p. 5), the Administrator concluded that the then-current primary O₃ standard (with its level of 75 ppb) was not requisite to protect public health with an adequate margin of safety, and should be revised to provide increased public health protection (80 FR 65346, October 26, 2015).

With regard to the most appropriate indicator for the revised standard, key considerations included the finding that O₃ is the only photochemical oxidant (other than nitrogen dioxide) that is routinely monitored and for which a comprehensive database exists, and the consideration that, since the precursor emissions that lead to the formation of O₃ also generally lead to the formation of other photochemical oxidants, measures leading to reductions in population exposures to O₃ can generally be expected to lead to reductions in other photochemical oxidants (2013 ISA, section 3.6; 80 FR 65347, October 26, 2015). The CASAC also indicated O₃ to be the appropriate indicator “based on its causal or likely causal associations with multiple adverse health outcomes and its representation of a class of pollutants known as photochemical oxidants” (Frey, 2014b, p. ii). Based on all of these considerations and public comments, the Administrator retained O₃ as the indicator for the primary standard (80 FR 65347, October 26, 2015).

With regard to averaging time, eight hours was the duration established in 1997 with the replacement of the then-existing 1-hour standard (62 FR 38856, July 18, 1997). The decision at that time was based on evidence from numerous controlled human exposure studies reporting adverse respiratory effects resulting from 6- to 8-hour exposures, as well as quantitative analyses indicating the control provided by an 8-hour averaging time of both 8-hour and 1-hour peak exposures and associated health risk (62 FR 38861, July 18, 1997; U.S. EPA, 1996b). The 1997 decision was also consistent with CASAC advice at that time (62 FR 38861, July 18, 1997; 61 FR 65727, December 13, 1996). For similar reasons, the 8-hour averaging time was retained in the subsequent 2008 review (73 FR 16436, March 27, 2008). In 2015, the decision, based on then-available health effects information, was to again retain the 8-hour averaging time, as appropriate for addressing health effects associated with short-term exposures to ambient air O₃, and based on the conclusion that it could effectively limit health effects attributable to both short- and long-term O₃ exposures (80 FR 65348, 65350, October 26, 2015).

With regard to the form for the standard, the existing n th-high metric form had been established in the 1997 review, when the form was revised from an expected exceedance form. At that time, it was recognized that a concentration-based form, by giving proportionally more weight to years when 8-hour O₃ concentrations are well above the level of the standard than years when concentrations are just above the level, better reflects the continuum of health effects associated with increasing O₃ concentrations than does an expected exceedance form (80 FR 65350-65352, October 26, 2015). The subsequent 2008 review also considered the potential value of a percentile-based form, but the EPA concluded that, because of the differing lengths of the monitoring season for O₃ across the U.S., such a form would not be effective in ensuring the same degree of public health protection across the country (73 FR 16474-75, March 27, 2008). Additionally, the EPA recognized the importance of a form that provides stability to ongoing control programs and insulation from the impacts of extreme meteorological events that are conducive to O₃ occurrence (73 FR 16474-16475, March 27, 2008). In the 2015 decision, based on all of these considerations, and including advice from the CASAC, which stated that this form “provides health protection while allowing for atypical meteorological conditions that can lead to abnormally high ambient ozone concentrations which, in turn, provides programmatic stability” (Frey, 2014b, p. 6), the existing form (the annual fourth-highest daily maximum 8-hour O₃ average concentration, averaged over three consecutive years) was retained (80 FR 65352, October 26, 2015).

As for the decision on adequacy of protection provided by the combination of all elements of the existing standard, the 2015 decision to set the level of the revised standard at 70 ppb placed the greatest weight on the results of controlled human exposure studies and on quantitative analyses based on information from these studies, particularly analyses of O₃ exposures of concern, consistent with CASAC advice and interpretation of the scientific evidence (80 FR 65362, October 26, 2015; Frey, 2014b). This weighting reflected the recognition that controlled human exposure studies provide the most certain evidence indicating the occurrence of health effects in humans following specific O₃ exposures, and, in particular, that the effects reported in the controlled human exposure studies are due solely to O₃ exposures, and are not complicated by the presence of co-occurring pollutants or pollutant mixtures (as is the case in epidemiologic studies) (80 FR 65362-65363, October 26, 2015). With regard to this evidence, the Administrator at that time recognized that: (1) The largest respiratory effects, and the broadest range of effects, have been studied and reported following exposures to 80 ppb O₃ or higher (i.e., decreased lung function, increased airway inflammation, increased respiratory symptoms, airway hyperresponsiveness, and decreased lung host defense); (2) exposures to O₃ concentrations somewhat above 70 ppb have been shown to both decrease lung function and to result in respiratory symptoms; and (3) exposures to O₃ concentrations as low as 60 ppb have been shown to decrease lung function and to increase airway inflammation (80 FR 65363, October 26, 2015). The Administrator also noted that 70 ppb was well below the O₃ exposure concentration documented to result in the widest range of respiratory effects (i.e., 80 ppb), and below the lowest O₃ exposure concentration shown in 6.6 hour exposures with quasi-continuous exercise to result in the combination of lung function decrements and respiratory symptoms (80 FR 65363, October 26, 2015).

In considering the degree of protection to be provided by a revised standard, and the extent to which that standard would be expected to limit population exposures to the broad range of O₃ exposures shown to result in health effects, the Administrator focused particularly on the HREA estimates of two or more exposures of concern. Placing the most emphasis on a standard that limits repeated occurrences of exposures at or above the 70 and 80 ppb benchmarks, while at elevated ventilation, the Administrator noted that a revised standard with a level of 70 ppb was estimated to eliminate the occurrence of two or more days with exposures at or above 80 ppb and to virtually eliminate the occurrence of two or more days with exposures at or above 70 ppb for all children and children with asthma, even in the worst-case year and location evaluated (80 FR 65363-65364, October 26, 2015). The Administrator's consideration of exposure estimates at or above the 60 ppb benchmark (focused most particularly on multiple occurrences), an exposure to which the Administrator was less confident would result in adverse effects, as discussed above, was primarily in the context of considering the extent to which the health protection provided by a revised standard included a margin of safety against the occurrence of adverse O₃-induced effects (80 FR 65364, October 26, 2015). In this context, the Administrator noted that a revised standard with a level of 70 ppb was estimated to protect the vast majority of children in urban study areas (i.e., about 96% to more than 99% of children in individual areas) from experiencing two or more days with exposures at or above 60 ppb (while at moderate or greater exertion). This represented a more than 60% reduction in repeated exposures over the estimates for the then-existing standard, with its level of 75 ppb.

Given the considerable protection provided against repeated exposures of concern for all three benchmarks, including the 60 ppb benchmark, the Administrator judged that a standard with a level of 70 ppb would incorporate a margin of safety against the adverse O₃-induced effects shown to occur in the controlled human exposure studies following exposures (while at moderate or greater exertion) to a concentration somewhat higher than 70 ppb (80 FR 65364, October 26, 2015). The Administrator also judged the HREA estimates of one or more exposures (while at moderate or greater exertion) at or above 60 ppb to also provide support for her somewhat broader conclusion that “a standard with a level of 70 ppb would incorporate an adequate margin of safety against the occurrence of O₃ exposures that can result in effects that are adverse to public health” (80 FR 65364, October 26, 2015). Although she placed less weight on the other HREA risk estimates and epidemiologic evidence for considering the standard level, in light of associated uncertainties, the Administrator judged that a standard with a level of 70 ppb would be expected to result in important reductions in the population-level risk of endpoints on which these types of information are focused and provide associated additional public health protection, beyond that provided by the then-existing standard (80 FR 65364, October 26, 2015). In summary, based on the evidence, exposure and risk information, advice from the CASAC, and public comments, the 2015 decision was to revise the primary standard to be 70 ppb, in terms of the 3-year average of annual fourth-highest daily maximum 8-hour average O₃ concentrations, to provide the requisite protection of public health, including the health of at-risk populations, with an adequate margin of safety (80 FR 65365, October 26, 2015).

2. Overview of Health Effects Information

The information summarized in this section is an overview of the scientific assessment of the health effects evidence available in this review; the assessment is documented in the ISA and its policy implications are further discussed in the PA. In this review, as in past reviews, the health effects evidence evaluated in the ISA for O₃ and related photochemical oxidants is focused on O₃ (ISA, section IS.1.1). Ozone is concluded to be the most prevalent photochemical oxidant present in the atmosphere and the one for which there is a very large, well-established evidence base of its health and welfare effects (ISA, section IS.1.1). Thus, the current health effects evidence and the Agency's review of the evidence, including the evidence newly available in this review, continues to focus on O₃. The subsections below briefly summarize the following aspects of the evidence: The nature of O₃-related health effects, the potential public health implications and populations at risk, and exposure concentrations associated with health effects.

a. Nature of Effects

The evidence base available in the current review includes decades of extensive evidence that clearly describes the role of O₃ in eliciting an array of respiratory effects and recent evidence indicates the potential for relationships between O₃ exposure and metabolic effects. As was established in prior reviews, the effects for which the evidence is strongest are transient decrements in pulmonary function and respiratory symptoms, such as coughing and pain on deep inspiration, as a result of short-term exposures particularly when breathing at elevated rates (ISA, section IS.4.3.1; 2013 ISA, p. 2-26). These effects are demonstrated in the large, long-standing evidence base of controlled human exposure studies (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, ISA). The epidemiologic evidence base documents consistent, positive associations of O₃ concentrations in ambient air with lung function effects in panel studies (2013 ISA, section 6.2.1.2; ISA, Appendix 3, section 3.1.4.1.3), and with more severe health outcomes, including asthma-related emergency department visits and hospital admissions (2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1 and 3.1.5.2). Extensive experimental animal evidence informs a detailed understanding of mechanisms underlying the short-term respiratory effects, and studies in animal models describe effects of longer-term O₃ exposure on the developing lung (ISA, Appendix 3, sections 3.1.11 and 3.2.6).

The full body of evidence continues to support the conclusions of a causal relationship of respiratory effects with short-term O₃ exposures and of a relationship of respiratory effects with longer-term exposures that is likely to be causal (ISA, sections IS.4.3.1 and IS.4.3.2). Further, the ISA determines that the relationship between short-term O₃ exposure and metabolic effects is likely to be causal, based primarily on newly available experimental animal evidence (ISA, section IS.4.3.3). The newly available evidence, particularly from controlled human exposure studies of cardiovascular endpoints, has altered conclusions from the last review with regard to relationships between short-term O₃ exposures and cardiovascular effects and mortality, such that the evidence no longer supports conclusions that the relationships are likely to be causal.

With regard to respiratory effects from short-term O₃ exposure, the strongest evidence comes from controlled human exposure studies, also available in the last review, demonstrating O₃-related respiratory effects in generally healthy adults (ISA, section IS.1.3.1). As in the last review, the key evidence comes from the body of controlled human exposure studies that document respiratory effects in people exposed for short periods (6.6 to 8 hours) during quasi-continuous exercise. The potential for O₃ exposure to elicit health outcomes more serious than those assessed in the controlled human exposure studies continues to be indicated by the epidemiologic evidence of associations of O₃ concentrations in ambient air with increased incidence of hospital admissions and emergency department visits for an array of health outcomes, including asthma exacerbation, chronic obstructive pulmonary disease (COPD) exacerbation, respiratory infection, and combinations of respiratory diseases (ISA, Appendix 3, sections 3.1.5 and 3.1.6). The strongest such evidence is for asthma-related outcomes and specifically asthma-related outcomes for children, indicating an increased risk for people with asthma and particularly children with asthma (ISA, Appendix 3, section 3.1.5.7).

Respiratory responses observed in human subjects exposed to O₃ for periods of 8 hours or less, while intermittently or quasi-continuously exercising, include lung function decrements (e.g., based on forced expiratory volume in one second [FEV₁] measurements), respiratory symptoms, increased airway responsiveness, mild bronchoconstriction (measured as an increase in specific airway resistance [sRaw]), and pulmonary inflammation, with associated injury and oxidative stress (ISA, Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through 6.2.4). The available mechanistic evidence, discussed in greater detail in the ISA, describes pathways involving the respiratory and nervous systems by which O₃ results in pain-related respiratory symptoms and reflex inhibition of maximal inspiration (inhaling a full, deep breath), commonly quantified by decreases in forced vital capacity (FVC) and total lung capacity. This reflex inhibition of inspiration combined with mild bronchoconstriction contributes to the observed decrease in FEV₁, the most common metric used to assess O₃-related lung function effects. The evidence also indicates that the additionally observed inflammatory response is correlated with mild airway obstruction, generally measured as an increase in sRaw (ISA, Appendix 3, section 3.1.3). As described below, the prevalence and severity of respiratory effects in controlled human exposure studies, including symptoms (e.g., pain on deep inspiration, shortness of breath, and cough), increases with increasing O₃ concentration, exposure duration, and ventilation rate of exposed subjects (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2).

Within the evidence base from controlled human exposure studies, the majority of studies involve healthy adult subjects (generally 18 to 35 years), although there are studies involving subjects with asthma, and a limited number of studies, generally of durations shorter than four hours, involving adolescents and adults older than 50 years. A summary of salient observations of O₃ effects on lung function, based on the controlled human exposure study evidence reviewed in the 1996 and 2006 AQCDs, and recognized in the 2013 ISA, continues to pertain to this evidence base as it exists today: “(1) young healthy adults exposed to ≥80 ppb ozone develop significant reversible, transient decrements in pulmonary function and symptoms of breathing discomfort if minute ventilation (Ve) or duration of exposure is increased sufficiently; (2) relative to young adults, children experience similar spirometric responses [i.e., as measured by FEV₁ and/or FVC] but lower incidence of symptoms from O₃ exposure; (3) relative to young adults, ozone-induced spirometric responses are decreased in older individuals; (4) there is a large degree of inter-subject variability in physiologic and symptomatic responses to O₃, but responses tend to be reproducible within a given individual over a period of several months; and (5) subjects exposed repeatedly to O₃ for several days experience an attenuation of spirometric and symptomatic responses on successive exposures, which is lost after about a week without exposure” (ISA, Appendix 3, section 3.1.4.1.1, p. 3-11). Repeated daily exposure studies at higher concentrations, such as 300 ppb, have found FEV₁ responses to be enhanced on the second day of exposure. This enhanced response is absent, however, with repeated exposure at lower concentrations, perhaps as a result of a more complete recovery or less damage to pulmonary tissues (2013 ISA, section pp. 6-13 to 6-14; Folinsbee et al., 1994).

With regard to airway inflammation and the potential for repeated occurrences to contribute to further effects, O₃-induced respiratory tract inflammation “can have several potential outcomes: (1) Inflammation induced by a single exposure (or several exposures over the course of a summer) can resolve entirely; (2) continued acute inflammation can evolve into a chronic inflammatory state; (3) continued inflammation can alter the structure and function of other pulmonary tissue, leading to diseases such as fibrosis; (4) inflammation can alter the body's host defense response to inhaled microorganisms, particularly in potentially at-risk populations such as the very young and old; and (5) inflammation can alter the lung's response to other agents such as allergens or toxins” (2013 ISA, p. 6-76; ISA Appendix 3, section 3.1.5.6). With regard to O₃-induced increases in airway responsiveness, the controlled human exposure study evidence for healthy adults generally indicates resolution within 18 to 24 hours after exposure, with slightly longer persistence in some individuals (ISA, Appendix 3, section 3.1.4.3.1; 2013 ISA, p. 6-74; Folinsbee and Hazucha, 2000).

The array of O₃-associated respiratory effects, including reduced lung function, respiratory symptoms, increased airway responsiveness, and inflammation are of increased significance to people with asthma given aspects of the disease that contribute to a baseline status that includes chronic airway inflammation and greater airway responsiveness than people without asthma (ISA, section 3.1.5). For example, O₃ exposure of a magnitude that increases airway responsiveness may put such people at potential increased risk for prolonged bronchoconstriction in response to asthma triggers (ISA, Appendix 3, p. 3-7, 3-28; 2013 ISA, section 6.2.9; 2006 AQCD, section 8.4.2). The increased significance of effects in people with asthma and risk of increased exposure for children (from greater frequency of outdoor exercise) is illustrated by the epidemiologic findings of positive associations between O₃ exposure and asthma-related emergency department visits and hospital admissions for children with asthma. Thus, the evidence indicates O₃ exposure to increase the risk of asthma exacerbation, and associated outcomes, in children with asthma.

With regard to an increased susceptibility to infectious diseases, the experimental animal evidence continues to indicate, as described in the 2013 ISA and past AQCDs, the potential role for O₃ exposures through effects on defense mechanisms of the respiratory tract (ISA, section 3.1.7.3; 2013 ISA, section 6.2.5). The evidence base regarding respiratory infections and associated effects has been augmented in this review by a number of epidemiologic studies reporting positive associations between short-term O₃ concentrations and emergency department visits for a variety of respiratory infection endpoints (ISA, Appendix 3, section 3.1.7).

Although the long-term exposure conditions that may contribute to further respiratory effects are less well understood, experimental studies, including with nonhuman infant primates, have provided evidence relating O₃ exposure to asthma-like effects, and epidemiologic cohort studies have reported associations of O₃ concentrations in ambient air with asthma development in children (ISA, IS.4.3.2 and Appendix 3, sections 3.2.4.1.3 and 3.2.6). The biological plausibility of such a role for O₃ has been indicated by animal toxicological evidence on biological mechanisms (ISA, Appendix 3, sections 3.2.3 and 3.2.4.1.2).

Overall, the respiratory effects evidence newly available in this review is consistent with the evidence base in the last review, supporting a generally similar understanding of the respiratory effects of O₃ (ISA, Appendix 3, section 3.1.4). A few recent studies provide insights in previously unexamined areas, both with regard to human study groups and animal models for different effects, while other studies confirm and provide depth to prior findings with updated protocols and techniques (ISA, Appendix 3, sections 3.1.11 and 3.2.6). Newly available epidemiologic studies of hospital admissions and emergency department visits for a variety of respiratory outcomes supplement the previously available evidence with additional findings of consistent associations with O₃ concentrations across a number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 3.1.6.1.1, 3.1.7.1 and 3.1.8). These studies include a number that report positive associations for asthma-related outcomes, as well as a few for COPD-related outcomes. Together these epidemiologic studies continue to indicate the potential for O₃ exposures to contribute to such serious health outcomes, particularly for people with asthma.

As was the case for the evidence available in the last review, the currently available evidence for health effects other than those of O₃ exposures on the respiratory system is more uncertain than that for respiratory effects. Further, the evidence now available has contributed to changes in conclusions for some of these effects. For example, the current evidence for cardiovascular effects and mortality, expanded from that in the last review, is no longer considered sufficient to conclude that the relationships of short-term exposure with these effects are likely to be causal (ISA, sections IS.4.3.4 and IS.4.3.5). These changes stem from newly available evidence in combination with the uncertainties recognized for the evidence available in the last review. Although there exists largely consistent evidence for a limited number of O₃-induced cardiovascular endpoints in animal toxicological studies and cardiovascular mortality in epidemiologic studies, there is a general lack of coherence between these results and findings in controlled human exposure and epidemiologic studies of cardiovascular health outcomes (ISA, section IS.1.3.1, Appendix 6, section 6.1.8). The relationships are now characterized as suggestive of, but not sufficient to infer, a causal relationship (ISA, Appendix 4, section 4.1.17; Appendix 6, section 6.1.8).

With regard to metabolic effects of short-term O₃ exposures, the evidence comes primarily from experimental animal study findings, with a limited number of epidemiologic studies (ISA, section IS.4.3.3 and Appendix 5, section 5.1.8 and Table 5-3). The exposure conditions from the animal studies generally involve much higher O₃ concentrations (e.g., 4-hour concentrations of 400 to 800 ppb [ISA, Appendix 5, Tables 5-8 and 5-10]) than those commonly occurring in areas of the U.S. where the current standard is met, and the concentration in the available controlled human exposure study is similarly high, at 300 ppb (ISA, sections 5.1.3, 5.1.5 and 5.1.8, Table 5-3). The evidence for metabolic effects and long-term exposures is concluded to be suggestive of, but not sufficient to infer, a causal relationship (ISA, section IS.4.3.6.2).

b. Public Health Implications and At-Risk Populations

The public health implications of the evidence regarding O₃-related health effects, as for other effects, are dependent on the type and severity of the effects, as well as the size of the population affected. Judgments or interpretative statements developed by public health experts, particularly experts in respiratory health, also inform consideration of public health implications.

With regard to O₃ in ambient air, the potential public health impacts relate most importantly to respiratory effects. Controlled human exposure studies have documented reduced lung function, respiratory symptoms, increased airway responsiveness, and inflammation, among other effects, in healthy adults exposed while at elevated ventilation, such as while exercising. Ozone effects in individuals with compromised respiratory function, such as individuals with asthma, are plausibly related to emergency department visits and hospital admissions for asthma which have been associated with ambient air concentrations of O₃ in epidemiologic studies (as summarized in section II.A.2.a above; 2013 ISA, section 6.2.7; ISA, Appendix 3, sections 3.1.5.1 and 3.1.5.2).

The clinical significance of individual responses to O₃ exposure depends on the health status of the individual, the magnitude of the responses, the severity of respiratory symptoms, and the duration of the response. While a particular reduction in FEV₁ or increase in inflammation or airway responsiveness may not be of concern for a healthy group, it may increase the risk of a more severe effect in a group with asthma. As a more specific example, the same increase in inflammation or airway responsiveness in individuals with asthma could predispose them to an asthma exacerbation event triggered by an allergen to which they may be sensitized (e.g., ISA, Appendix 3, section 3.1.5.6.1; 2013 ISA, sections 6.2.3 and 6.2.6). Duration and frequency of documented effects is also reasonably expected to influence potential adversity and interference with normal activity. In summary, consideration of differences in magnitude or severity, and also the relative transience or persistence of the responses (e.g., FEV₁ changes) and respiratory symptoms, as well as pre-existing sensitivity to effects on the respiratory system, and other factors, are important to characterizing implications for public health effects of an air pollutant such as O₃ (ATS, 2000; Thurston et al., 2017).

Decisions made in past reviews of the O₃ primary standard and associated judgments regarding adversity or health significance of measurable physiological responses to air pollutants have been informed by guidance, criteria or interpretative statements developed within the public health community, including the ATS, an organization of respiratory disease specialists, as well as the advice from the CASAC. The ATS released its initial statement (titled Guidelines as to What Constitutes an Adverse Respiratory Health Effect, with Special Reference to Epidemiologic Studies of Air Pollution) in 1985 and updated it in 2000 (ATS, 1985; ATS, 2000). The ATS described its 2000 statement, considered in the last review of the O₃ standard, as being intended to “provide guidance to policy makers and others who interpret the scientific evidence on the health effects of air pollution for the purposes of risk management” (ATS, 2000). The recent statement further notes that it does not offer “strict rules or numerical criteria, but rather proposes considerations to be weighed in setting boundaries between adverse and nonadverse health effects,” providing a general framework for interpreting evidence that proposes a “set of considerations that can be applied in forming judgments” for this context (Thurston et al., 2017). Similarly, in the 2000 statement, the ATS describes it as proposing “principles to be used in weighing the evidence and setting boundaries” and states that “the placement of dividing lines should be a societal judgment” (ATS, 2000). The ATS explicitly states that it does “not attempt to provide an exact definition or fixed list of health impacts that are, or are not, adverse,” providing instead “a number of generalizable `considerations'” (ATS, 2000). The ATS state there “cannot be precise numerical criteria, as broad clinical knowledge and scientific judgments, which can change over time, must be factors in determining adversity” (ATS, 2000).

With regard to pulmonary function decrements, the earlier ATS statement concluded that “small transient changes in forced expiratory volume in 1 s[econd] (FEV₁) alone were not necessarily adverse in healthy individuals but should be considered adverse when accompanied by symptoms” (ATS, 2000). The more recent ATS statement continues to support this conclusion and also gives weight to findings of small lung function changes in the absence of respiratory symptoms in individuals with pre-existing compromised function, such as that resulting from asthma (Thurston et al., 2017). In keeping with the intent of these statements to avoid specific criteria, neither statement provides more specific descriptions of such responses, such as with regard to magnitude, duration or frequency, for consideration of such conclusions. The earlier ATS statement, in addition to emphasizing clinically relevant effects, also emphasized both the need to consider changes in “the risk profile of the exposed population,” and effects on the portion of the population that may have a diminished reserve that puts its members at potentially increased risk if affected by another agent (ATS, 2000). These concepts, including the consideration of the magnitude of effects occurring in just a subset of study subjects, continue to be recognized as important in the more recent ATS statement (Thurston et al., 2017) and continue to be relevant to the evidence base for O₃.

The information newly available in this review regarding O₃ exposure and health effects among sensitive populations, thoroughly evaluated in the ISA, has not altered our understanding of human populations at particular risk of health effects from O₃ exposures (ISA, section IS.4.4). The respiratory effects evidence, extending decades into the past and augmented by new studies in this review, supports the conclusion that “individuals with pre-existing asthma are at greater risk of ozone-related health effects based on the substantial and consistent evidence within epidemiologic studies and the coherence with toxicological studies” (ISA, p. IS-57). Numerous epidemiologic studies document associations of O₃ with asthma exacerbation. Such studies indicate the associations to be strongest for populations of children which is consistent with their generally greater time outdoors while at elevated exertion. Together, these considerations indicate people with asthma, including particularly children with asthma, to be at relatively greater risk of O₃-related effects than other members of the general population (ISA, section IS.4.4.2 and Appendix 3).

With respect to people with asthma, the limited evidence from controlled human exposure studies (which are primarily in adult subjects) indicates similar magnitude of FEV₁ decrements as in people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Across studies of other respiratory effects of O₃ (e.g., increased respiratory symptoms, increased airway responsiveness and increased lung inflammation), the responses observed in study subjects generally do not differ due to the presence of asthma, although the evidence base is more limited with regard to study subjects with asthma (ISA, Appendix 3, section 3.1.5.7). However, the features of asthma (e.g., increased airway responsiveness) contribute to a risk of asthma-related responses, such as asthma exacerbation in response to asthma triggers, which may increase the risk of more severe health outcomes (ISA, section 3.1.5). For example, a particularly strong and consistent component of the epidemiologic evidence is the appreciable number of epidemiologic studies that demonstrate associations between ambient O₃ concentrations and hospital admissions and emergency department visits for asthma (ISA, section IS.4.4.3.1). The strongest associations (e.g., highest effect estimates) or associations more likely to be statistically significant are those for childhood age groups, which are age groups most likely to spend time outdoors during afternoon periods (when O₃ may be highest) and at activity levels corresponding to those that have been associated with respiratory effects in the human exposure studies (ISA, Appendix 3, sections 3.1.4.1 and 3.1.4.2). The epidemiologic studies of hospital admissions and emergency department visits are augmented by a large body of individual-level epidemiologic panel studies that demonstrated associations of short-term ozone concentrations with respiratory symptoms in children with asthma. Additional support comes from epidemiologic studies that observed O₃-associated increases in indicators of airway inflammation and oxidative stress in children with asthma (ISA, section IS.4.3.1). Together, this evidence continues to indicate the increased risk of population groups with asthma, including particularly, children (ISA, Appendix 3, section 3.1.5.7).

Children, and also outdoor adult workers, are at increased risk largely due to their generally greater time spent outdoors while at elevated exertion rates (including in summer afternoons and early evenings when O₃ levels may be higher). This behavior makes them more likely to be exposed to O₃ in ambient air, under conditions contributing to increased dose, e.g., elevated ventilation taking greater air volumes into the lungs (2013 ISA, section 5.2.2.7). In light of the evidence summarized in the prior paragraph, children and outdoor workers with asthma may be at increased risk of more severe outcomes, such as asthma exacerbation. Further, there is experimental evidence from early life exposures of nonhuman primates that indicates potential for effects in childhood when human respiratory systems are under development (ISA, section IS.4.4.4.1). Overall, the evidence available in the current review, while not increasing our knowledge about susceptibility or at-risk status of these population groups, is consistent with that in the last review (ISA, section IS.4.4).

The ISA also expressly considered the evidence regarding O₃ exposure and health effects among populations with several other potential risk factors. As in the last review, the evidence for low income and minority populations, remains “suggestive” of increased risk, and includes several inconsistencies (ISA, Tables IS-9 and IS-10). The ISA in the last review additionally identified a role for dietary anti-oxidants such as vitamins C and E in influencing risk of O₃-related effects, such as inflammation, as well as a role for genetic factors to also confer either an increased or decreased risk (2013 ISA, sections 8.1 and 8.4.1). No newly available evidence has been evaluated that would inform or change these prior conclusions (ISA, section IS.4.4 and Table IS-10).

The magnitude and characterization of a public health impact is dependent upon the size and characteristics of the populations affected, as well as the type or severity of the effects. As summarized above, a population most at risk of health effects associated with O₃ in ambient air is people with asthma. The National Center for Health Statistics data for 2017 indicate that approximately 7.9% of the U.S. populations has asthma (CDC, 2019; PA, Table 3-1) and this is one of the principal populations that the primary O₃ NAAQS is designed to protect (80 FR 65294, October 26, 2015). Children under the age of 18 account for 16.7% of the total U.S. population, with 6.2% of the total population being children under 5 years of age (U.S. Census Bureau, 2019). Another at-risk population group, also due to time and activity outdoors, is outdoor workers. Population groups with relatively greater asthma prevalence, such as populations in poverty and children (CDC, 2019, Tables 3-1 and 4-1; PA, Table 3-1), might be expected to have a relatively greater potential for O₃-related health impacts.

c. Exposure Concentrations Associated With Effects

The extensive evidence base for O₃ health effects, compiled over several decades, continues to indicate respiratory responses to short-term exposures as the most sensitive effects. As at the time of the last review, our conclusions regarding O₃ exposure concentrations associated with respiratory effects reflect the extensive longstanding evidence base of controlled human exposure studies of short-term exposures of people with and without asthma (ISA, Appendix 3). As summarized in section II.A.2.a above, these studies have documented an array of respiratory effects, including reduced lung function, respiratory symptoms, increased airway responsiveness, and inflammation, in study subjects following 1- to 8-hour exposures, primarily while exercising.

The current evidence, including that newly available in this review, does not alter the scientific conclusions reached in the last review on exposure duration and concentrations associated with O₃-related health effects. These conclusions were largely based on the body of evidence from the controlled human exposure studies. A limited number of controlled human exposure studies are newly available in the current review, with none involving lower exposure concentrations than those previously studied or finding effects not previously reported (ISA, Appendix 3, section 3.1.4).

The severity of observed responses, the percentage of individuals responding, and strength of statistical significance at the study group level have been found to increase with increasing exposure (ISA; 2013 ISA; 2006 AQCD). For example, the magnitude of respiratory response (e.g., size of lung function reductions and magnitude of symptom scores) documented in the controlled human exposure studies is influenced by ventilation rate, exposure duration, and exposure concentration. When performing physical activities requiring elevated exertion, ventilation rate is increased, leading to greater potential for health effects due to an increased internal dose (2013 ISA, section 6.2.1.1, pp. 6-5 to 6-11). Accordingly, the exposure concentrations eliciting a given level of response after a given exposure duration is lower for subjects exposed while at elevated ventilation, such as while exercising (2013 ISA, pp. 6-5 to 6-6; ISA Appendix 3, section 3.1.4.2). For example, in studies of healthy young adults exposed while at rest for 2 hours, 500 ppb is the lowest concentration eliciting a statistically significant O₃-induced group mean lung function decrement, while a 1- to 2-hour exposure to 120 ppb produces a statistically significant response in lung function when the ventilation rate of the group of study subjects is sufficiently increased with exercise (2013 ISA, pp. 6-5 to 6-6).

The exposure conditions (e.g., duration and exercise) given primary focus in the past several O₃ NAAQS reviews are those of the 6.6-hour study design, which involves six 50-minute exercise periods during which subjects maintain a moderate level of exertion to achieve a ventilation rate of approximately 20 L/min per m body surface area while exercising. The 6.6 hours of exposure in these quasi-continuous exercise studies has generally occurred in an enclosed chamber and the study design includes three hours in each of which is a 50-minute exercise period and a 10-minute rest period, followed by a 35-minute lunch (rest) period, which is followed by three more hours of exercise and rest, as before lunch. Most of these studies performed to date involve exposure maintained at a constant (unchanging) concentration for the full duration, although a subset of studies have concentrations that vary (generally in a stepwise manner) across the exposure period and are selected so as to achieve a specific target concentration as the exposure average.

Evidence from studies with similar duration and quasi-continuous exercise aspects (6.6-hour duration with six 50-minute exercise periods) demonstrates an exposure-response (E-R) relationship for O₃-induced reduction in lung function (Table 1; ISA, Appendix 3, Figure 3-3 PA, Figure 3-2). No studies of the 6.6-hour design are newly available in this review. The previously available studies of this design document statistically significant O₃-induced reduction in lung function (FEV₁) and increased pulmonary inflammation in young healthy adults exposed to O₃ concentrations as low as 60 ppb. Statistically significant group mean changes in FEV₁, also often accompanied by statistically significant increases in respiratory symptoms, become more consistent across such studies of exposures to higher O₃ concentrations, such as somewhat above 70 ppb (73 ppb), and 80 ppb (Table 1 and Appendix 3A, Table 3A-1). The lowest exposures concentration for which these studies document a statistically significant increase in respiratory symptoms is somewhat above 70 ppb, at 73 ppb (Schelegle et al., 2009). In the 6.6-hour studies, the group means of O₃-induced FEV₁ reductions for target exposure concentrations at or below 70 ppb are approximately 6% or lower (Table 1). For example, the group means of O₃-induced FEV₁ decrements reported in these studies that are statistically significantly different from the responses in filtered air are 6.1% for 70 ppb and 1.7% to 3.5% for 60 ppb (Table 1).

The group mean O₃-induced FEV₁ decrements generally increase with increasing O₃ exposures, reflecting increases in both the number of the individuals experiencing FEV₁ reductions and the magnitude of the FEV₁ reduction (Table 1; ISA, Appendix 3, Figure 3-3; PA, Figure 3-2). For example, following 6.6-hour exposures to a lower concentration (40 ppb), for which decrements were not statistically significant at the group mean level, none of 60 subjects across two separate studies experienced an O₃-induced FEV₁ reduction as large as 15% or more (Table 1; PA, Appendix 3D, Table 3D-19). The group mean O₃-induced FEV₁ decrements generally increase with increasing O₃ exposures, reflecting increases in both the number of the individuals experiencing FEV₁ reductions and the magnitude of the FEV₁ reduction (Table 1; ISA, Appendix 3, Figure 3-3; PA, Figure 3-2). For example, following 6.6-hour exposures to a lower concentration (40 ppb), for which decrements were not statistically significant at the group mean level, none of 60 subjects across two separate studies experienced an O₃-induced FEV₁ reduction as large as 15% or more (Table 1; PA, Appendix 3D, Table 3D-19). Across the four experiments (with number of subjects ranging from 30 to 59) that have reported results for a 60 ppb target exposure, the number of subjects experiencing this magnitude of FEV₁ reduction (at or above 15%) varied (zero of 30, one of 59, two of 31 and two of 30 exposed subjects), while, together, they represent 3% of all 150 subjects. This percentage of subjects (with reductions of 15% or more) increased to 10% (three of 31 subjects) for the study at 73 ppb (70 ppb target) (PA, Appendix 3D, Table 3D-19; Schelegle et al., 2009), and is higher still (16%) in a variable exposure study at 80 ppb (PA, Appendix 3D, Table 3D-20; Schelegle et al., 2009). In addition to illustrating the E-R relationship, these findings also illustrate the considerable variability in magnitude of responses observed among study subjects (ISA, Appendix 3, section 3.1.4.1.1; 2013 ISA, p. 6-13).

For shorter exposure periods (e.g., one to two hours), with heavy intermittent or very heavy continuous exercise, higher exposure concentrations, ranging up from 80 ppb up to 400 ppb, have been studied (ISA, section 3.1; 2013 ISA, section 6.2.1.1; 2006 AQCD, chapter 6; PA, Appendix 3A, Table 3A-3). Across these shorter-duration studies (which involved ventilation rates 2-3 times greater than in the prolonged [6.6- or 8-hour] exposure studies) the lowest exposure concentration for which statistically significant respiratory effects were reported is 120 ppb, for a 1-hour exposure combined with continuous very heavy exercise and a 2-hour exposure with intermittent heavy exercise. As recognized above, the increased ventilation rate associated with increased exertion increases the amount of O₃ entering the lung, where depending on dose and the individual's susceptibility, it may cause respiratory effects (2013 ISA, section 6.2.1.1). Thus, for exposures involving a lower exertion level, a comparable response would not be expected to occur without a longer exposure duration (ISA, Appendix 3, Figure 3-3; PA, Appendix 3A, Table 3A-1).

With regard to the epidemiologic studies reporting associations between O₃ and respiratory health outcomes such as asthma-related emergency department visits and hospitalizations, these studies are generally focused on investigating the existence of a relationship between O₃ occurring in ambient air and specific health outcomes. Accordingly, while as a whole, this evidence base of epidemiologic studies provides strong support for the conclusions of causality, these studies provide less information on details of the specific O₃ exposure circumstances that may be eliciting health effects associated with such outcomes, and whether these occur under air quality conditions that meet the current standard. Further, the vast majority of these studies were conducted in locations and during time periods that would not have met the current standard. The extent to which reported associations with health outcomes in the resident populations in these studies are influenced by the periods of higher concentrations during times that did not meet the current standard is unknown. While this does not lessen their importance in the evidence base documenting the causal relationship between O₃ and respiratory effects, it means they are less informative in considering O₃ exposure concentrations occurring under air quality conditions allowed by the current standard.

With regard to the experimental animal evidence (largely in rodents) and exposure conditions associated with respiratory effects, the exposure concentrations are generally much greater than those examined in the controlled human exposure studies (summarized above), and higher than concentrations commonly occurring in ambient air in areas of the U.S. where the current standard is met. This is also true for the small number of early life studies in nonhuman primates that reported O₃ to contribute to asthma-like effects in infant primates. The exposures eliciting the effects in these studies included multiple 5-day periods with O₃ concentrations of 500 ppb over 8-hours per day (ISA, Appendix 3, section 3.2.4.1.2).

Thus, as in the last review the exposures given greatest attention in this review, particularly with regard to considering O₃ exposures expected under air quality conditions that meet the current standard, are those informed by the controlled human exposure studies. The full body of evidence continues to indicate respiratory effects as the effects associated with lowest exposures, with conditions of exposure (duration, ventilation rate, as well as concentration) influencing dose and associated response. Evidence for other categories of effects does not indicate effects at comparably low exposures.

3. Overview of Exposure and Risk Information

Consideration of the scientific evidence available in the current review, as at the time of the last review, is informed by results from quantitative analyses of estimated population exposure and consequent risk of respiratory effects. These analyses in this review have focused on exposure-based risk analyses, producing two types of risk metrics. The first metric estimates population occurrences of daily maximum 7-hour average exposure concentrations (during periods of elevated breathing rates) at or above concentrations of potential concern (benchmark concentrations). The second metric (lung function risk) uses E-R information for O₃ exposures and FEV₁ decrements to estimate the portion of the simulated at-risk population expected to experience one or more days with an O₃-related FEV₁ decrement of at least 10%, 15% or 20%. Both of these metrics were used to characterize health risk associated with O₃ exposures among the simulated population during periods of elevated breathing rates. Similar risk metrics were also derived in the 2014 HREA for the last review and the associated estimates informed the Administrator's 2015 decision on the current standard (80 FR 65292, October 26, 2015).

The currently available evidence in this review continues to demonstrate a causal relationship between short-term O₃ exposures and respiratory effects, with the current evidence base for respiratory effects largely consistent with that for the last review, as summarized in section II.A.2 above. Accordingly, the exposure-based analyses performed in this review, summarized below, are conceptually similar to those in the last review while also incorporating a number of updates that contribute to reduced uncertainty. Drawing on the summary in section II.C of the proposal, while giving relatively greater focus on the comparison-to-benchmarks analysis, the short sections below provide an overview of key aspects of the assessment design (II.A.3.a), key limitations and uncertainties (II.A.3.b), and exposure/risk estimates (II.A.3.c).

a. Key Design Aspects

Exposure and risk estimates were derived for air quality conditions just meeting the current primary O₃ standard, and for two additional scenarios reflecting conditions just meeting design values just lower and just higher than the level of the current standard (65 and 75 ppb). The analyses estimated population exposure and risk for simulated populations in eight urban study areas which represent a variety of circumstances with regard to population exposure to short-term concentrations of O₃ in ambient air. The areas (Atlanta, Boston, Dallas, Detroit, Philadelphia, Phoenix, Sacramento and St. Louis) range in total population size from approximately two to eight million and are distributed across seven regions of the U.S.: Northeast, Southeast, Central, East North Central, South, Southwest and West (PA, Appendix 3D, Table 3D-1). Study-area-specific characteristics contribute to variation in the estimated magnitude of exposure and associated risk across the urban study areas that reflect an array of air quality, meteorological, and population exposure conditions. The current set of study areas, streamlined compared to the 15-area set in the last review, was chosen to ensure it reflects the full range of air quality and exposure variation expected in major urban areas in the U.S. with air quality that just meets the current standard. Seven of the eight study areas were also included in the 2014 HREA; the eighth study area (Phoenix) is newly added in the current assessment to insure representation of a large city in the southwest. Additionally, the O₃ concentrations simulated in these areas are somewhat nearer the current standard than was the case for the 2014 HREA (PA, Appendix 3C, Table 3C and 2014 HREA, Table 4-1). This contributes to a reduction in the uncertainty associated with development of the air quality scenarios of interest, particularly the one reflecting air quality conditions that just meet the current standard.

With regard to the objectives for the analysis approach, the analyses and the use of a case study approach are intended to provide assessments of air quality scenarios, including particularly one just meeting the current standard, for a diverse set of areas and associated exposed populations. These analyses are not intended to provide a comprehensive national assessment (PA, section 3.4.1). Nor is the objective to present an exhaustive analysis of exposure and risk in the areas that currently just meet the current standard and/or of exposure and risk associated with air quality adjusted to just meet the current standard in areas that currently do not meet the standard. Rather, the purpose is to assess, based on current tools and information, the potential for exposures and risks beyond those indicated by the information available at the time the standard was established. Accordingly, use of this approach recognizes that capturing an appropriate diversity in study areas and air quality conditions is an important aspect of the role of the exposure and risk analyses in informing the Administrator's conclusions on the public health protection afforded by the current standard.

Consistent with the health effects evidence in this review (summarized in section II.A.2 above), the focus of the quantitative assessment is on short-term exposures of individuals in the population during times when they are breathing at an elevated rate. Exposure and risk are characterized for four population groups. Two are populations of school-aged children, aged 5 to 18 years: All children and children with asthma; two are populations of adults: All adults and adults with asthma. Estimates for adults, in terms of percentages, are generally lower due to the lesser amount and frequency of time spent outdoors at elevated exertion (PA, Appendix 3D, section 3D.3.2). The exception is outdoor workers who, due to the requirements of their job, spend more time outdoors at elevated exertion. For a number of reasons, including the appreciable data limitations (e.g., related to specific durations of time spent outdoors and activity data), and associated uncertainties summarized in Table 3D-64 of Appendix 3D of the PA, the group was not simulated in these analyses, a decision also made for past exposure assessments. Asthma prevalence estimates for the full populations in the eight study areas range from 7.7 to 11.2%; the rates for children in these areas range from 9.2 to 12.3% (PA, Appendix 3D, section 3D.3.1).

The approach for this analysis incorporates an array of models and data (PA, section 3.4.1). Ambient air O₃ concentrations were estimated in each study area for the air quality conditions of interest by adjusting hourly ambient air concentrations, from monitoring data for the years 2015-2017, using a photochemical model-based approach and then applying a spatial interpolation technique to produce air quality surfaces with high spatial and temporal resolution (PA, Appendix 3C). The final products were datasets of ambient air O₃ concentration estimates with high temporal and spatial resolution (hourly concentrations in 500 to 1,700 census tracts) for each of the eight study areas (PA, section 3.4.1 and Appendix 3C, section 3C.7) representing the three air quality scenarios assessed.

Population exposures were estimated using the EPA's Air Pollutant Exposure model (APEX) version 5, which probabilistically generates a large sample of hypothetical individuals from population demographic and activity pattern databases and simulates each individual's movements through time and space to estimate their time series of O₃ exposures occurring within indoor, outdoor, and in-vehicle microenvironments (PA, Appendix 3D, section 3D.2). The APEX model accounts for the most important factors that contribute to human exposure to O₃ from ambient air, including the temporal and spatial distributions of people and ambient air O₃ concentrations throughout a study area, the variation of ambient air-related O₃ concentrations within various microenvironments in which people conduct their daily activities, and the effects of activities involving different levels of exertion on breathing rate (or ventilation rate) for the exposed individuals of different sex, age, and body mass in the study area (PA, Appendix 3D, section 3D.2). By incorporating individual activity patterns, and estimating physical exertion for each exposure event, the model addresses an important determinant of their exposure (2013 ISA, section 4.4.1). For each exposure event, the APEX model tracks activity performed, ventilation rate, exposure concentration, and duration for all simulated individuals throughout the assessment period, and then utilizes the time-series of exposure events in derivation of the exposure and risk estimates.

The general approach and methodology for the exposure-based assessment used in this review is similar to that used in the last review, although a number of updates and improvements, related to the air quality, exposure, and risk aspects of the assessment, have been implemented (Appendices 3C and 3D). These include (1) a more recent period (2015-2017) of ambient air monitoring data in which O₃ concentrations in the eight study areas are at or near the current standard; (2) the most recent version of the photochemical air quality model, CAMx (comprehensive air quality model with extensions), with updates to the treatment of atmospheric chemistry and physics within the model; (3) a significantly expanded CHAD, that now has nearly 180,000 diaries, with over 25,000 school aged children; (4) updated National Health and Nutrition Examination Survey data (2009-2014), which are the basis for the age- and sex-specific body weight distributions used to specify the individuals in the modeled populations; (5) updated algorithms used to estimate age- and sex-specific resting metabolic rate, a key input to estimating a simulated individual's activity-specific ventilation (or breathing) rate; (6) updates to the ventilation rate algorithm itself; and (7) an approach that better matches the simulated exposure estimates with the 6.6-hour duration of the controlled human exposure studies and with the study subject ventilation rates. Further, the current APEX model uses the most recent U.S. Census demographic and commuting data (2010), NOAA Integrated Surface Hourly meteorological data to reflect the assessment years studied (2015-2017), and updated estimates of asthma prevalence for all census tracts in all study areas based on 2013-2017 National Health Interview Survey and Behavioral Risk Factor Surveillance System data. Additional details are described in the PA (e.g., PA, section 3.4.1, Appendices 3C and 3D).

The comparison-to-benchmarks analysis characterizes the extent to which individuals in at-risk populations could experience O₃ exposures, while engaging in their daily activities, with the potential to elicit the effects reported in controlled human exposure studies for concentrations at or above specific benchmark concentrations. Results are characterized through comparison of exposure concentrations to three benchmark concentrations of O₃: 60, 70, and 80 ppb. These are based on the three lowest concentrations targeted in studies of 6- to 6.6-hour exposures, with quasi-continuous exercise, and that yielded different occurrences, of statistical significance, and severity of respiratory effects, as summarized in section II.A.2.c above and section II.C.1 of the proposal (PA, section 3.3.3; PA, Appendix 3A, section 3A.1; PA, Appendix 3D, section 3D.2.8.1). The lowest benchmark, 60 ppb, represents the lowest exposure concentration for which controlled human exposure studies have reported statistically significant respiratory effects, as summarized in section II.A.2.c above. Exposure to approximately 70 ppb averaged over 6.6 hours resulted in a larger group mean lung function decrement, as well as a statistically significant increase in prevalence of respiratory symptoms (Table 1; ISA, Appendix 3, Figure 3-3 and section 3.1.4.1.1; Schelegle et al., 2009). Studies of exposures to approximately 80 ppb have reported larger lung function decrements at the study group mean than following exposures to 60 or 70 ppb, in addition to an increase in airway inflammation, increased respiratory symptoms, increased airway responsiveness, and decreased resistance to other respiratory effects (ISA, Appendix 3, sections 3.1.4.1—3.1.4.4; PA, Figure 3-2 and section 3.3.3).

The APEX-generated exposure concentrations for comparison to these benchmark concentrations is the average of concentrations encountered by an individual while at an activity level that elicits the specified elevated ventilation rate. The incidence of such exposures above the benchmark concentrations are summarized for each simulated population, study area, and air quality scenario in Appendix 3D of the PA.

The lung function risk analysis estimates the extent to which individuals in exposed populations could experience O₃-induced lung function decrements of different sizes in two different ways. The population-based E-R function approach uses quantitative descriptions of the E-R relationships for study group incidence of different magnitudes of lung function decrements based on individual study subject observations (PA, Appendix 3D, section 3D.2.8.2.1). The individual-based McDonnell-Smith-Stewart (MSS) model uses quantitative estimates of biological processes identified as important in eliciting the different sizes of decrements at the individual level, with a factor that also provides a representation of intra- and inter-individual response variability (PA, Appendix 3D, section 3D.2.8.2.2; McDonnell et al., 2013). The two approaches, summarized in sections II.C and II.D.1 of the proposal and described in detail in Appendix 3D of the PA, utilize evidence from the 6.6-hour controlled human exposure studies in different ways, and accordingly, differ in strengths, limitations and uncertainties.

While the lung function risk analysis focuses only on the specific O₃ effect of FEV₁ reduction, the comparison-to-benchmark analysis, with its use of multiple benchmark concentrations, provides for risk characterization of the array of respiratory effects elicited by O₃ exposure, the type and severity of which increase with increased exposure concentration. In this way, the comparison-to-benchmark analysis (involving comparison of daily maximum 7-hour average exposure concentrations that coincide with 7-hour average elevated ventilation rates at or above the target rate to benchmark concentrations) provides perspective on the extent to which the air quality being assessed could be associated with discrete exposures to O₃ concentrations reported to result in an array of respiratory effects. For example, estimates of such exposures can indicate the potential for O₃-related effects in the exposed population, including effects for which we do not have E-R functions that could be used in quantitative risk analyses. Thus, the comparison-to-benchmark analysis provides for a broader risk characterization with consideration of the array of O₃-related respiratory effects.

b. Key Limitations and Uncertainties

Uncertainty in the exposure and risk analyses was characterized using a largely qualitative approach adapted from the World Health Organization approach for characterizing uncertainty in exposure assessment (WHO, 2008) augmented by several quantitative sensitivity analyses for key aspects of the assessment approach (PA, section 3.4.4 and Appendix 3D, section 3D.3.4). This characterization and associated analyses build on information generated from a previously conducted quantitative uncertainty analysis of population-based O₃ exposure modeling (Langstaff, 2007), considering the various types of data, algorithms, and models that together yield exposure and risk estimates for the eight study areas. In this way, we considered the limitations and uncertainties underlying these data, algorithms, and models and the extent of their influence on the resultant exposure/risk estimates using the general approach applied in past risk and exposure assessments for O₃, nitrogen dioxide, carbon monoxide, and sulfur dioxide (U.S. EPA, 2008; U.S. EPA, 2010; U.S. EPA, 2014a; U.S. EPA, 2018).

Key uncertainties and limitations in data and tools that affect the quantitative estimates of exposure and risk and their interpretation in the context of considering the current standard are summarized here. These include uncertainty related to estimation of the concentrations in ambient air for the current standard and the additional air quality scenarios; lung function risk approaches that rely, to varying extents, on extrapolating from controlled human exposure study conditions to lower exposure concentrations, lower ventilation rates, and shorter durations; and characterization of risk for particular population groups that may be at greatest risk, particularly for people with asthma, and particularly children with asthma. Areas in which uncertainty has been reduced by new or updated information or methods include the use of updated air quality modeling, with a more recent model version and model inputs, applied to study areas with design values near the current standard, as well as updates to several inputs to the exposure model, including changes to the exposure duration to better match those in the controlled human exposure studies and an alternate approach to characterizing periods of activity while at moderate or greater exertion for simulated individuals.

With regard to the analysis approach overall, two updates since the 2014 HREA reduce uncertainty in the results. The first relates to identifying when simulated individuals may be at moderate or greater exertion, with the new approach reducing the potential for overestimation of the number of people achieving the associated ventilation rate, which was an important uncertainty in the 2014 HREA. Additionally, the current analysis focus on exposures of 7 hours duration better represents the 6.6-hour exposures from the controlled human exposure studies (than the 8-hour exposure durations used for the 2014 HREA and prior assessments).

Additional aspects of the analytical design pertaining to both exposure-based risk metrics include the estimation of ambient air O₃ concentrations for the air quality scenarios, and main components of the exposure modeling. Uncertainties include the modeling approach used to adjust ambient air concentrations to meet the air quality scenarios of interest and the method used to interpolate monitor concentrations to census tracts. While the adjustment to conditions near, just above, or just below the current standard is an important area of uncertainty, the size of the adjustment needed to meet a given air quality scenario is minimized with the selection of study areas for which recent O₃ design values were near the level of the current standard. Also, more recent data are used as inputs for the air quality modeling, such as more recent O₃ concentration data (2015-2017), meteorological data (2016) and emissions data (2016), as well as a recently updated air quality photochemical model which includes state-of-the-science atmospheric chemistry and physics (PA, Appendix 3C). Further, the number of ambient monitors sited in each of the eight study areas provides a reasonable representation of spatial and temporal variability for the air quality conditions simulated in those areas. Among other key aspects, there is uncertainty associated with the simulation of study area populations (and at-risk populations), including those with particular physical and personal attributes. As also recognized in the 2014 HREA, exposures could be underestimated for some population groups that are frequently and routinely outdoors during the summer (e.g., outdoor workers, children). In addition, longitudinal activity patterns do not exist for these and other potentially important population groups (e.g., those having respiratory conditions other than asthma), limiting the extent to which the exposure model outputs reflect information that may be particular to these groups. Important uncertainties in the approach used to estimate energy expenditure (i.e., metabolic equivalents of work or METs used to estimate ventilation rates), include the use of longer-term average MET distributions to derive short-term estimates, along with extrapolating adult observations to children. Both of these approaches are reasonable based on the availability of relevant data and appropriate evaluations conducted to date, and uncertainties associated with these steps are somewhat reduced in the current analyses (compared to the 2014 HREA) because of the added specificity, and use of redeveloped METs distributions (based on newly available information), which is expected to more realistically estimate activity-specific energy expenditure.

There are some uncertainties that apply to the estimation of lung function risk and not to the comparison-to-benchmarks analysis. For example, both lung function risk approaches utilized in the risk analyses incorporate some degree of extrapolation beyond the exposure circumstances evaluated in the controlled human exposure studies. Accordingly, the uncertainty in the lung function risk estimates increases with decreasing exposure concentration and is particularly increased for concentrations below those evaluated in controlled human exposure studies (85 FR 49857-49859, PA, section 3.4.4 and Appendix 3D, section 3D.3.4). The two lung function risk approaches differ in how they extrapolate beyond the controlled human exposure study conditions and in the impact on the estimates. The E-R function approach generates nonzero predictions from the full range of nonzero concentrations for 7-hour average durations in which the average exertion levels meets or exceeds the target. The MSS model, which draws on evidence-based concepts of how human physiological processes respond to O₃, extrapolates beyond the controlled experimental conditions with regard to exposure concentration, duration and ventilation rate (both magnitude and duration). Differences in percent of the risk estimates for days for which the highest 7-hour average concentration is below the lowest 6.6-hour exposure concentration tested, as presented in the PA, Tables 3-6 and 3-7, illustrate the impact.

An overarching area of uncertainty, remaining from the last review and important to consideration of the exposure and risk analysis results, relates to the underlying health effects evidence base. Although the quantitative analysis focuses on the evidence providing the “strongest evidence” of O₃ respiratory effects (ISA, p. IS-1), the controlled human exposure studies, and on the array of respiratory responses documented in those studies, evidence is lacking from controlled human exposure studies at the lower concentrations (e.g., 60, 70 and 80 ppb) for children and for people of any age with asthma. While the limited evidence informing our understanding of potential risk to people with asthma is uncertain, it indicates the potential for this group, given their disease status, to be at great risk, as summarized in section II.A.2 above. Such a conclusion is consistent with the epidemiologic study findings of positive associations of O₃ concentrations with asthma-related ED visits and hospital admissions (and the higher effect estimates from these studies).

c. Summary of Exposure and Risk Estimates

The benchmark-based risk metric results are summarized in terms of the percent of the simulated populations of all children and children with asthma estimated to experience at least one day per year with a 7-hour average exposure concentration at or above the different benchmark concentrations while breathing at elevated rates under air quality conditions just meeting the current standard (Table 2). Given the recognition of people with asthma as an at-risk population and the relatively greater amount and frequency of time spent outdoors at elevated exertion of children, this summary focuses on the estimates from the comparison-to-benchmarks analysis for children, including children with asthma, which were the focus of the Administrator's proposed decision. Under air quality conditions just meeting the current standard, less than 0.1% of any study area's children with asthma, on average, were estimated to experience any days per year with a 7-hour average exposure at or above 80 ppb, while breathing at elevated rates (Table 3; PA, section 3.4 and Appendix 3D). With regard to the 70 ppb benchmark, the study areas' estimates for children with asthma range up to 0.7 percent (0.6% for all children), on average across the 3-year period, and range up to 1.0% in a single year. Approximately 3% to nearly 9% of each study area's simulated children with asthma, on average across the 3-year period, are estimated to experience one or more days per year with a 7-hour average exposure at or above 60 ppb. This range is very similar for the populations of all children.

Regarding multiday occurrences, the analyses indicate that no children would be expected to experience more than a single day with a 7-hour average exposure at or above 80 ppb in any year simulated in any location (Table 2). For the 70 ppb benchmark, the estimate is less than 0.1% of any area's children (on average across 3-year period), both those with asthma and all children. The estimates for the 60 ppb benchmark are slightly higher, with up to 3% of children estimated to experience more than a single day with a 7-hour average exposure at or above 60 ppb, on average (and more than 4% in the highest year across all eight study area locations).

Framed from the perspective of estimated protection provided by the current standard, these results indicate that, in the single year with the highest concentrations across the 3-year period, 99% of the population of children with asthma would not be expected to experience such a day with an exposure at or above the 70 ppb benchmark; 99.9% would not be expected to experience such a day with exposure at or above the 80 ppb benchmark. The estimates, on average across the 3-year period, indicate that over 99.9%, 99.3% and 91.2% of the population of children with asthma would not be expected to experience a day with a 7-hour average exposure while at elevated ventilation that is at or above 80 ppb, 70 ppb and 60 ppb, respectively (Table 1). Further, more than approximately 97% of all children or children with asthma are estimated to be protected against multiple days of exposures at or above 60 ppb.

These estimates are of generally similar magnitude to those which were the focus in the 2015 decision establishing the current standard (Table 3; PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4, Table 3D-38). The differences observed are generally slight, likely reflecting influences of a number of the differences in the quantitative modeling and analyses performed in the current assessment from those for the 2014 HREA, summarized in section II.A.3.a above (e.g., 2015-2017 vs. 2006-2010 distribution of ambient air O₃ concentrations, better matching of simulated exposure estimates with the 6.6-hour duration of the controlled human exposure studies and with the study subject ventilation rates). Much larger differences are seen between different air quality scenario results for the same benchmark. For example, for the 70 ppb benchmark, the differences between the 75 ppb and current standard scenario (or between the 65 ppb and current standard scenarios) in either assessment are appreciably larger than the slight differences between the two assessments for any one air quality scenario.

B. Conclusions on the Primary Standard

In drawing conclusions on the adequacy of the current primary standard, in view of the advances in scientific knowledge and additional information now available, the Administrator has considered the currently available health effects evidence and exposure/risk information. He additionally has considered the evidence base, information, and policy judgments that were the foundation of the last review, to the extent they remain relevant in light of the currently available information. The Administrator has taken into account both evidence-based and exposure- and risk-based considerations discussed in the PA, as well as advice from the CASAC and public comments. Evidence-based considerations draw upon the EPA's assessment and integrated synthesis of the scientific evidence, particularly that from controlled human exposure studies and epidemiologic studies evaluating health effects related to O₃ exposures as presented in the ISA, with a focus on policy-relevant considerations as discussed in the PA (summarized in sections II.B and II.D.1 of the proposal and section II.A.2 above). The exposure- and risk-based considerations draw from the results of the quantitative analyses presented and considered in the PA (as summarized in section II.C of the proposal and section II.A.3 above).

The consideration of the evidence and exposure/risk information in the PA informed the Administrator's proposed conclusions and judgments in this review, and his associated proposed decision. Section II.B.1 below briefly summarizes the basis for the Administrator's proposed decision, drawing from section II.D of the proposal. Section II.B.1.a provides a brief overview of key aspects of the policy evaluations presented in the PA, and the advice and recommendations of the CASAC are summarized in section II.B.1.b. An overview of the Administrator's proposed conclusions is presented in section II.B.1.c. Public comments on the proposed decision are addressed in section II.B.2, and the Administrator's conclusions and decision in this review regarding the adequacy of the current primary standard and whether any revisions are appropriate are described in section II.B.3.

1. Basis for Proposed Decision

a. Policy-Relevant Evaluations in the PA

The main focus of the policy-relevant considerations in the PA is consideration of the question: Does the currently available scientific evidence- and exposure/risk-based information support or call into question the adequacy of the protection afforded by the current primary O₃ standard? The PA response to this overarching question takes into account discussions that address the specific policy-relevant questions for this review, focusing first on consideration of the evidence, as evaluated in the ISA, including that newly available in this review, and the extent to which it alters key conclusions supporting the current standard. The PA also considers the quantitative exposure and risk estimates drawn from the exposure/risk analyses (presented in detail in Appendices 3C and 3D of the PA), including associated limitations and uncertainties, and the extent to which they may indicate different conclusions from those in the last review regarding the magnitude of risk, as well as level of protection from adverse effects, associated with the current standard. The PA additionally considers the key aspects of the evidence and exposure/risk estimates that were emphasized in establishing the current standard, as well as the associated public health policy judgments and judgments about the uncertainties inherent in the scientific evidence and quantitative analyses that are integral to the Administrator's consideration of whether the currently available information supports or calls into question the adequacy of the current primary O₃ standard (PA, section 3.5).

As summarized in section II.D.1 of the proposal, based on the evidence in the ISA, the PA concludes that the respiratory effects evidence newly available in this review is consistent with the evidence base in the last review, supporting a generally similar understanding of the respiratory effects of O₃ (PA, section 3.5.4; ISA, Appendix 3). As was the case for the evidence available in the last review, the currently available evidence for health effects other than those of O₃ exposures on the respiratory system is more uncertain than that for respiratory effects. Such effects include metabolic effects, for which the evidence available in this review is sufficient to conclude there to likely be a causal relationship with short-term O₃ exposures and suggestive of, but not sufficient to infer, such a relationship between long-term O₃ exposure (ISA, section IS.1.3.1). These new determinations are based on evidence largely from experimental animal studies, that is newly available in this review (ISA, Appendix 5). Additionally, newly available evidence regarding cardiovascular effects and mortality, in combination with uncertainties in the previously available evidence that had been identified in the last review, contributes to conclusions that the evidence is suggestive of, but not sufficient to infer, causal relationships with O₃ exposures (ISA, Appendix 4, section 4.1.17 and Appendix 6, section 6.1.8). As in the last review, the evidence is also suggestive of such relationships for reproductive and developmental effects, and nervous system effects (ISA, section IS.1.3.1).

In evaluating the policy implications of the current evidence, the PA observes that within the respiratory effects evidence base, the most certain evidence comes from controlled human exposure studies, the majority of which involve healthy adult subjects (generally 18 to 35 years), although there are studies (generally not at the lowest studied exposures) involving subjects with asthma, and a limited number of studies, generally of durations shorter than four hours, involving adolescents and adults older than 50 years. Respiratory responses observed in human subjects exposed to O₃ for periods of 8 hours or less, while intermittently or quasi-continuously exercising, include lung function decrements (e.g., based on FEV₁ measurements), respiratory symptoms, increased airway responsiveness, mild bronchoconstriction (measured as an increase in sRaw), and pulmonary inflammation, with associated injury and oxidative stress (ISA, Appendix 3, section 3.1.4; 2013 ISA, sections 6.2.1 through 6.2.4). Newly available epidemiologic studies of hospital admissions and emergency department visits for a variety of respiratory outcomes supplement the previously available evidence with additional findings of consistent associations with O₃ concentrations across a number of study locations (ISA, Appendix 3, sections 3.1.4.1.3, 3.1.5, 3.1.6.1.1, 3.1.7.1 and 3.1.8). Together, the clinical and epidemiological bodies of evidence, in combination with the insights gained from the experimental animal evidence, continue to indicate the potential for O₃ exposures to contribute to serious health outcomes and to indicate the increased risk of population groups with asthma, including particularly, children (ISA, Appendix 3, section 3.1.5.7).

The PA concludes that the newly available evidence in this review does not alter conclusions from the last review on exposure duration and concentrations associated with O₃-related effects, observing that the 6.6-hour controlled human exposure studies of respiratory effects remain the focus for our consideration of exposure circumstances associated with O₃ health effects. The PA additionally recognizes that while the evidence clearly demonstrates that short-term O₃ exposures cause respiratory effects, as was the case in the last review, uncertainties remain in several aspects of our understanding of these effects. These include uncertainties related to exposures likely to elicit effects (and the associated severity and extent) in population groups not studied, or less well studied (including individuals with asthma and children) and also the severity and prevalence of responses to short (e.g., 6.6- to 8-hour) O₃ exposures, at and below 60 ppb, while at increased exertion levels.

The PA additionally includes exposure/risk analyses of air quality scenarios in eight study areas, with a focus on the scenario for air quality that just meets the current standard, as described in section II.C of the proposal and summarized in section II.A.3 above. In considering the results of these analyses, the PA gives particular emphasis to the comparison-to-benchmarks analysis, which provides a characterization of the extent to which population exposures to O₃ concentrations, similar to those evaluated in controlled human exposure studies, have the potential to occur in areas of the U.S. when air quality just meets the current standard (PA, section 3.4). The policy evaluations of the exposure/risk analyses focus on children and children with asthma as key at-risk populations, and consideration of the potential for one versus multiple exposures to occur. The PA recognizes that consideration of differences in magnitude or severity of responses (e.g., FEV₁ changes) including the relative transience or persistence of the responses and respiratory symptoms, as well as pre-existing sensitivity to effects on the respiratory system, and other factors, are important to characterizing implications for public health effects of an air pollutant such as O₃ (PA, sections 3.3.2, 3.4.5 and 3.5).

In summary, the PA concludes that the newly available health effects evidence, critically assessed in the ISA as part of the full body of evidence, reaffirms conclusions on the respiratory effects recognized for O₃ in the last review on which the current standard is based. The PA additionally draws on the quantitative exposure and risk estimates for conditions just meeting the current standard (PA, sections 3.4 and 3.5.2). Limitations and uncertainties associated with the available information remain (PA, sections 3.5.1 and 3.5.2). The PA recognizes that the newly available quantitative exposure/risk estimates for conditions just meeting the current standard indicate a generally similar level of protection for at-risk populations from respiratory effects, as that described in the last review for the now-current standard (section II.A.3, Table 3, above; PA, sections 3.1 and 3.4, Appendix 3D, section 3D.3.2.4, Table 3D-38). Collectively, in consideration of the evidence and quantitative exposure/risk information available in the current review, as well as advice from the CASAC, the PA concludes that it is appropriate to consider retaining the current primary standard of 0.070 ppm O₃, as the fourth-highest daily maximum 8-hour concentration averaged across three years, without revision.

b. CASAC Advice in This Review

In comments on the draft PA, the CASAC agreed with the draft PA findings that the health effects evidence newly available in this review does not substantially differ from that available in the 2015 review, stating that, “[t]he CASAC agrees that the evidence newly available in this review that is relevant to setting the ozone standard does not substantially differ from that of the 2015 Ozone NAAQS review” (Cox, 2020a, Consensus Responses to Charge Questions p. 12). With regard to the adequacy of the current standard, views of individual CASAC members differed. Part of the CASAC “agree with the EPA that the available evidence does not call into question the adequacy of protection provided by the current standard, and thus support retaining the current primary standard” (Cox, 2020a, p. 1). Another part of the CASAC indicated its agreement with the previous CASAC's advice, based on review of the 2014 draft PA, that a primary standard with a level of 70 ppb may not be protective of public health with an adequate margin of safety, including for children with asthma (Cox, 2020a, p. 1 and Consensus Responses to Charge Questions p. 12). Additional comments from the CASAC in the “Consensus Responses to Charge Questions” on the draft PA attached to the CASAC letter provide recommendations on improving the presentation of the information on health effects and exposure and risk estimates in completing the final PA. The EPA considered these comments, making a number of revisions to address them in completing the PA. The comments from the CASAC also took note of uncertainties that remain in this review of the primary standard and identified a number of additional areas for future research and data gathering that would inform the next review of the primary O₃ NAAQS (Cox, 2020a, Consensus Responses to Charge Questions p. 14). The recommendations from the CASAC were considered in the proposed decision and have been considered by the Administrator in his decision in this review, summarized in section II.B.3 below.

c. Administrator's Proposed Conclusions

In reaching conclusions on the adequacy and appropriateness of protection provided by the current primary standard and his proposed decision to retain the standard, the Administrator carefully considered: (1) The assessment of the current evidence and conclusions reached in the ISA; (2) the currently available exposure and risk information, including associated limitations and uncertainties, described in detail in the PA; (3) the considerations and staff conclusions and associated rationales presented in the PA, including consideration of commonly accepted guidelines or criteria within the public health community, including the ATS, an organization of respiratory disease specialists; (4) the advice and recommendations from the CASAC; and (5) public comments that had been offered up to that point (85 FR 49830, August 14, 2020). In so doing, he considered the evidence base on health effects associated with exposure to photochemical oxidants, including O₃, in ambient air, noting the health effects evidence newly available in this review, and the extent to which it alters key scientific conclusions in the last review. He additionally considered the quantitative exposure and risk estimates developed in this review, including associated limitations and uncertainties, and what they indicate regarding the magnitude of risk, as well as level of protection from adverse effects, associated with the current standard. The Administrator also considered the key aspects of the evidence and exposure/risk estimates from the 2015 review that were emphasized in establishing the standard at that time. Further, he considered uncertainties in the current evidence and the exposure/risk information, as a part of public health judgments that are essential and integral to his decision on the adequacy of protection provided by the standard, similar to the judgments made in establishing the current standard. Such judgments include public health policy judgments and judgments about the uncertainties inherent in the scientific evidence and quantitative analyses. The Administrator drew on the considerations and conclusions in the current PA, taking note of key aspects of the associated rationale, and he considered the advice and conclusions of the CASAC, including particularly its overall agreement that the currently available evidence does not substantially differ from that which was available in the 2015 review when the current standard was established.

As an initial matter, the Administrator recognized the continued support in the current evidence for O₃ as the indicator for photochemical oxidants, taking note that no newly available evidence has been identified in this review regarding the importance of photochemical oxidants other than O₃ with regard to abundance in ambient air, and potential for health effects. For such reasons, described with more specificity in the ISA and PA and summarized in the proposal, he proposed to conclude it is appropriate for O₃ to continue to be the indicator for the primary standard for photochemical oxidants and focused on the current information for O₃ (85 FR 49830, August 14, 2020).

With regard to O₃ health effects, the Administrator recognized the long-standing evidence that has established there to be a causal relationship between respiratory effects and short-term O₃ exposures. He recognized that the strongest and most certain evidence for this conclusion, as in the last review, is that from controlled human exposure studies that report an array of respiratory effects in study subjects (which are largely generally healthy adults) engaged in quasi-continuous or intermittent exercise. He also recognized the supporting experimental animal and epidemiologic evidence, including the epidemiologic studies reporting positive associations for asthma-related hospital admissions and emergency department visits, which are strongest for children, with short-term O₃ exposures (85 FR 49830, August 14, 2020).

Regarding the current evidence and EPA conclusions for populations at increased risk of O₃-related health effects (ISA, section 4.4), the Administrator took particular note of the robust evidence that continues to identify people with asthma as being at increased risk of O₃ related respiratory effects, including specifically asthma exacerbation and associated health outcomes, and also children, particularly due to their generally greater time outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). Based on this evidence and related factors, the Administrator proposed to conclude it appropriate to give particular focus to people with asthma and children (population groups for which the evidence of increased risk is strongest) in evaluating whether the current standard provides requisite protection based on the judgment that such a focus will also provide protection of other population groups, identified in the ISA, for which the current evidence is less robust and clear as to the extent and type of any increased risk, and the exposure circumstances that may contribute to it.

The Administrator additionally recognized newly available evidence and conclusions regarding O₃ exposures and metabolic effects. In so doing, he also noted that the basis for the conclusions is largely experimental animal studies in which the exposure concentrations were well above those in the controlled human exposure studies for respiratory effects, and also above those likely to occur in areas of the U.S. that meet the current standard. In light of these considerations, he further proposed to judge the current standard to be protective of such circumstances, leading him to continue to focus on respiratory effects in evaluating whether the current standard provides requisite protection (85 FR 49830, August 14, 2020).

With regard to exposure circumstances of interest for respiratory effects, the Administrator focused particularly on the 6.6-hour controlled human exposure studies involving exposure, with quasi-continuous exercise, that examine exposures from 60 to 80 ppb. In so doing, he recognized that this information on exposure concentrations that have been found to elicit effects in exercising study subjects is unchanged from what was available in the last review. He additionally recognized that while, as a whole, the epidemiologic studies of associations between O₃ and respiratory effects and health outcomes (e.g., asthma-related hospital admission and emergency department visits) provide strong support for the conclusions of causality, they are less useful for his consideration of the potential for O₃ exposures associated with air quality conditions allowed by the current standard to contribute to such health outcomes, taking note of the scarcity of U.S. studies conducted in locations in which and during time periods when the current standard would have been met (85 FR 49830, August 14, 2020).

In reaching his proposed decision to retain the 2015 standard, the Administrator took note of several aspects of the rationale by which it was established, giving weight to the considerations summarized here. The 2015 decision considered the breadth of the O₃ respiratory effects evidence, recognizing the relatively greater significance of effects reported for exposures while at elevated exertion to average O₃ concentrations at and above 80 ppb, as well as to the greater array of effects elicited. The decision also recognized the significance of effects observed at the next lower studied exposures (slightly above 70 ppb) that included both lung function decrements and respiratory symptoms. The standard level was set to provide a high level of protection from such exposures. The decision additionally emphasized consideration of lower exposures down to 60 ppb, particularly with regard to consideration of a margin of safety in setting the standard. In this context, the 2015 decision identified the appropriateness of a standard that provided a degree of control of multiple or repeated occurrences of exposures, while at elevated exertion, at or above 60 ppb (80 FR 65365, October 26, 2015). The controlled human exposure study evidence as a whole provided context for consideration of the 2014 HREA estimates for the comparison-to-benchmarks analysis (80 FR 65363, October 26, 2015). The current Administrator proposed to similarly consider the currently available exposure and risk analyses in this review (85 FR 49830, August 14, 2020).

The Administrator also recognized some uncertainty, reflecting limitations in the evidence base, with regard to the exposure levels eliciting effects (as well as the severity of the effects) in some population groups not well represented in the available controlled human exposure studies, such as children and individuals with asthma. In so doing, the Administrator recognizes that the controlled human exposure studies, primarily conducted in healthy adults, on which the depth of our understanding of O₃-related health effects is based, provide limited, but nonetheless important information with regard to responses in people with asthma or in children. Additionally, some aspects of our understanding continue to be limited, as in the 2015 review; among these aspects are the risk posed to these less studied population groups by 7-hour exposures with exercise to concentrations as low as 60 ppb that are estimated in the exposure analyses. Collectively, these aspects of the evidence and associated uncertainties contribute to a recognition that for O₃, as for other pollutants, the available evidence base in a NAAQS review generally reflects a continuum, consisting of ambient levels at which scientists generally agree that health effects are likely to occur, through lower levels at which the likelihood and magnitude of the response become increasingly uncertain.

As in the 2015 decision, the Administrator's proposed decision in this review recognized that the exposure and risk estimates developed from modeling exposures to O₃ in ambient air are critically important to consideration of the potential for exposures and risks of concern under air quality conditions of interest, and consequently are critically important to judgments on the adequacy of public health protection provided by the current standard. Thus taking into consideration related information, limitations and uncertainties recognized in the proposal, the Administrator considered the exposure and risk estimates across the eight study areas (with their array of exposure conditions) for air quality conditions just meeting the current standard. In light of factors recognized above and summarized in section II.D.4 of the proposal, the Administrator, in his consideration of the exposure and risk analyses, focused in the proposal on the results for children and children with asthma. In considering the public health implications of estimated occurrences of exposures, while at increased exertion, at or above the three benchmark concentrations (60, 70, and 80 ppb), the Administrator considered the effects reported in controlled human exposure studies of this range of concentrations during 6.6 hours of quasi-continuous exercise. While the Administrator noted reduced uncertainty in several aspects of the exposure and risk approaches as compared to the analyses in the last review, he recognized the relatively greater uncertainty associated with the lung function risk estimates compared to the results of the comparison-to-benchmarks analysis. In light of these uncertainties, as well as the recognition that the comparison-to-benchmarks analysis provides for characterization of risk for the broad array of respiratory effects compared to a narrower focus limited to lung function decrements, the Administrator focused in the proposal primarily on the estimates of exposures at or above different benchmark concentrations that represent different levels of significance of O₃-related effects, both with regard to the array of effects and severity of individual effects (85 FR 49830, August 14, 2020).

In his consideration of the exposure analysis estimates for exposures at or above the different benchmark concentrations (with reduced associated uncertainty compared to the analysis available in 2015) and based on the greater severity of responses reported in controlled human exposures, with quasi-continuous exercise, at and above 73 ppb, the Administrator focused in the proposal first on the higher two benchmark concentrations (which at 70 and 80 ppb are, respectively, slightly below and above this level) and the estimates for one-or-more-day occurrences. In this context, he proposed to judge it desirable that the standard provide a high level of protection against one or more occurrences of days with exposures, while breathing at an elevated rate, to concentrations at or above 70 ppb. With regard to the 60 ppb benchmark, the Administrator gave greater weight to estimates of occurrences of two or more (rather than one or more) days with an exposure at or above that benchmark, taking note of the lesser severity of responses observed in studies of the lowest benchmark concentration of 60 ppb and other considerations summarized in the proposal, including potential risks for at-risk populations. Based on this weighting of the exposure analysis results for the eight urban study areas, the Administrator noted what was indicated by the exposure estimates for air quality conditions just meeting the current standard with regard to protection for the simulated at-risk populations. Some 97% to more than 99% of all children (including those with asthma), on average, and more than 95% in the single highest year, are estimated to be protected from experiencing two or more days with exposures at or above 60 ppb while at elevated exertion. More than 99% of children with asthma (and of all children), on average per year, are estimated to be protected from a day or more with an exposure at or above 70 ppb. Lastly, the percentage (for both population groups) for at least one day with such an exposure at or above 80 ppb is 99.9% or more in each of the three years simulated, with no simulated children estimated to experience more than a single such day. The Administrator proposed to judge that protection from this set of exposures provides a strong degree of protection to at-risk populations, such as children with asthma. In so doing, he found that the updated exposure and risk analyses continue to support a conclusion of a high level of protection, including for at-risk populations, from O₃-related effects of exposures that might be expected with air quality conditions that just meet the current standard (85 FR 49830, August 14, 2020).

In reaching his proposed conclusion, the Administrator additionally took note of the comments and advice from the CASAC, including the CASAC conclusion that the newly available evidence does not substantially differ from that available in the last review, and the conclusion expressed by part of the CASAC, that the currently available evidence supports retaining the current standard (85 FR 49873, August 14, 2020). He also noted that another part of the CASAC indicated its agreement with the prior CASAC comments on the 2014 draft PA, in which the prior CASAC opined that a standard set at 70 ppb may not provide an adequate margin of safety (Cox, 2020a, p. 1). With regard to the latter view (that referenced 2014 comments from the prior CASAC), the Administrator additionally noted that the 2014 advice from the prior CASAC also concluded that the scientific evidence supported a range of standard levels that included 70 ppb and recognized the choice of a level within its recommended range to be “a policy judgment under the statutory mandate of the Clean Air Act” (Frey, 2014b, p. ii).

In reflecting on all of the information currently available, the Administrator also considered the extent to which the currently available information might indicate support for a less stringent standard, noting that the CASAC advice did not convey support for such a standard. He additionally considered the current exposure and risk estimates for the air quality scenario for a design value just above the level of the current standard (at 75 ppb), in comparison to the scenario for the current standard, with its level of 70 ppb. In so doing, he found the markedly increased estimates of exposures to the higher benchmarks under air quality for a higher standard level to be of concern and indicative of less than the requisite protection. Thus, in light of considerations raised in the proposal, including the need for an adequate margin of safety, the Administrator proposed to judge that a less stringent standard would not be appropriate to consider (85 FR 49830, August 14, 2020).

Similarly, the Administrator also considered whether it would be appropriate to consider a more stringent standard that might be expected to result in reduced O₃ exposures. As an initial matter in this regard, he considered the advice from the CASAC (summarized in section II.B.1.b above). With regard to the CASAC advice, he noted that while part of the Committee concluded that the evidence supported retaining the current standard without revision, another part of the Committee reiterated advice from the prior CASAC, which while including the current standard level among the range of recommended standard levels, also provided policy advice to set the standard at a lower level (85 FR 49873, August 14, 2020). In considering the reference to the 2014 CASAC advice, the Administrator noted the slight differences of the current exposure and risk estimates from the 2014 HREA estimates considered by the prior CASAC. The Administrator additionally recognized the PA finding that the factors contributing to these differences, which include the use of air quality data reflecting concentrations much closer to the now-current standard than was the case in the 2015 review, also contribute to a reduced uncertainty in the estimates. Thus, he noted that the current exposure analysis estimates indicate the current standard to provide appreciable protection against multiple days with a maximum exposure at or above 60 ppb. He considered this in the context of the adequacy of protection provided by the standard and of the CAA requirement that the standard protect public health, including the health of at-risk populations, with an adequate margin of safety, and proposed to conclude that the current standard provides an adequate margin of safety, and that a more stringent standard is not needed (85 FR 49873, August 14, 2020).

In light of all of the above, including advice from the CASAC, the Administrator proposed to judge the current exposure and risk analysis results to describe appropriately strong protection of at-risk populations from O₃-related health effects. Thus, based on his consideration of the evidence and exposure/risk information, including that related to the lowest exposures studied and the associated uncertainties, the Administrator proposed to judge that the current standard provides the requisite protection, including an adequate margin of safety, and thus should be retained, without revision (85 FR 49874, August 14, 2020). In so doing, he recognized that the protection afforded by the current standard can only be assessed by considering its elements collectively, including the standard level of 70 ppb, the averaging time of eight hours and the form of the annual fourth-highest daily maximum concentration averaged across three years. The Administrator proposed to judge that the current evidence presented in the ISA and considered in the PA, as well as the current air quality, exposure and risk information presented and considered in the PA, provide continued support to these elements, as well as to the current indicator.

In summary, in the proposal the Administrator recognized that the ISA found the newly available health effects evidence, critically assessed in the ISA as part of the full body of evidence, consistent with the conclusions on the respiratory effects recognized for O₃ in the last review. He additionally noted that the evidence newly available in this review, such as that related to metabolic effects, does not include information indicating a basis for concern for exposure conditions associated with air quality conditions meeting the current standard. Further, the Administrator noted the quantitative exposure and risk estimates for conditions just meeting the current standard that indicate a high level of protection for at-risk populations from respiratory effects. Collectively, these considerations (including those discussed more completely in the proposal) provided the basis for the Administrator's proposed judgments regarding the public health protection provided by the current primary standard of 0.070 ppm O₃, as the fourth-highest daily maximum 8-hour concentration averaged across three years. On this basis, the Administrator proposed to conclude that the current standard is requisite to protect the public health with an adequate margin of safety, and that it is appropriate to retain the standard without revision (85 FR 49874, August 14, 2020).

2. Comments on the Proposed Decision

Over 50,000 individuals and organizations indicated their views in public comments on the proposed decision. Most of these are associated with mass mail campaigns or petitions. Approximately 40 separate submissions were also received from individuals, and 75 from organizations and groups of organizations; forty elected officials also submitted comments. Among the organizations commenting were state and local agencies and organizations of state agencies, organizations of health professionals and scientists, environmental and health protection advocacy organizations, industry organizations and regulatory policy-focused organizations. The comments on the proposed decision to retain the current primary standard are addressed here. Those in support of the proposed decision are addressed in section II.B.2.a and those in disagreement are addressed in section II.B.2.b. Comments related to aspects of the process followed in this review of the O₃ NAAQS (described in section I.D above), as well as comments related to other legal, procedural or administrative issues, and those related to issues not germane to this review are addressed in the separate Response to Comments document.

a. Comments in Support of Proposed Decision

Of the commenters supporting the Administrator's proposed decision to retain the current primary standard, without revision, all generally note their agreement with the rationale provided in the proposal, with the CASAC conclusion that the current evidence is generally consistent with that available in the last review, and with the CASAC members that conclude the evidence does not call into question the adequacy of the current standard. Some commenters further remarked that the primary standard was upheld in the litigation following its 2015 establishment (Murray Energy Corp. v. EPA, 936 F.3d 597 [D.C. Cir. 2019]) and that this review is based largely on the same body of respiratory effects evidence. These commenters all find the process for the review to conform to Clean Air Act requirements and the proposed decision to retain the current standard to be well supported, noting that the there are no new controlled human exposure studies (of the type given primary focus in the establishment of the current standard) and concurring with the proposed judgment that at-risk populations are protected with an adequate margin of safety. Some commenters also variously cited EPA statements that the recent metabolic studies, as well as the epidemiologic and toxicological studies newly available in this review for other health endpoints, do not demonstrate effects of O₃ when the current standard is met and thus do not call into question the protection provided by the standard. The EPA agrees with these commenters' conclusion on the current standard.

Further, these comments concur with the EPA's consideration of epidemiologic and toxicological studies of respiratory effects, and with the weight the proposed decision placed on the evidence for other effects, including metabolic and cardiovascular effects, and total mortality. Some of these comments also express the view that health benefits of a more restrictive O₃ standard are highly uncertain, while such a standard would likely cause an increase in nonattainment areas and socioeconomic impacts that the EPA should consider and find to outweigh the uncertain benefits. While, as discussed in section II.B.3 below, the Administrator does not find a more stringent standard necessary to provide requisite public health protection, he does not consider the number of nonattainment areas or economic impacts of alternate standards in reaching this judgment. As summarized in section I.A. above, in setting primary and secondary standards that are “requisite” to protect public health and welfare, respectively, as provided in section 109(b), the EPA may not consider the costs of implementing the standards. See generally, Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards” (American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 [D.C. Cir. 1981]; accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 [D.C. Cir. 2019]). Arguments such as the views on socioeconomic impacts expressed by these commenters have been rejected by the courts, as summarized in section I.A above, including in Murray Energy, with the reasoning that consideration of such impacts was precluded by Whitman' s holding that the “plain text of the Act `unambiguously bars cost considerations from the NAAQS-setting process' ” (Murray Energy Corp. v. EPA, 936 F.3d at 621, quoting Whitman, 531 U.S. at 471).

We also note that some commenters that stated their support for retaining the current standard without revision additionally claimed that, based on the results of the exposure and risk analyses in this review, the current standard provides somewhat more public health protection than the EPA recognized in the 2015 decision establishing it. As support for this view, these commenters cite conclusions (including those in the PA) that the exposure and risk estimates are equivalent or slightly lower than those from the 2014 HREA. In generally agreeing with the commenters' observation with regard to the differences in exposure/risk estimates from analyses in this review compared to those from 2014, we note that the current exposure/risk estimates, while based on conceptually similar approaches to those used in the 2014 HREA, reflect a number of improvements to input data and modeling approaches, summarized in section II.A.3 above, which have reduced uncertainties. These updated analyses inform the Administrator's judgments in this review.

b. Comments in Disagreement With Proposed Decision

Of the commenters that disagreed with the proposal to retain the current standard, some recommend tightening the standard, while one submission recommends a less stringent standard. The commenters supporting a less stringent standard generally assert that the current standard is overprotective, stating that information they provide supports returning to the pre-2015 standard of 75 ppb and/or revising the form from the 4th highest daily maximum to the seventh highest daily maximum. The commenters that recommended a more stringent standard describe a need for revision to provide greater public health protection, generally claiming that the current standard is inadequate and does not provide an adequate margin of safety for potentially vulnerable groups. We address these sets of comments in turn below.

(i) Comments in Disagreement With Proposed Decisions—Calling for Less Stringent Standard

The commenters recommending revision to a less stringent standard generally expressed the view that the current standard is more stringent than necessary to protect public health. In support of this view the commenters argue (1) that in this review the EPA “discredited” a cardiovascular mortality study on which commenters assert the 2015 decision had placed especially heavy weight; (2) that in light of limitations they assert for the exposure and risk estimate analyses conducted in this review, a 75 ppb standard would meet 2015 objectives; and, (3) that additional factors they identify indicate that the current standard of 70 ppb is too close to background levels while a standard of 75 ppb or one with a form that uses the seventh (versus fourth) highest daily maximum 8-hour O₃ concentration would not be.

With regard to the first argument, the EPA knows of no cardiovascular mortality study, much less any health study, that was relied on in the 2015 review that has been discredited, and the commenters provide no citation for such a study. To the extent that the commenter may be intending to refer to the difference of the current review from the 2015 review with regard to the Agency's causality determinations for cardiovascular effects and all-cause mortality, we note that these changes did not involve “discrediting” of any studies in the 2013 ISA. Rather, as summarized in section II.A.2.a above, since the time of the last review the controlled human exposure study evidence base has been appreciably expanded from one study to several, none of which report O₃-induced cardiovascular endpoints. This update to the evidence base for cardiovascular effects, which also includes epidemiologic studies, has contributed to a change in the weight of evidence that supports the Agency's causality determinations for both cardiovascular effects and mortality. To the extent that the commenters intend to suggest that these changes in causality determinations indicate that the current standard is more stringent than necessary to protect public health, the Agency disagrees. The Administrator's reasons for concluding that the current standard provides the requisite public health protection are explained in section II.B.3 below.

With regard to the risk and exposure analyses, the comment argues that 2019 O₃ ambient air monitoring data for locations meeting a design value of 75 ppb indicate that a 75 ppb standard could achieve comparable exposure estimates to those derived for air quality just meeting the current standard by the EPA's exposure/risk analyses. The comment also asserts that uncertainty in the controlled human exposure evidence base with regard to children with asthma suggests “some latitude” is needed in the risk calculations. The analysis provided in the comment appears to focus on counties in designated nonattainment areas with 2019 design values ranging from 71 to 75. For these counties, the commenters' analysis appears to sum the population of the subset of these counties with at least one daily maximum 8-hour average concentration in 2019 falling in the range from 73 to 79 ppb (and, separately, the population of counties with at least one such value above 80 ppb). From these population counts, the analysis derives estimates of the subpopulations of children with asthma spending afternoons outdoors (using national estimates for representation of children in the total population, of children with asthma in the total child population, and of children in asthma spending afternoons outdoors using analysis of CHAD diaries for children). The analysis divides the two values by the commenters' estimate of children with asthma in the U.S. (304 million [total population of the U.S.] × 10.5% [percentage representing children] × 9.7% [percentage representing children with asthma]).

There are many aspects of the analysis submitted with the comment that are not focused on the objective of estimating exposures of concern that might be expected to be experienced by at-risk populations in U.S. areas that just meet a standard with an alternative level of 75 ppb. As just one example of these aspects, the denominator in the final step of the commenters' calculation is inflated by population counts for areas of the U.S. excluded from the commenters' analysis (with this larger population multiplied by a national estimates of percent that are children, 10.5%, and a national estimate of percent of children that have asthma, 9.7%), yielding a percentage of unclear relevance to consideration of exposures occurring in areas just meeting an alternative standards of 75 ppb. If the population of the nonattainment areas on which the commenters' focus is substituted in the calculation for the total population of the U.S. as the denominator (29.5 million × 10.5% × 9.7% = 146,664), with the commenters' estimates of children in those areas that may experience an exposure at or above 80 ppb (4,788) or below 80 ppb and at or above 73 ppb (12,641), the percentages are 3.3% and 8.6%, respectively (and the percentage for at or above 73 ppb would be 5.8%). Thus, contrary to the commenters' assertion, their analytical approach, with use of a denominator that reflects the commenters' focus areas, results in higher estimates of the percentage of at-risk children that may experience particular exposures of concern in areas meeting a 75 ppb standard than does the EPA's analysis, which takes into account a number of factors in much greater detail (e.g., through the use of exposure modeling and human activity data to estimate time series contributing to 7-hour exposure periods with average O₃ concentrations at or above benchmarks), and focuses on temporal and spatial patterns of air quality in areas just meeting a standard of 75 ppb. The commenters analysis is not focused on the factors that are key determinants of population exposures of concern, leading to results that are inconsistent with and less informative than the findings of EPA's more detailed, extensive and technically sound exposure and risk analyses (summarized in section II.A.3 above and Appendices 3C and 3D of the PA). Based on consideration of these analyses, among other factors, as described in section II.B.3 below, the EPA disagrees that the available evidence and quantitative analyses supports the conclusion that the current standard is overprotective and that a standard of 75 ppb would protect public health with an adequate margin of safety.

In support of the commenters' additional argument that the current standard is too close to background and that a 75 ppb standard (or a standard using the seventh highest form) would not be, the commenters (1) state that just because a D.C. Circuit decision has stated that EPA is not required to take U.S. background O₃ (USB) into consideration in NAAQS decisions does not mean that such considerations are precluded; (2) cite the lower number of counties (and associated population) that would be in nonattainment for a 75 ppb standard as compared to the current standard (while also suggesting that revision of the form to a seventh highest would appropriately allow for additional high O₃ days due to wildfires); and (3) suggest that the EPA is underestimating USB by a factor of three.

With regard to the legal point, the EPA agrees that while it is not required to take USB into account in NAAQS decisions, it may do so when such consideration is consistent with the Clean Air Act and prior court decisions. The EPA is not relying on consideration of background O₃ levels to support its decision in this review. Moreover, given the differences in public health protection, as noted in the Administrator's proposed conclusions and described in his conclusions in section II.B.3 below, we do not believe that we could use proximity to background concentrations as a basis for revising the current 70 ppb standard to a potential 75 ppb standard. On the commenters' second point, the EPA notes that the number of counties that would or would not be in nonattainment, the size of population living in them, and the increasing number of days for high O₃ due to wildfires are not relevant factors in judging whether a particular standard is requisite under the Clean Air Act. Regardless of such implications of a decision to retain or revise a NAAQS, the key consideration for the review of a primary standard is whether the standard is judged to provide the requisite protection of public health with an adequate margin of safety. The commenters have provided no evidence suggesting that the current standard provides more than the requisite public health protection under the CAA or indicating that an alternate standard with a level of 75 ppb or with a seventh highest form would provide requisite protection. For these reasons, we do not find these comments persuasive in supporting consideration of revising the current standard to an alternate standard with a level of 75 ppb or with a seventh highest form.

With regard to USB, the commenters present an argument focused on an urban/“rural” comparison and one focused on a 1-month analysis of O₃ concentrations in response to population mobility changes attributed to restrictions placed to manage infections of Corona virus 19 disease (COVID-19). We find there to be limitations in both arguments that undercut the conclusions reached by the commenter. As a result, we disagree that the observations made by the commenters support their statements regarding USB and with the implication that they contradict the EPA's findings from the detailed and extensive analyses presented in the PA (PA, section 2.5 and Appendix 2B).

With regard to the urban/“rural” comparison, the commenters' first cite EPA's analysis in the PA which indicated, based on daily maximum 8-hour (MDA8) concentrations for the nation as a whole, that from one quarter (10 out of 42 ppb) to one third (14 out of 45 ppb) of average MDA8 concentrations in spring and summer, respectively, are derived from anthropogenic sources. They then state that differences in monthly mean MDA8 concentration between two sets of monitoring sites in the Philadelphia metropolitan area that they identify as the three highest and the three most rural was 3.3 ppb in April 2020. The commenters suggest that this amount is much smaller than the 10 to 14 ppb that EPA estimated to be from anthropogenic sources. Based on these two statements, they contend that USB is being underestimated by a factor of three.

We find the commenters' analysis to have several flaws that undercut their conclusion. First, the difference between the two sets of sites, all of which fall in the Philadelphia metropolitan area, are not indicative of either USA (i.e., U.S. anthropogenic) or USB contributions. There is no evidence that this difference is indicative of either USB or USA, and it is especially anomalous given that the commenters' analysis is based on 2020 data (affected by reduced emissions during the reduced travel during the initial months of the COVID-19 epidemic in the U.S.) while EPA's is based on 2016 data. Second, the authors cite a country-wide seasonal average despite the fact that the U.S. anthropogenic contributions are clearly higher in the nonattainment area (than a U.S. average) being referenced. Further, the conclusions about USB underestimation appear quantitatively incorrect and to perhaps confuse USA and USB in the calculations. Even if all USA anthropogenic contributions cited (10 USA and 30 USB of total 40 ppb) in spring of 2016 were actually USB, the underestimation of USB would be 25% at most (0 USA and 40 USB of total 40 ppb; (40−30)/40 = 25%), thus it is unclear how the commenter concluded a factor of three (300%) under-estimation of USB. In addition, the commenter's dataset is for the Philadelphia-Wilmington-Atlantic City CSA, where O₃ more frequently exceeds the level of the standard in May through September (e.g., PA, Appendix 3C, Figure 3C-79), months that have lower USB and higher US anthropogenic than month of April, which the commenters analyzed. Finally, the commenter has focused on low concentration days (averaging ~40-45ppb) that the PA shows tend to be different than high days (PA, section 2.5 and Appendix 2B).

The second argument is based on data on Apple Mobility data and O₃ and NO₂ concentrations for the period from 3/22/2020 to 4/20/2020 (when transportation activity was affected by the behavioral changes in response to COVID-19) and differences from the same period in prior years. Based on the differences, the commenters conclude that O₃ concentrations were less responsive to the 40 to 60% reduction in mobility than were NO₂ concentrations (7% vs 22% difference), indicating to the commenters that society is reaching a period of diminishing returns of actions to control O₃ concentrations. We note, however, that the period of the commenters' analysis is April, while the majority of days with MDA8 greater than 70 ppb in the Philadelphia nonattainment area occur in May to September. In the mid to late summer period, local production of O₃ is increased (see PA section 2.5.3.2) and MDA8 concentrations in the Philadelphia nonattainment area more frequently are above the level of the standard. Thus, the analysis does not support the commenters' argument for a less stringent standard.

(ii) Comments in Disagreement With Proposed Decision and Calling for More Stringent Standard

Among the commenters that disagree with the proposed decision and call for a more stringent standard, most express concerns regarding the process for reviewing the criteria and standards in this review and assert that the proposal must be withdrawn, and a new review conducted. The commenters expressing the view that a more stringent standard is needed variously cite a number of concerns. Some state that EPA cannot, as some commenters imply it does, simply base its decision on a judgment that the available evidence is similar to that when the standard was established in a prior review, and some argue that the available health effects evidence indicates that adverse health effects occur from exposures allowed by the current standard. Further some commenters express their views that the combined consideration of the complete evidence base indicates that sensitive or vulnerable populations are not protected by the current standard; and/or that the standard does not provide an adequate margin of safety. Additionally, in support of their view that the standard should be made more stringent, some commenters disagree with the conclusions of the exposure and risk analyses, characterizing the analyses as deficient, and contending that other quantitative analyses they cite indicate health impacts that would be avoided by a lower standard level. Most of the commenters advocating a more stringent standard recommend revision of the level to a value at or below 60 ppb and others support a level at or below 65 ppb. Some of these commenters additionally note they had raised similar concerns during the 2015 review. Some commenters also express the view that the EPA should establish a separate long-term standard.

With regard to the process by which this review has been conducted, we disagree with the commenters that it is arbitrary and capricious or that it does not comport with legislative requirements. The review process, summarized in section I.D, implemented a number of features, some of which have been employed in past reviews and others which have not, and several which represent efficiencies in consideration of the statutorily required time frame for completion of the review. The comments that raise concerns regarding specific aspects of the process are addressed in the separate Response to Comments document. As indicated there, the EPA disagrees with these comments. The EPA finds the review to have been lawfully conducted, the process reasonably explained, and thus finds no reason to withdraw the proposal.

We disagree with some commenters' contention that the EPA based its proposed decision simply on the similarity of the health effects evidence to that available in the last review. While the health effects information is generally similar to that available in the last review, particularly with regard to respiratory effects (the effects causally related to O₃ exposure), the current health effects evidence base includes hundreds of new health studies. Based on consideration of the full evidence base, including that the newly available in the current review, the EPA has reached different conclusions regarding some categories of effects (as summarized in II.A.2.a above). The EPA's observation that the nature of the evidence has not substantially changed with regard to effects causally related to O₃ exposure, was not, as implied by the comment, the primary consideration in the Administrator's proposed decision. The Administrator considered a number of factors in reaching his proposed decision, including the full extent of the currently available health effects evidence, and the details in which it is, and is not, similar to the last review, which has led to conclusions similar to prior conclusions for some categories of O₃ effects and resulted in changes to others (85 FR 49868-49874, August 14, 2020). Further, in reaching his final decision in this review, as described in section II.B.3 below, he has again considered the currently available information, now in light of the public comments received on the proposal, among other factors. In sum, while we have noted the similarities in the health effects information between this review and the last review (particularly for respiratory effects), we have engaged in independent analysis and assessment of the health effects information in this review, and the Administrator has exercised his independent judgment based on the current health effects assessment, in combination with current exposure/risk information, advice from the CASAC and public comment. Thus, contrary to the suggestion by these commenters, the decision on the primary standard has been made in consideration of the current health effects evidence, current analyses of air quality, exposure and risk, advice from the CASAC, and public comments, consistent with requirements under the CAA.

In support of their position that the available health effects evidence indicates that O₃ exposures occurring in areas that meet the current standard are causing adverse effects, some commenters cite studies that investigate associations of O₃ concentrations and effects, such as respiratory effects, mortality, and preterm birth. These studies include some already evaluated in the air quality criteria, some published subsequent to the literature cutoff date for the ISA, and some which some commenters claim the EPA arbitrarily dismissed or inconsistently weighed in reaching the proposed decision. As discussed in I.D above, we have provisionally considered these “new” studies that have not already been evaluated in the air quality criteria and that were cited by commenters in support of their comments on the proposed decision (Luben et al., 2020). Based on this consideration, we conclude that these studies do not materially change the broad conclusions of the ISA with regard to these health effects, including the conclusions that there is a causal relationship of short-term respiratory effects with O₃ exposures; a relationship of long-term respiratory effects with O₃ exposure that is likely to be causal; evidence that is suggestive of, but not sufficient to infer, causal relationships of cardiovascular effects and total mortality with short- or long-term O₃ exposure; evidence that is suggestive of, but not sufficient to infer, causal relationships of central nervous system effects with short- or long-term O₃ exposure; and, evidence that is suggestive of, but not sufficient to infer, causal relationships of reproductive and developmental effects with long-term O₃ exposure (ISA, section IS.1.3.1). Nor do we find that these studies warrant reopening the air quality criteria for further review (Luben et al., 2020). Thus, we do not find these publications to be contrary to the discussions and associated conclusions in the PA and proposal or to indicate the current standard to be inadequate. We disagree that studies cited by commenters show these categories of effects to be caused by O₃ exposures associated with O₃ air quality that meets the current standard. We continue to focus on the studies of respiratory effects as most important to the Administrator's judgments concerning the public health protection provided by the current standard.

The epidemiologic studies of respiratory effects identified by the commenters include some investigating associations of O₃ exposure with hospital admissions or emergency department visits for respiratory outcomes, or with various respiratory effects for selected population groups. Studies of O₃ and respiratory effects cited by these commenters in support of their comment include studies that have already been evaluated in the air quality criteria (Goodman et al., 2017; O'Lenick et al., 2017; Jerrett et al., 2009; Lin et al., 2008; Islam et al., 2009; Galizia et al., 1999; Peters et al., 1999; Wendt et al., 2014), and also several “new” studies, including four that investigate a relationship between O₃ and COVID-19 (Ware et al., 2016; Strosnider et al., 2019; Wang et al., 2019a; Adhikari and Yin, 2020; Zhu et al., 2020; Zoran et al., 2020; Petroni et al., 2020). We do not find these studies to contradict any of the scientific conclusions on respiratory effects described in the ISA.

With regard to the four studies on COVID-19, we disagree with the commenters that they provide evidence that O₃ exposure contributes to COVID-19 incidence, much less that they indicate that O₃ concentrations occurring when the current standard is met would do so. These studies investigate an association between O₃ and COVID-19 cases or deaths. We note, however, that the time-series study design used in three of these studies (Zhu et al., 2020 [incorrectly cited by some commenters as Yongiian et al, 2020]; Adhikari and Yin, 2020; Zoran et al., 2020) is not appropriate for infectious disease cases, which do not follow a Poisson distribution, as they increase exponentially with community spread. The fourth study, an ecological study (Petroni et al. 2020), is also limited by its study design, which is susceptible to confounding or other biases related to ecologic fallacy, as well as its manner of assigning exposure to the population. Further, the time periods in none of the four studies is long enough to rule out a coincidental increase in the community spread of COVID-19 with the increased O₃ concentrations expected with the beginning of O₃ season in these areas (e.g., March-April). Lastly, the biological basis by which a gaseous pollutant such as O₃ would be expected to contribute to incidence of this disease is unclear. Thus, we do not find these studies to support a conclusion that O₃ exposure causes COVID-19 morbidity or mortality.

With regard to the commenters' claims that effects other than respiratory effects (see above) are occurring as a result of O₃ concentrations allowed by the current standard, we note that the standard is exceeded in nearly all of the locations and time periods analyzed in these studies. Although some studies analyzed multiple cities or locations in which the current standard was met during some time periods, air quality during other time periods or locations in the dataset does not meet the current standard. As noted in past reviews, compared to single-city studies, there is additional uncertainty in interpreting relationships between O₃ air quality in individual study cities and reported O₃ multicity effect estimates. Specifically, as recognized in section II.A.2.c above, the available multicity effect estimates in studies of short-term O₃ do not provide a basis for considering the extent to which O₃ health effect associations are influenced by individual locations with ambient O₃ concentrations low enough to meet the current O₃ standards versus locations with O₃ concentrations that violate this standard (85 FR 49853, August 14, 2020; 80 FR 65344, October 26, 2015). Thus, based on this information and the full health effects evidence base for O_3, we disagree with commenters about the implications of the cited epidemiologic studies regarding health risks of O₃ exposures resulting from the O₃ concentrations in ambient air allowed by the current standard.

Protection of Sensitive Groups: Commenters expressing the view that the current standard does not protect sensitive or at-risk populations, variously state that the EPA does not consider risks to a number of population groups the commenters identify as at higher risk for O₃-related health effects, and that retaining the current standard “creates additional and unacceptable risks” for Black and low-income communities. Further, some commenters express the views that together the evidence from controlled human exposure studies and from epidemiologic studies indicates adverse effects associated with exposures allowed by the current standard; and that the EPA has not appropriately considered a number of aspects of the evidence related to risks to people with asthma.

Some commenters, in addition to contending that the current standard will not protect populations for which the EPA has concluded there is adequate evidence for identification of increased risk (e.g., people with asthma, children, and outdoor workers), additionally assert that the current standard will not protect populations of color, American Indian/American Native groups, low SES communities, people of any age with respiratory issues other than asthma, diabetes or atrial fibrillation and pregnant women. As described in section I.A. above, primary NAAQS are intended to protect the public health, including at-risk populations, with an adequate margin of safety. Accordingly, in reviewing the air quality criteria, the EPA evaluates the evidence with regard to factors that place some populations at increased risk of harm from the subject pollutant. In this review, the populations for which the evidence indicates increased risk include people with asthma, children and outdoor workers, among other groups, as summarized in section II.A.2.b above (ISA, section IS.4.4).

In support of their argument that individuals with atrial fibrillation are at increased risk of O₃-related health effects, the commenter cited a study of O₃ exposure and total mortality that has been evaluated in the ISA (Medina-Ramon and Schwartz, 2008). It was initially evaluated in the last review and explicitly discussed again as part of the evidence base available in the current review (ISA, section 6.1.5.2 and Table IS-10; 2013 ISA, sections 6.6.2.2 and 8.2.4). Based on consideration of that study and others investigating a potential for increased risk among populations with cardiovascular disease (CVD), the 2013 ISA concluded that the evidence was “inadequate to classify pre-existing CVD as a potential at-risk factor for O₃-related health effects” (2013 ISA, sections 8.2.4). In the current review, while a limited number of recent studies add to the evidence available in the 2013 ISA, collectively the evidence remains inadequate to conclude whether individuals with pre-existing CVD are at greater risk of O₃-related health effects (ISA, Table IS-10, section IS.4.4.3.5). Thus, the evidence does not support the commenters assertion that populations with atrial fibrillation are at increased risk of O₃-related effects and that the current standard does not protect these groups.

The commenters who contend pregnant women are at increased risk do not provide supporting evidence, and the ISA does not reach such a conclusion based on the currently available evidence. Further, the ISA determined the evidence to be suggestive of, but not sufficient to infer, a causal relationship between O₃ exposure and reproductive effects (ISA, section IS.4.3.6.3). Thus, we disagree with the commenters that pregnant women may be at increased risk of O₃-related effects and disagree that the current standard does not protect these groups.

With regard to a potential for increased risk of O₃-related health effects based upon race or ethnicity, including American Indians or Native Americans), the available evidence is inadequate to make such a determination (ISA, section IS.4.4, Tables IS-9 and IS-10). Additionally, the evidence of increased O₃ risk based on SES has been evaluated in the ISA and concluded to be “suggestive,” but the evidence is limited by inconsistencies (ISA, section IS.4.4). Thus, contrary to the view expressed by some commenters, the EPA has considered this factor in this review and the evidence was not adequate to identify SES as a risk factor for O₃ related health effects. As noted by the commenters, the evidence for low SES populations is “suggestive” of increased risk (ISA, section IS.4.4), in part because it includes several inconsistencies (as summarized in section II.A.2.b above), including studies that did not find O₃-related risk to be higher in lower SES communities. While we agree with the commenters that populations of some particular races or ethnic backgrounds or with low SES have higher rates of some health conditions, including asthma, the available evidence is not adequate to conclude an increased risk status based solely on racial, ethnic or income variables alone (ISA, section IS.4.4). Thus, we disagree with commenters that EPA has arbitrarily not considered such factors in reaching the decision on the primary standard.

Some commenters further claim that tribal populations and communities of color are at increased risk of O₃-related health effects due to increased impacts of COVID-19. We disagree with commenters that the studies they cite provide support for the role of O₃ exposure in the observed increase in prevalence. The studies cited simply describe greater prevalence of COVID-19 among such communities and do not investigate and therefore do not provide evidence for a role for O₃ exposure. An additional study cited by one commenter in support of their statement that people with COVID-19 are more susceptible to effects of O₃, does not include any analyses with O₃ among its analyses (Wu et al, 2020). With regard to diabetes, we note that the evidence related to a potential for this to affect risk of O₃-related effects has been explicitly evaluated and found to be inadequate, thus indicating a lack of basis in the evidence for the statement by some commenters that diabetes prevalence in a community increases the risk of O₃-related effects (ISA, Table IS-10).

Additionally, commenters that contend that retaining the current standard “creates additional and unacceptable risks” for minority and low-income populations variously cite higher rates of asthma and other preexisting conditions in these populations and higher levels of pollution. In making this claim, these commenters state that non-Hispanic Blacks have been found to be more likely to live in counties with higher O₃ pollution. To the extent that such patterns in the distribution of certain population groups and O₃ concentrations result in these populations residing in areas that do not currently meet the current standard, we note that they are at greater risk than populations residing in areas that meet the current standard, and implementing the standard will reduce their risks. But we disagree with the commenters' conclusion that retaining the current standard, without any change, creates additional risks for these populations.

Thus, contrary to statements by some commenters, the EPA's proposed decision to retain the current standard did consider evidence regarding risk to and thus protection of specific populations, such as those of particular races or ethnicities or low-income populations. The proposed decision, and the Administrator's decision described in section II.B.3 below, are based on consideration of the currently available evidence, particularly that with regard to populations that may be at greater risk of O₃-related health effects than the general population. As described in section II.B.3 below, the Administrator judges that by basing his decision on consideration of these populations, including adults and children with asthma, the at-risk population groups for which the evidence is strongest and most extensive, will also provide protection for other at-risk populations for which the evidence is less certain and less complete.

The commenters who express the view that the current standard does not provide sufficient protection of people with asthma raise concerns with the EPA's consideration of this group and O₃-related effects. Further, some commenters state that the EPA has not adequately explained how its approach for decision-making in this review protects at-risk populations, such as people with asthma. Such commenters state that the EPA does not explain how the proposed decision accounts for the greater vulnerability of people with asthma, given the attention to evidence from controlled human exposure studies of largely healthy subjects. Some commenters contend that the EPA arbitrarily focuses on lung function decrements and respiratory symptoms ahead of lung inflammation, and/or that the EPA has not rationally considered the most recent ATS statement with regard to consideration of effects in people with respiratory disease, such as asthma (which the commenters describe as a difference from past reviews).

We disagree with these commenters. In this review, as in past reviews, the EPA has fully considered the health effects evidence in this review, including for sensitive populations, such as people with asthma, and explained its conclusions regarding the adequacy of public health protection offered by the current standard, including for such populations. Thus, the decision in this review, as described in section II.B.3 below, is based on the current scientific information. Further, our approach in this review does not differ appreciably from our approach in the last review. This approach is consistent with the applicable legal requirements for this review, including with provisions of the CAA related to the review of the NAAQS, and with how the EPA and the courts have historically interpreted the CAA. The approach is based fundamentally on the current health effects evidence in the ISA and quantitative analyses of exposure and risk in the PA. The policy implications of this information, along with guidance, criteria or interpretive statements developed within the public health community, including, also, statements from the ATS, in addition to advice from the CASAC are evaluated in the PA for consideration by the Administrator. The PA evaluations inform the Administrator's public health policy judgments and conclusions. Thus, as in past reviews, the Administrator's decision on the adequacy of the current primary standard draws upon the scientific evidence for health effects, quantitative analyses of population exposures and health risks, CASAC advice, and judgments about how to consider the uncertainties and limitations that are inherent in the scientific evidence and quantitative analyses, as well as public comments on the proposed decision.

As described in section II.B.3 below, key aspects of the evidence informing the Administrator's decision-making in this review include: (1) The causal relationship of O₃ with respiratory effects, based on the full health effects evidence base, including both the controlled human exposure studies conducted primarily in largely healthy adult subjects, and the epidemiologic studies of health outcomes for people with asthma, and particularly children with asthma; (2) the increased risk to children and people with asthma, among other groups (3) the respiratory effects reported at the lowest exposures in the controlled human exposure studies; and (4) features of asthma that contribute to the susceptibility of people with asthma to O₃-related effects. As a whole, the evidence base in this NAAQS review generally reflects a continuum, consisting of exposure levels at which scientists generally agree that health effects are likely to occur, through lower levels at which the likelihood and magnitude of the response become increasingly uncertain. As summarized in section I.A above, the CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels (see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351), but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety. The Administrator's consideration of the scientific evidence is informed by the quantitative estimates of exposure and risk for air quality allowed by the current standard, and associated judgments on the adequacy of public health protection provided by the current standard are informed by advice from the CASAC and statements from ATS on adversity.

With regard to the most recent ATS statement, the commenters' claim that the EPA does not adequately consider the implications of the sentence that “small lung function changes should be considered adverse in individuals with extant compromised function, such as that resulting from asthma, even without accompanying respiratory symptoms” and to consider the importance of examining effects in susceptible subsets of broader populations (Thurston et al., 2017). We disagree. The ATS statements (from the initial statement in 1985 to the recent 2017 statement) and their role in primary O₃ standard reviews, summarized in section II.A.2.b above, occupy a prominent role in consideration of public health implications in the PA and the proposal (PA, section 3.3.2; 85 FR 49848, 49866, 49871, August 14, 2020), and the Administrator considers them in his decision, as described in section II.B.3 below. The PA presentation includes summaries of the purpose and intentions articulated by the ATS, and of the evolution and areas of consistency across the statements. The PA gave particular attention to the ATS emphasis on consideration of the significance or adversity of effects, particularly for more susceptible individuals. It recognized both the 2000 ATS statement concluding that “small transient changes in forced expiratory volume in 1 s[econd] (FEV₁) alone were not necessarily adverse in healthy individuals, but should be considered adverse when accompanied by symptoms” (ATS, 2000), and also the more recent statement that also gives weight to findings of such lung function changes in the absence of respiratory symptoms in individuals with pre-existing compromised function, such as that resulting from asthma (Thurston et al., 2017). With regard to population risk (another aspect of the ATS statement cited by commenters), the PA and proposal summarized the 2000 and 2017 ATS statements, recognizing that the 2017 statement references and further describes concepts described in the 2000 statement, such as its discussion of considering effects on the portion of the population that may have a diminished reserve that puts its members at potentially increased risk if affected by another agent (ATS, 2000). As described in section II.B.3 below, the Administrator considers the ATS statements in reaching his conclusions in this review.

In support of their claim that the EPA has not appropriately considered the ATS statements, some commenters additionally take issue with the EPA's use of the number of subjects experiencing at least a 15% FEV₁ decrement in its description in the proposal of the increased response evident by comparing from the lowest exposure levels studied (40 ppb) up to 70 ppb (85 FR 49851, August 14, 2020). These comments also state that EPA did not discuss the clinical significance of FEV₁ decrements of 10% or higher for people with existing lung disease, while stating that the ATS statement mentions this magnitude of decrement. The ATS statement references decrements at or above 10% in illustrating a point about variation of subject responses beyond a group mean, noting that while the mean of an exposed group of study subjects may be small, some group members have larger reductions and can have passed a threshold for clinical importance. It does not provide a discussion of thresholds of clinical importance. In claiming that EPA's discussion on this represents a difference from the last review, the commenters cite the 2014 HREA and state that we have not considered FEV₁ decrements at or above 10% in the current review, however this is not the case. Furthermore, the PA states that the mid- to upper-end of the range of moderate levels of functional responses and higher (i.e., FEV₁ decrements ≥15% and ≥20%) are included to generally represent potentially adverse lung function decrements in active healthy adults, while for people with asthma or lung disease, a focus on moderate functional responses (FEV₁ decrements down to 10%) may be appropriate (PA, Appendix 3D, p. 3D-76).

In objecting to the EPA's approach to considering the ATS statement, these commenters cite a reference to the ATS statement in CASAC's advice as additional evidence that the EPA approach to considering the ATS statement is arbitrary. This comment was made within the context of the CASAC comments on the draft PA that emphasized the need to improve discussion of the susceptibility of people with asthma, including giving attention to the occurrence of lung function decrements in susceptible groups, specifically children with asthma. This section of the CASAC letter also cautions against too great a focus on lung function decrements and emphasizes the need for fuller consideration of respiratory effects that are likely to be important in people with asthma due to features of that disease. In consideration of these comments, the final PA includes an improved discussion on the unique vulnerability of people with asthma (PA, sections 3.3.1.1, 3.3.2, 3.3.4, and 3.5.1) that contributes to due consideration of this population group in decision-making on the primary O₃ standard. Further, in considering the exposure and risk analysis results, we recognize the comparison-to-benchmarks analysis as providing a more robust consideration of risk to sensitive groups as it provides the ability to consider O₃ effects more broadly, with each benchmark representing the array of effects, at different severities, associated with that exposure level. This is one of the reasons (consistent with the CASAC advice) that this analysis (rather than the lung function risk analysis) receives greater emphasis in the PA, consistent with the CASAC advice in this area.

In light of the above discussion, we note that the PA, the proposal, and the decision described in section II.B.3 below, focus specifically on consideration of people with asthma, and particularly children with asthma. While the evidence regarding the susceptibility of people with asthma to the effects of O₃ is robust, our understanding of the exposures at which various effects (of varying severity) would be elicited is less defined. For example, the inherent characteristics of asthma contribute to a risk of asthma-related responses, such as asthma exacerbation in response to asthma triggers, which may increase the risk of more severe health outcomes (ISA, section 3.1.5). This is supported by the strong and consistent epidemiologic evidence that demonstrates associations between ambient O₃ concentrations and hospital admissions and emergency department visits for asthma (ISA, section IS.4.4.3.1). In moving to consideration of the potential specific exposure scenarios (e.g., multiple-hour exposures to 60 to 80 ppb O₃ during quasi-continuous exercise), we note that the evidence is for largely healthy adult subjects. With regard to lung function decrements, the limited evidence from controlled human exposure studies (primarily at higher exposures and in adult subjects) indicates similar magnitude of O₃-related FEV₁ decrements for people with as for people without asthma (ISA, Appendix 3, section 3.1.5.4.1). Further, across other respiratory effects of O₃ (e.g., increased respiratory symptoms, increased airway responsiveness and increased lung inflammation), the evidence has also found the observed responses to generally not differ due to the presence of asthma, although the evidence base is more limited with regard to study subjects with asthma (ISA, Appendix 3, section 3.1.5.7). Thus, in light of the uncertainties in the evidence base with respect to people with asthma and exposures eliciting effects and the severity of those effects, other aspects of the evidence are informative to the necessary judgments. Accordingly, the advice from the CASAC and the statements from the ATS are important to the judgments made by the Administrator in basing his decision on the current evidence and ensuring a primary standard that protects at-risk populations, such as people with asthma.

Contrary to the claim from some commenters, our consideration of effects in people with asthma did not focus solely on lung function responses. As noted above, we recognize that the inherent characteristics of asthma as a disease provide the potential for O₃ exposures to trigger asthmatic responses, such as through causing an increase in airway responsiveness. Based on the available evidence, we consider the potential for such a response to be greater, in general, at relatively higher, versus lower, exposure concentrations, noting 80 ppb to be the lowest exposure concentration at which increased airway responsiveness has been reported in generally healthy adults. We recognize that this evidence and the evidence represented by the three benchmark concentrations used in the exposure/risk analyses (60, 70 and 80 ppb) is for largely healthy adults and does not include data for people with asthma. In reaching his decision in this review, the Administrator gives additional consideration to the effects of particular concern for people with asthma, such as asthma exacerbation, in light of the limitations of the evidence represented by the benchmarks in this regard, as discussed in section II.B.3 below.

In support of their view that the EPA gives too little weight to effects reported in studies of 60 ppb, some commenters assert that the EPA arbitrarily focused on the evidence for lung function decrements and respiratory symptoms, and does not explain how the proposed decision protects against the harm posed by inflammatory responses to O₃. In making this statement they cite the study by Kim et al. (2011) and discussions in the ISA regarding studies documenting the role of O₃ in eliciting inflammatory responses and regarding possible conceptual mechanisms by which inflammatory responses can contribute to other effects (including cardiovascular effects). In so doing, they contend that exposures lower than those for which the current standard is intended can cause inflammation resulting in permanent lung damage and the development of severe lung disease. They additionally state that airway inflammation of O₃ is of particular concern for people with asthma as airway inflammation is a feature in the definition of asthma.

Contrary to the view of some commenters, the Administrator has given significant consideration (in the proposal and in section II.B.3 below) to the exposure estimates for the 60 ppb benchmark. In considering the O₃ inflammatory response, we note that inflammation induced by a single exposure (or several exposures over the course of a summer) can resolve entirely (2013 ISA, p. 6-76). Thus, the inflammatory response observed following the single exposure to 60 ppb in the study by Kim et al. (2011) of largely healthy subjects is not necessarily an adverse response. We further consider the comments from the CASAC regarding airway inflammation as an important aspect of asthma, including the CASAC's description of increased airway inflammation in people with asthma as having the potential to increase the risk of an asthma exacerbation. As described in section II.B.3 below, the Administrator also considers this, while noting also the lack of evidence from studies of people with asthma at 60 ppb. In so doing, he recognizes that, due to interindividual variability in responsiveness, both in regard to O₃ and in regard to asthma exacerbation triggering events, not every occurrence of an exposure considered to have the potential to increase airway inflammation will result in such an adverse effect. We find it important to note, however, that continued acute inflammation can contribute to a chronic inflammatory state, with the potential to affect the structure and function of the lung (2013 ISA, p. 6-76; ISA, sections 3.1.4.4.2 and 3.1.5.6.2). In light of this evidence, the Administrator, in his consideration of the exposure/risk estimates of exposures at or above the 60 ppb benchmark (described in section II.B.3 below), is less concerned about such estimates representing a single occurrence, and gives weight to estimates of multiple occurrences and their associated greater risk. Thus, rather than a sole focus on a single exposure level or type of effect (such as lung function decrements), the Administrator considers the quantitative estimates for all three benchmarks (with regard to single and multiple occurrences), recognizing that they represent differing levels of significance and severity of O₃-related effects, both with regard to the array of effects and severity of each type of effects, as well as the implications for the at-risk populations, including people with asthma. The comparison-to-benchmarks analysis provides for this full characterization of risk for the broad array of respiratory effects, including inflammation and airway responsiveness, thus avoiding an inadequate and narrower focus, e.g., limited to lung function decrements (85 FR 49872, August 14, 2020).

Contrary to the commenters' claims, the Administrator, in reaching his proposed decision, and in his final decision, as described in section II.B.3 below, placed primary focus on what the evidence indicates with regard to health effects in the at-risk population of people with asthma, particularly children with asthma, and on results of the exposure and risk analysis for this population. In so doing, he recognizes key aspects of the evidence, as summarized in section II.A.2.a above, that indicate the array of O₃-associated respiratory effects to be of increased significance to people with asthma given aspects of the disease that may put such peoples at increased risk for prolonged bronchoconstriction in response to asthma triggers. The increased significance of effects in people with asthma and risk of increased exposure for children (from greater frequency of outdoor exercise) is illustrated by the epidemiologic findings of positive associations between O₃ exposure and asthma-related emergency department visits and hospital admissions for children with asthma. In this context, the Administrator focuses on the breadth of O₃ respiratory effects evidence at the lowest exposures tested in the controlled human exposure studies which provide the most certain evidence, considering this in light of the fuller evidence base which provides a foundation for necessary judgments in light of uncertainties.

Thus, we disagree with commenters that we have not considered the full body of evidence and quantitative information available in this review with regard to exposures that might be expected to elicit effects in at-risk populations. In so doing, as summarized in section II.A.2.a above, section II.B.1.a of the proposal, and the PA, we recognize that the currently available evidence supports the conclusion of a causal relationship between short-term O₃ exposure and respiratory effects, with the strongest evidence coming from controlled human exposure studies that document subtle reversible effects in 6.6-hour exposures of largely healthy adult subjects, engaged in quasi-continuous exercise, to average concentrations as low as 60 ppb. The epidemiologic evidence of associations of O₃ concentrations in ambient air with increased incidence of hospital admissions and emergency department visits for an array of respiratory health outcomes further indicates the potential for O₃ exposures to elicit health outcomes more serious than those assessed in the experimental studies, particularly for children with asthma, and the evidence base of such epidemiologic studies as a whole provides strong support for the conclusion of causality for respiratory effects. Further as described in the PA and the proposal and summarized in section II.A.2.a above, very few of these studies were conducted in locations during periods when the current standard was met. While some commenters cite the low values of some of the air quality metrics analyzed in such studies, those metrics are not in the form of the design value for the current standard and so, contrary to commenters' assertion, cannot show that serious health effects are occurring under air quality conditions allowed by the current 70 ppb standard.

Protection With an Adequate Margin of Safety: Some commenters expressed the view that the current standard does not provide an adequate margin of safety variously argue that the EPA is ignoring precedent and CAA requirements for considering scientific uncertainty in its judgments regarding an adequate margin of safety, and that statements from the prior CASAC and new evidence suggests that the current 70 ppb standard provides little margin of safety for protection of sensitive subpopulations from harm. These commenters generally advocate revision to a 60 ppb standard to address this concern. In support of their views, some state that the EPA is ignoring findings of a statistically significant lung function response to 6.6-hour exposure to 60 ppb during quasi-continuous exercise while others cite the EPA consideration of epidemiologic evidence, claiming that the EPA is inappropriately using identified uncertainties as a basis for not revising the standard. We disagree with these characterizations.

As an initial matter, we note that, contrary to the statements made by these commenters, the Administrator, in reaching his proposed decision, as in reaching his final decision, has considered legal precedent and CAA requirements for a primary standard that protects public health, including the health of sensitive groups, with an adequate margin of safety. With regard to scientific uncertainty, as summarized in section I.A above, the CAA requirement that primary standards provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. It was also intended to provide a reasonable degree of protection against hazards that research has not yet identified. See Lead Industries Ass'n v. EPA, 647 F.2d 1130, 1154 (D.C. Cir. 1980); American Petroleum Institute v. Costle, 665 F.2d at 1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir. 2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Thus, in considering whether the primary standard includes an adequate margin of safety, the Administrator is seeking to ensure that the standard not only prevents pollution levels that have been demonstrated to be harmful but also prevents lower pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. In so doing, however, the CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels (see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351), but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

In the proposed decision, as in the decision described in section II.B.3 below, the Administrator's consideration of the kind and degree of uncertainties associated with the current information (as some of the factors the EPA considers in addressing the requirement for an adequate margin of safety) involved a number of judgments. With regard to his consideration of the epidemiologic evidence, for example, the Administrator recognizes that, as a whole, investigations of associations between O₃ and respiratory effects and health outcomes (e.g., asthma-related hospital admission and emergency department visits) provide strong support for the overarching conclusion of that O₃ causes respiratory effects, and its risks to people with asthma. In his consideration of O₃ exposures of concern, the Administrator, agrees with staff evaluations in the PA, that such studies available in this review are less informative to his judgments related to air quality conditions allowed by the current standard (85 FR 49870, August 14, 2020). For example, as summarized in section II.A.2.c above, none of the U.S. studies that show associations between O₃ and the clearly adverse health outcomes of hospital admissions or emergency department visits for respiratory causes were based in locations during time periods when the current standard was always met (PA, section 3.3.3). While there were two such studies based in single cities in Canada, as discussed above, the interpretation of individual single-city results is complicated by the presence of co-occurring pollutants or pollutant mixtures (PA, section 3.3.3). Thus, as in reaching his decision in this review, the Administrator has fully considered conclusions reached in the ISA regarding the epidemiologic evidence and the policy evaluations in the PA, and does not find the currently available epidemiologic studies to provide insights regarding exposure concentrations associated with health outcomes that might be expected under air quality conditions that meet the current standard (85 FR 49870, August 14, 2020). Thus, the EPA's decision on the standard in this review fully and appropriately considers the full evidence base, including the epidemiologic evidence, and associated uncertainties and limitations.

With regard to the controlled human exposure studies, and the nature and degree of effects that might be expected at exposures lower than those studied or in unstudied population groups, the Administrator has considered first what the evidence base indicates with regard to the lowest exposures as well as differences and similarities between the studied populations and the less well studied population groups recognized to be at increased risk. In so doing, he considers the findings of statistically significant respiratory responses in the studies of 60 ppb exposures in largely healthy subjects, particularly in his consideration of the exposure and risk estimates. For example, in reaching his decision in section II.B.3 below, as for his proposed decision, he finds it appropriate to consider the level of protection provided by the current standard from single exposures, but to give greater weight to multiple exposures, in judging adequacy of the margin of safety provided by the current standard. Such considerations contributed to the Administrator's proposed judgments with regard to the requisite level of protection needed to protect at-risk populations with an adequate margin of safety, as required by the Act and consistent with the factors recognized in the relevant case law. Thus, consistent with the CAA requirements and prior judicial decisions, the Administrator based his proposed decision, and bases his final decision (as summarized in II.B.4 below) on the scientific evidence, our current understanding of it, and his judgments concerning associated uncertainties, both those associated with inconclusive scientific and technical information, and those associated with hazards that research has not yet identified. These judgments, along with the factors recognized above that the EPA generally considers in each NAAQS review, contribute to his reasoned decision making in this review, as described in section II.B.3 below.

With regard to advice provide by CASAC in the last review as a general matter, we disagree with the commenters' presumption that it is necessary for EPA to address in this review each statement a prior CASAC made in a prior review. The Clean Air Act does not impose such a requirement. We further note that a prior CASAC's advice would be based on review of the prior air quality criteria, exposure/risk analyses and standard, as well as considerations pertinent in the prior review (which may, depending on the issue, differ from the pertinent evidence, information and considerations before a current CASAC). We note, however, that this specific advice from the prior CASAC on the adequacy of the margin of safety was cited by part of the CASAC in the current review. As summarized in the proposal and in section II.B.1.b above, while the prior CASAC advised that the size of the margin of safety provided varied across different standard levels within the range from 70 to 60 ppb that the prior CASAC recommended, it found a level of 70 ppb could be supported by the scientific evidence. Further, the prior CASAC recognized, as summarized in section II.B.1.b above, that with regard to the “size” of the margin of safety, the selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator's judgment (Lead Industries Ass'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d at 1353; section I.A above). Thus, in reaching his proposed decision, the Administrator explicitly considered the advice provided by the prior CASAC to the extent it was represented in advice from the current CASAC as emphasized by part of the current CASAC (85 FR 49873, August 14, 2020), and he has similarly again considered it reaching his final decision, as described in section II.B.3 below, in light of public comments raising it.

Some commenters also express the view that EPA is using limited data in sensitive population groups as an excuse for not establishing a level at which there is “an absence of adverse effect” in sensitive groups. In support of their view, some commenters claim that the EPA has not addressed a statement of the prior CASAC regarding the potential for lung function decrements and respiratory symptoms to occur in people with asthma with exposure to 70 ppb (while at elevated exertion). Contrary to this claim, the EPA considered in the last review the point made by the prior CASAC in the statement highlighted by the commenters. The statement from the prior CASAC that the commenters reference was provided in the CASAC review of a draft PA in the last review, fully considered in completing the final 2014 PA, and, along with the totality of the prior CASAC advice, taken into account in establishing the current primary standard (80 FR 65292, October 26, 2020). It is not necessary for the EPA to address in this review each statement a prior CASAC made in a prior review.

We agree with commenters who express the view that to protect sensitive populations from effects reported in some largely healthy subjects from the 6.6 hour exposure to 73 ppb (with quasi-continuous exercise), the standard should provide protection against somewhat lower exposures. As summarized in section II.B.3 below, this is an objective the Administrator identifies for the standard and, based on the exposure/risk estimates, he finds the standard to provide strong protection from such exposures (and associated risk of such effects). In addition, in highlighting this isolated statement from the last review, the commenters fail to distinguish CASAC advice on the primary standard from consideration of the exposure benchmark for comparison to a multi-hour exposure while engaged in quasi-continuous exercise.

With regard to the prior CASAC's scientific and policy advice on the primary standard, the prior CASAC concluded that the scientific evidence supported a range of standard levels that included 70 ppb, and also recognized the choice of a level within that range to be “a policy judgment under the statutory mandate of the Clean Air Act” (85 FR 49873). We further note that the current CASAC concludes in this review that newly available evidence relevant to standard setting does not substantially differ from that available in the last review (Cox, 2020a, Consensus Responses to Charge Questions p. 12; 85 FR 49873, August 14, 2020). As discussed further below, we note that the CAA does not require the Administrator to establish a primary NAAQS at a zero-risk level or at background concentration levels (see Lead Industries Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3d at 1351), but rather at a level that reduces risk sufficiently so as to protect public health with an adequate margin of safety.

Some commenters also state that the primary NAAQS must be set at a level at which there is an absence of adverse effects in sensitive populations. While the EPA agrees that the NAAQS must be set to protect sensitive populations with an adequate margin of safety, it is well established that the NAAQS are not meant to be zero risk standards. See Lead Industries v. EPA, 647 F.2d at 1156 n.51; ATA III, 283 F. 3d at 360 (“[t]he lack of a threshold concentration below which these pollutants are known to be harmless makes the task of setting primary NAAQS difficult, as EPA must select standard levels that reduce risks sufficiently to protect public health even while recognizing that a zero-risk standard is not possible”); Mississippi, 744 F. 3d at 1351 (same); see also id. at 1343 (“[d]etermining what is `requisite' to protect the `public health' with an `adequate' margin of safety may indeed require a contextual assessment of acceptable risk. See Whitman, 531 U.S. at 494-95 (Breyer J. concurring)”). As the Court of Appeals for the D.C. Circuit said in reviewing the 2015 O<sub>3</sub> NAAQS, “the primary standard for a non-threshold pollutant like ozone is not required to produce zero risk, and `[t]he task of determining what standard is `requisite' to protect the qualitative value of public health or what margin of safety is `adequate' to protect sensitive subpopulations necessarily requires the exercise of policy judgment.' ” Murray Energy, 936 F.3d at 610 (quoting Mississippi v. EPA). The Administrator's judgments in this review are rooted in his evaluation of the evidence, which reflects the scientific uncertainty as to the O₃ concentrations at which sensitive subpopulations would experience adverse health effects, and his judgments weigh both the risks and the uncertainties. This is a legitimate, and well recognized, exercise of “reasoned decision-making.” ATA III. 283 F. 3d at 370; see also id. at 370 (“EPA's inability to guarantee the accuracy or increase the precision of the . . . NAAQS in no way undermines the standards' validity. Rather, these limitations indicate only that significant scientific uncertainty remains about the health effects of fine particulate matter at low atmospheric concentration. . . .”); Mississippi, 744 F. 3d at 1352-53 (appropriate for EPA to balance scientific uncertainties in determining level of revised O₃ NAAQS).

Exposure/risk Analyses: In expressing the view that the standard should be made more stringent, some commenters disagree with EPA conclusions based on the exposure/risk analyses, and point to other analyses that they state show that a lower standard level (e.g. 65 ppb or lower) would avoid important health effects. These commenters' claims of deficiencies with the exposure/risk analyses include claims that the study area selection is not explained, that population size of the study areas analyzed is too small to support conclusions and does not include particular areas; that the analysis does not include adults, and other groups of interest, and that selection of study areas with air quality close to the current standard contributed to underestimates of population exposures. We disagree with these commenters' claims.

With regard to study area selection and population size for the analysis, we note that an exposure and risk analysis based on eight study areas, all of which are major metropolitan areas provides a robust foundation for population exposure estimates. The eight study areas included reflect the full range of air quality and exposure variation expected across major urban areas in the U.S and seven different NOAA climate regions (PA, section 3.4.1). This number of areas (8) and combined population size (more than 45 million in the combined metropolitan areas [PA, Appendix 3D, Table 3D-1]) are much larger than similar analyses in recent NAAQS reviews for other pollutants (e.g., sulfur dioxide [U.S. 2018]), and not that dissimilar to similar analyses in past O₃ NAAQS reviews. Some commenters claim that the exclusion of specific urban areas in which O₃ concentrations are much higher than those analyzed resulted in underestimates of exposure. We disagree with this claim as the air quality analyzed across all study areas was adjusted to just meet the current standard (or alternative scenarios). Thus, an urban area that currently has O₃ concentrations well in exceedance of the current standard would not necessarily have been found to have higher exposure estimates if it were simulated to have air quality just meeting the current standard. Such estimates would, however, have greater uncertainty, which is the reason such study areas as those identified by commenters (e.g., Los Angeles) were excluded. Areas included were those for which only small adjustments were required for the air quality to just meet the current standard (and alternative scenarios), yielding reduced uncertainty (e.g., given the need for larger air quality adjustments to achieve conditions that just meet the current standard) in these estimates compared to those from the 2014 HREA (PA, sections 3.4.1 and 3.4.4, and Appendix 3D). In selecting such areas, however, we considered a number of other characteristics in order to achieve a varied set of study areas, including with regard to air quality patterns. This variation contributed to variation in exposure estimates, even for the same air quality scenario (PA, Appendix 3D, Tables 3D-26, 3D-28 and 3D-30). Thus, in addition to focusing on study areas with ambient air concentrations close to conditions that reflect air quality that just meets the current standard that would be more informative to evaluating the health protection provided by the current standard than areas with much higher (or much lower) concentrations, the approach employed recognizes that capturing an appropriate diversity in study areas and air quality conditions (that reflect the current standard scenario) is an important aspect of the role of the exposure and risk analyses in informing the Administrator's conclusions on the public health protection afforded by the current standard.

Contrary to one commenter's assertion that adults were not included in the exposure assessment, the populations assessed included two adult populations groups: All adults and adults with asthma. The results for these groups and all of the populations assessed are presented in detail in the PA (PA, Appendix 3D). As described in the PA and proposal, the estimates for adults as a percentage of the study populations were generally lower than those for children. Thus, we focused discussion on the estimates for children, including particularly children with asthma. As recognized by the Administrator in section II.B.3 below, his judgments on the adequacy of protection provided by the current standard takes into account protection provided to the U.S. population, including those population groups at increased risk, which includes children and people of all ages with asthma, among other groups.

Some commenters claimed that the EPA should have separately assessed exposure for certain additional population subgroups, such as children at summer camps, adults with lung impairments other than asthma or outdoor workers. As an initial matter, we recognize appreciably increased uncertainty regarding key aspects of the information necessary for such simulations for all of these groups. Of the three groups, only outdoor workers are identified as an at-risk population in this review (ISA, Table IS-10), and accordingly this group was explicitly considered in designing the exposure analyses. The information available, however, was considered to be too uncertain to produce estimates for this population, as a separate group, with confidence. As described in the PA, important uncertainties exist in generating the simulated activity patterns for this group, including the limited number of CHAD diary days available for outdoor workers, assignment of diaries to proper occupation categories, and in approximating number of days/week and hours/day outdoors, among other pertinent aspects (PA, Appendix 3D, Table 3D-64). We note that these appreciable data limitations and associated uncertainties were also recognized in the 2014 HREA in which a limited sensitivity analyses was conducted for this subgroup. Those limited analyses, conducted for a single area with air quality just meeting the prior 75 ppb standard, indicated that when diaries were selected to mimic exposures that could be experienced by outdoor workers, the percentages of modeled individuals estimated to experience exposures of concern were generally similar to the percentages estimated for children (i.e., using the full database of diary profiles) in the urban study areas and years with the largest exposure estimates (2014 HREA, section 5.4.3.2, Figure 5-14). Accordingly, in this review, in recognition of the data limitations that remain in the current review, outdoor workers were not assessed as a separate population group, and in light of our consideration of conclusions from the sensitivity analyses in the last review, we have generally given primary focus to the estimates for the populations of children.

In summary, we disagree with comments stating that the exposure/risk analyses were deficient and do not provide support for their conclusions. As summarized above, in planning and conducting the exposure/risk analyses, we have appropriately considered issues raised by the commenters, such that the analyses reasonably reflect current understanding, information, tools and methodologies. Further, in presenting the analyses in the PA, we have recognized any associated limitations and uncertainties in an uncertainty characterization that utilized a largely qualitative approach adapted from the World Health Organization approach (and commonly utilized in NAAQS exposure/risk assessments, as discussed in the PA and proposal [85 FR 49857, August 14, 2020]), accompanied by a number of quantitative sensitivity analyses. This characterization and accompanying analyses build on previously conducted work in the 2014 HREA and provide a transparent and explicit recognition of strengths, limitations and uncertainties of the current exposure/risk analysis that were described the PA, considered in the proposal and also in the Administrator's decision described in section II.B.3 below. Thus, the exposure/risk analyses conducted for this review appropriately and soundly reflect current information and methodologies; and we have interpreted their results appropriately in light of any associated limitations and uncertainties.

With regard to other quantitative analyses identified by some commenters and described as showing health impacts that would be avoided by a more stringent standard (e.g. with a level of 65 ppb or lower), we note that these analyses utilize epidemiologic study effect estimates as concentration-response functions to generate predictions of the occurrence of health outcomes, primarily mortality, under different air quality conditions (characterized by the metric used in the epidemiologic study). As an initial matter, we note that our understanding of the relationship between O₃ exposures and total mortality is different in this review than it was in the last review, based on the more extensive evidence base now available. As summarized in section II.A.2.a above, and noted earlier in this section, while our conclusion in the last review was that the relationship of O₃ exposure with mortality was likely to be causal, the current evidence base does not support that conclusion because of limited evidence for cardiovascular mortality, which is by far the largest contributor to total mortality. Rather, the EPA has concluded the evidence in this review to be suggestive of, but not sufficient to infer, causal relationships of total mortality with short- or long-term O₃ exposure, as summarized in section II.A.2.a above (ISA, Appendix 6). Thus, we do not find the weight the commenter is suggesting we place on predictions of total mortality from the epidemiologic study based risk analyses cited by commenters to appropriately reflect the current evidence base for O₃ and mortality, or the evidence base for O₃ and cardiovascular effects (the primary contributor to mortality in the U.S.).

With regard to estimates of avoided respiratory mortality from the analyses cited by these commenters, we note that, while the epidemiologic studies that are inputs to the quantitative analyses cited by the commenters are part of the evidence base that supports our conclusion of a causal relationship between short-term O₃ exposure and respiratory effects, there are uncertainties inherent in the derivation of estimates of mortality ascribed to O₃ exposures using effect estimates from these studies. For example, in planning for analyses in this review, the IRP recognized several important uncertainties associated with aspects of the O₃ epidemiologic study-based approach used in the 2014 HREA (one of the analyses cited by commenters), and similar to the approach used in other analyses cited by commenters, that the EPA concluded to have a moderate or greater impact on risk estimates (IRP, Appendix 5A). Such uncertainties include complications posed by the presence of co-occurring pollutants or pollutant mixtures, as well as those involving the correlation of population O₃ exposures and ambient air monitor concentrations (including the use of area wide average O₃ concentrations) and uncertainties in the derived concentration-response functions (IRP, Appendix 5A; PA, Appendix 34D, section 3D.1.4). Specifically with regard to the 2014 HREA estimates of respiratory mortality, the EPA has recognized uncertainty about the extent to which mortality estimates based on the long-term metric in Jerrett et al. (2009) (i.e., seasonal average of 1-hour daily maximum concentrations) reflect associations with long-term average O₃ versus repeated occurrences of elevated short-term concentrations; and given potential nonlinearity of the C-R function to reflect a threshold of the mortality response, these estimates should be viewed with caution (IRP, Appendix 5A). Accordingly, the 2014 HREA concluded that lower confidence should be placed in the results of the assessment of respiratory mortality risks associated with long-term O₃, primarily because that analysis relies on just one study (Jerrett et al., 2009), and because of the uncertainty in that study about the existence and identification of a potential threshold in the concentration-response function (2014 HREA, section 9.6; 80 FR 65316, October 26, 2015). The other analysis cited by the commenters for predictions of respiratory mortality is also based its estimates on Jerrett et al. (2009). Thus, we find the conclusions regarding uncertainty and low confidence recognized for the 2014 HREA estimates to also apply to the other analysis by commenters and disagree with the conclusions reached by these commenters on this analysis. Further, we do not find that the 2014 HREA or other analyses cited by the commenters, in combination with the full body of evidence currently available, support a conclusion of significant health outcomes for O₃ air quality that meets the current standard.

Long-term Standard Consideration: In support of their view that EPA should establish an additional primary standard that targets long-term exposure, some commenters stated that recent epidemiologic studies indicate causal linkages between long-term exposures and adverse health outcomes, while also suggesting there was support for such a standard in a statement made by the CASAC in reviewing the draft PA. With regard to the epidemiologic studies, these commenters cite several studies published after the literature cut-off date for the ISA that they describe as showing linkages of long-term O₃ exposure to a number of outcomes, including mortality, smokers progression to COPD, hospital admissions for acute respiratory disease syndrome and emergency department visits (Dominici et al., 2019; Seltzer et al., 2020; Limaye and Knowlton, 2020; Dedoussi et al., 2020; Lim et al., 2019; Paulin et al,. 2020; Rhee et al., 2019; Strosnider et al., 2019). We have provisionally considered these “new” studies in addressing these comments consistent with section I.D above (Luben et al., 2020). Of the studies focused on mortality that these commenters cite as providing new information in support of a long-term standard, just three represent new evidence related to investigation of associations of long-term O₃ exposure with mortality (Lim et al., 2019) or respiratory morbidity (Paulin et al., 2020 and Rhee et al., 2019). The study by Lim et al. (2019) analyzes associations between long-term O₃ exposure and respiratory mortality in a U.S. population of older adults in the U.S., reporting a positive association with an effect estimate lower than a previously published study included in the ISA. These results contribute to the evidence base for respiratory effects, e.g., with an additional high-quality study of a previously studied population group (Lim et al., 2019) or with studies investigating additional populations and respiratory outcomes (Paulin et al., 2020; Rhee et al., 2019), albeit with limitations, without reducing uncertainties in the evidence base as a whole. These studies are generally consistent with the evidence assessed in the ISA, and they do not materially change the broad conclusions in the ISA regarding the scientific evidence.

We additionally note that the O₃ concentrations did not meet the current standard in all locations and time periods analyzed in these three multicity studies. Although two of these studies include some locations across the U.S. in which the current standard was likely met during some portions of the study period, air quality during other time periods of locations in the dataset did not meet the current standard. Further, the multicity effect estimates in these studies do not provide a basis for considering the extent to which O₃ health effect associations are influenced by individual locations with ambient O₃ concentrations low enough to meet the current O₃ standards versus locations with O₃ concentrations that violate this standard. Thus, while these more recent studies may be consistent with the existing evidence base evaluated in the ISA, they do not provide a basis for conclusions regarding whether the O₃ exposures occurring under air quality conditions allowed by the current standard may be eliciting the effects analyzed.

We additionally note that while epidemiologic studies evaluate the relationship between health effects and specific O₃ concentrations during a defined study period and the generally consistent and coherent associations observed in these epidemiologic studies contribute to the causality determinations and conclusions regarding the causal nature of the effect of O₃ exposure on health effects, “they do not provide information about which averaging times or exposure metrics may be eliciting the health effects under study” (ISA, section IS.6.1). As noted in the ISA, “disentangling the effects of short-term ozone exposure from those of long-term ozone exposure (and vice-versa) is an inherent uncertainty in the evidence base,” as “the populations included in epidemiologic studies have long-term, variable, and uncharacterized exposures to ozone and other ambient pollutants” (ISA, section IS.6.1). As summarized in the proposal, however, we have also considered the toxicological studies of effects associated with long-term exposures and note that they involve much higher exposures than those occurring at the current standard (85 FR, 49853, August 14, 2020).

Lastly, we disagree with the commenters' implication that the EPA has not addressed a CASAC recommendation. The commenters appear to be asserting that the CASAC recommended that EPA consider a long-term standard. However, the CASAC did not make such a recommendation (Cox, 2020a). In making this assertion, the commenter cites a comment the CASAC makes with regard to a sentence in the draft PA that is drawn from the Administrator's conclusions section of the 2015 decision. Rather than ignoring this CASAC comment, as asserted by the commenters, we made a revision to that section of the PA (moving the statement from the draft PA to a footnote in the final PA with the objective of retaining an accurate description of a consideration related to that 2015 decision, while lessening the potential for confusion of a 2015 consideration with considerations in the current review). Notwithstanding sentences pertaining to the last review, we note the PA evaluates the information in the current review with regard to the protection offered by the current standard (and that the Administrator considered the PA evaluation, as well as the CASAC advice in his proposed decision [summarized in section II.B.1.c above] as in his final decision below). We further note that the description of the Administrator's conclusion in the last review, which is also summarized in the proposal (and in section II.A.1 above), does not describe health effects associated with long-term average concentrations likely under the current standard.

Further, in considering an implication of the commenters' claim that a “long-term standard” is needed in order to provide protection from health effects that may be elicited by long-term exposures to O₃, we note that the impact of standards with short averaging times, such as eight hours, is not limited to reducing short-term exposures. This is because a reduction in magnitude of short-term exposure concentrations (e.g., daily maximum 8-hour concentrations) is also a reduction in exposure to such concentrations over the longer term. For example, a standard, such as the current one, that limits daily maximum 8-hour concentrations to not exceed 70 ppb as a 3-year average of the annual fourth highest value, in addition to limiting the magnitude of concentrations to which a population is exposed in eight hour periods, also limits the frequency to which the population is exposed to such concentrations over the long term. That is, the reduction in frequency of the higher concentrations reduces exposures to such concentrations over the short and long term. Thus, given that, as indicated by the current and established evidence, the O₃ concentrations most likely to contribute to health effects are the higher concentrations, the current standard provides control of exposures to such concentrations over both the short and long term. In light of all of the considerations raised here, we disagree with the commenters assertion of the need for a long-term O₃ standard.

4. Administrator's Conclusions

Having carefully considered the public comments, as discussed above, the Administrator believes that the fundamental scientific conclusions on effects of photochemical oxidants, including O_3, in ambient air that were reached in the ISA and summarized in the PA, and estimates of potential O₃ exposures and risks described in the PA, and summarized above and in sections II.B and II.C of the proposal, remain valid. Additionally, the Administrator believes the judgments he proposed to reach in the proposal (section II.D.3) with regard to the evidence and the quantitative exposure/risk information remain appropriate. Thus, as described below, the Administrator concludes that the current primary O₃ standard provides the requisite protection of the public health with an adequate margin of safety, including for at-risk populations, and should be retained. In considering the adequacy of the current primary O₃ standard, the Administrator has carefully considered the assessment of the available health effects evidence and conclusions contained in the ISA; the evaluation of policy-relevant aspects of the evidence and quantitative analyses in the PA (summarized in sections II.A.2 and II.A.3 above); the advice and recommendations from the CASAC (summarized in section II.B.1.b above); and public comments (discussed in section II.B.2 above and in the separate RTC document).

In the discussion below, the Administrator considers the key aspects of the evidence and exposure/risk estimates important to his judgment regarding the adequacy of protection provided by the current standard. First, the Administrator considers the evidence base on health effects associated with exposure to photochemical oxidants, including O₃, in ambient air. He additionally considers the quantitative exposure and risk estimates developed in this review, including associated limitations and uncertainties, and what they indicate regarding the magnitude of risk, as well as degree of protection from adverse health effects, associated with the current standard. He additionally considers uncertainties in the evidence and the exposure/risk information, as a part of public health judgments that are essential and integral to his decision on the adequacy of protection provided by the standard. Such judgments include public health policy judgments and judgments about the implications of the uncertainties inherent in the scientific evidence and quantitative analyses. The Administrator draws on the PA considerations, and PA conclusions in the current review, taking note of key aspects of the rationale presented for those conclusions. Further, the Administrator considers the advice and conclusions of the CASAC, including particularly its overall agreement that the currently available evidence does not substantially differ from that which was available in the 2015 review when the current standard was established.

As an initial matter, the Administrator recognizes the continued support in the current evidence for O₃ as the indicator for photochemical oxidants (as recognized in section II.D.1 of the proposal). As recognized in the proposal, no newly available evidence has been identified in this review regarding the importance of photochemical oxidants other than O₃ with regard to abundance in ambient air, and potential for health effects, and the “the primary literature evaluating the health and ecological effects of photochemical oxidants includes ozone almost exclusively as an indicator of photochemical oxidants” (ISA, p. IS-3). Accordingly, the information relating health effects to photochemical oxidants in ambient air is also focused on O₃. Thus, the Administrator concludes it is appropriate for O₃ to continue to be the indicator for the primary standard for photochemical oxidants.

With regard to the extensive evidence base for health effects of O₃, the Administrator gives particular attention to the longstanding evidence of respiratory effects causally related to short-term O₃ exposures (summarized in section II.A.2.a above). He recognizes that the strongest and most certain evidence for this conclusion, as in the last review, is that from controlled human exposure studies that report an array of respiratory effects in study subjects (largely generally healthy adults) engaged in quasi-continuous or intermittent exercise. He additionally recognizes the supporting experimental animal and epidemiologic evidence. In so doing, he takes note of the epidemiologic evidence of positive associations for increased incidence of hospital admissions and emergency department visits for an array of respiratory outcomes, with the strongest such evidence being for asthma-related outcomes and specifically asthma-related outcomes for children, with short-term O₃ exposures. As a whole, this strong evidence base continues to demonstrate a causal relationship between short-term O₃ exposures and respiratory effects, including in people with asthma. The Administrator also notes the ISA conclusion that the relationship between long-term exposures and respiratory effects is likely to be causal. These conclusions are also consistent with the conclusions in the last review and reflect a general similarity in the underlying evidence base for such effects.

With regard to conclusions regarding the health effects evidence that differ from those in the last review, the Administrator recognizes the new conclusions regarding metabolic effects, cardiovascular effects and mortality (as summarized in section II.A.2.a above; ISA, Table ES-1). As an initial matter, he takes note of the fact that while the 2013 ISA considered the evidence available in the last review sufficient to conclude that the relationships for short-term O₃ exposure with cardiovascular health effects and mortality were likely to be causal, that conclusion is no longer supported by the now more expansive evidence base which the current ISA determines to be suggestive of, but not sufficient to infer, a causal relationship for these health effect categories (ISA, Appendix 4, section 4.1.17; Appendix 6, section 6.1.8). Further, the Administrator recognizes the new ISA determination that the relationship between short-term O₃ exposure and metabolic effects is likely to be causal (ISA, section IS.4.3.3). In so doing, he takes note that the basis for this conclusion is largely experimental animal studies in which the exposure concentrations were well above those in the controlled human exposure studies for respiratory effects as well as above those likely to occur in areas of the U.S. that meet the current standard (as summarized in section II.A.2.c above). Thus, while recognizing the ISA's conclusion regarding this potential hazard of O_3, he also recognizes that the evidence base is largely focused on circumstances of elevated concentrations above those occurring in areas that meet the current standard. In light of these considerations, he judges the current standard to be protective of such circumstances leading him to continue to focus on respiratory effects in his evaluation of whether the current standard provides requisite protection.

With regard to populations at increased risk of O₃-related health effects, the Administrator notes the populations and lifestages identified in the ISA and summarized in section II.A.2.b above. In so doing, he takes note of the longstanding and robust evidence that supports identification of people with asthma as being at increased risk of O₃-related respiratory effects, including specifically asthma exacerbation and associated health outcomes, and also children, particularly due to their generally greater time outdoors while at elevated exertion (PA, section 3.3.2; ISA, sections IS.4.3.1, IS.4.4.3.1, and IS.4.4.4.1, Appendix 3, section 3.1.11). This tendency of children to spend more time outdoors while at elevated exertion than other age groups, including in the summer when O₃ levels may be higher, makes them more likely to be exposed to O₃ in ambient air under conditions contributing to increased dose due to greater air volumes taken into the lungs. Based on these considerations, the Administrator concludes it is appropriate to give particular focus to people with asthma and children, population groups for which the evidence of increased risk is strongest, in evaluating whether the current standard provides requisite protection. He judges that such a focus will also provide protection of other potentially at-risk population groups, identified in the ISA, for which the current evidence is less robust and clear as to the extent and type of any increased risk, and the exposure circumstances that may contribute to it.

With regard to exposures of interest for respiratory effects, the Administrator refers to the controlled human exposure studies of 6.6-hour exposures, with quasi-continuous exercise, to concentrations ranging from as low as approximately 40 ppb to 120 ppb (as considered in the PA, and summarized in sections II.A.2.c above). He also notes that, as in the last review, these studies, and particularly those that examine exposures from 60 to 80 ppb, are the primary focus of the PA consideration of exposure circumstances associated with O₃ health effects important to the Administrator's judgments regarding the adequacy of the current standard. The Administrator further recognizes that this information on exposure concentrations that have been found to elicit effects in exercising study subjects is unchanged from what was available in the last review.

With regard to the epidemiologic studies of respiratory effects, the Administrator recognizes that, as a whole, these investigations of associations between O₃ and respiratory effects and health outcomes (e.g., asthma-related hospital admission and emergency department visits) provide strong support for the conclusions of causality (as summarized in section II.A.2.a above). He additionally takes note of the PA observation that these studies are generally focused on investigating the existence of relationships between O₃ in ambient air and specific health outcomes and not on detailing the specific exposure circumstances eliciting such effects (PA, section 3.3.3). In so doing, he takes note of the PA conclusions in this regard, including the scarcity of U.S. studies conducted in locations in which and during time periods when the current standard would have been met (as summarized in sections II.A.2.c above). He also recognizes the additional considerations raised in the PA and summarized in section II.A.2.c above regarding information on exposure concentrations in these studies during times and locations that would not have met the current standard, including considerations such as complications in disentangling specific O₃ exposures that may be eliciting effects (PA, section 3.3.3; ISA, p. IS-86 to IS-88). He takes note that such considerations do not lessen the importance of these studies in the evidence base documenting the causal relationship between O₃ and respiratory effects. With regard to his consideration of exposure concentrations associated with O₃ air quality conditions that meet the current standard, based on information cited here and discussed in the PA and section II.B.2.b(ii) above, he judges these studies that are available in the current review to be less informative. Thus, the Administrator agrees with the PA conclusions in consideration of this evidence from controlled human exposure and epidemiologic studies (as assessed in the ISA and summarized in the PA), and in consideration of public comments in section II.B.2.b(ii) above, that the evidence base in this review does not include new evidence of respiratory effects associated with appreciably different exposure circumstances than the evidence available in the last review, including particularly any circumstances that would also be expected to be associated with air quality conditions likely to occur under the current standard. In light of these considerations, he finds it appropriate to focus on the studies of 6.6-hour exposures with quasi-continuous exercise, and particularly on study results for concentrations ranging from 60 to 80 ppb.

In considering the significance of responses documented in these studies and the full evidence base for his purposes in judging implications of the current information on public health protection provided by the current standard, notes that the responses reported from exposures ranging from 60 to 80 ppb are transient and reversible in the study subjects. In so doing, he also notes that these studies are in largely healthy adult subjects, that such data are lacking at these exposure levels for children and people with asthma, and that the evidence indicates that such responses, if repeated or sustained, particularly in people with asthma, pose risks of effects of greater concern, including asthma exacerbation, as cautioned by the CASAC. The Administrator also takes note of statements from the ATS (summarized in section II.A.2.b above), as well as judgments made by the EPA in considering similar effects (and ATS statements) in previous NAAQS reviews (80 FR 65343, October 26, 2015). With regard to the ATS statements, including the one newly available in this review (Thurston et al., 2017), the Administrator recognizes the role of such statements, as described by the ATS, as proposing principles or considerations for weighing the evidence rather than offering “strict rules or numerical criteria” (ATS, 2000, Thurston et al., 2017).

The more recent ATS statement is generally consistent with the prior statement (that was considered in the last O₃ NAAQS review) and the attention that statement gives to at-risk or vulnerable population groups, while also broadening the discussion of effects, responses and biomarkers to reflect the expansion of scientific research in these areas. In this way, the most recent statement updates the prior statement, while retaining previously identified considerations, including, for example, its emphasis on consideration of vulnerable populations, thus expanding upon (e.g., with some increased specificity), while retaining core consistency with, the earlier ATS statement (Thurston et al., 2017; ATS, 2000). One example of this increased specificity that was raised in public comments and discussed in section II.B.2 above, is in the discussion of small changes in lung function (in terms of FEV₁) in people with compromised function, such as people with asthma (Thurston et al., 2017). In considering these statements, the Administrator notes that, in keeping with the intent of these statements to avoid specific criteria, the statements, in discussing what constitutes an adverse health effect, do not comprehensively describe all the biological responses raised, e.g., with regard to magnitude, duration or frequency of small pollutant-related changes in lung function. In so doing, he also recognizes the limitations in the current evidence base with regard to our understanding of these aspects of such changes that may be associated with exposure concentrations of interest. Notwithstanding these limitations and associated uncertainties, he takes note of the emphasis of the ATS statement on consideration of individuals with pre-existing compromised function, such as that resulting from asthma (an emphasis which is reiterated and strengthened in the current statement), and agrees that these are important considerations in his judgment on the adequacy of protection provided by the current standard for at-risk populations, as recognized below.

The Administrator recognizes some uncertainty, reflecting limitations in the evidence base, with regard to the exposure levels eliciting effects (as well as the severity of the effects) in some population groups not included in the available controlled human exposure studies, such as children and individuals with asthma. In so doing, the Administrator recognizes that the controlled human exposure studies, primarily conducted in healthy adults, on which the depth of our understanding of O₃-related health effects is based, in combination with the larger evidence base, informs our conceptual understanding of O₃ responses in people with asthma and in children. Aspects of our understanding continue to be limited, however, including with regard to the risk of particular effects and associated severity for these less studied population groups that may be posed by 7-hour exposures with exercise to concentrations as low as 60 ppb that are estimated in the exposure analyses. Collectively, these aspects of the evidence and associated uncertainties contribute to the Administrator's recognition that for O₃, as for other pollutants, the available evidence base in a NAAQS review generally reflects a continuum, consisting of levels at which scientists generally agree that health effects are likely to occur, through lower levels at which the likelihood and magnitude of the response become increasingly uncertain.

In light of these uncertainties in the evidence, as well as those associated with the exposure and risk analyses, the Administrator notes that, as is the case in NAAQS reviews in general, his decision regarding the primary O₃ standard in this review depends on a variety of factors, including his science policy judgments and public health policy judgments. These factors include judgments regarding aspects of the evidence and exposure/risk estimates, such as judgments concerning his interpretation of the different benchmark concentrations, in light of the available evidence and of associated uncertainties, as well as judgments on the public health significance of the effects that have been observed at the exposures evaluated in the health effects evidence. These judgments are rooted in his interpretation of the evidence, which reflects a continuum of health-relevant exposures, with less confidence and greater uncertainty in the existence of adverse health effects as one considers lower O₃ exposures. The factors relevant to judging the adequacy of the standards also include the interpretation of, and decisions as to the relative weight to place on, different aspects of the results of the exposure and risk assessment for the eight areas studied and the associated uncertainties. Together, factors described here inform the Administrator's judgment about the degree of protection that is requisite to protect public health with an adequate margin of safety, including the health of sensitive groups, and, accordingly, his conclusion that the current standard is requisite to protect public health with an adequate margin of safety.

As in prior O₃ NAAQS reviews, the Administrator considers the exposure estimates developed from modeling exposures to O₃ in ambient air in this review to be critically important to consideration of the potential for exposures and risks of concern under air quality conditions of interest, and consequently important to his judgments on the adequacy of public health protection provided by the current standard. The exposure/risk analysis provides a framework within which to consider implications of the health effects evidence with regard to protection afforded by the current standard. In his consideration of the exposure/risk estimates, the Administrator places greater weight and gives primary attention to the comparison-to-benchmarks analysis. This focus reflects his recognition of multiple factors, including the relatively greater uncertainty associated with the lung function risk estimates compared to the results of the comparison-to-benchmarks analysis. Additionally, he recognizes that, as noted in the PA, the comparison-to-benchmarks analysis provides for characterization of risk for the broad array of respiratory effects documented in the controlled human exposure studies. Accordingly, this analysis facilitates consideration of an array of respiratory effects, including but not limited to lung function decrements. Accordingly, the Administrator focuses primarily on the estimates of exposures at or above different benchmark concentrations that represent different levels of significance of O₃-related effects, both with regard to the array of effects and severity of individual effects. In so doing, he notes that this assures his consideration of the protection provided by the standard from the array of respiratory effects documented in the currently available evidence base.

In considering the public health implications of estimated occurrences of exposures (while at increased exertion) to the three benchmark concentrations (60, 70 and 80 ppb), the Administrator considers the respiratory effects reported in controlled human exposure studies of this range of concentrations (during quasi-continuous exercise). Accordingly, the controlled human exposure study evidence base, as a whole, provides context for consideration of the exposure/risk estimates. The Administrator recognizes the three benchmarks to represent exposure conditions associated with different levels of respiratory response in the subjects studied and to inform his judgments on different levels of risk that might be posed to unstudied members of at-risk populations. The highest benchmark concentration (80 ppb) represents an exposure where multiple controlled human exposure studies involving 6.6-hour exposures during quasi-continuous exercise demonstrate a range of O₃-related respiratory effects including inflammation and airway responsiveness, as well as respiratory symptoms and lung function decrements in healthy adult subjects. Findings for this O₃ exposure include: A statistically significant increase in multiple types of respiratory inflammation indicators in multiple studies; statistically significantly increased airway resistance and responsiveness; statistically significant FEV₁ decrements; and statistically significant increases in respiratory symptoms (Table 1). In one variable exposure study for which this (80 ppb) was the exposure period average concentration, the study subject mean FEV₁ decrement was nearly 8%, with individual decrements of 15% or greater (moderate or greater) in 16% of subjects and decrements of 10% or greater in 32% of subjects (Schelegle et al 2009); the percentages of individual subjects with decrements great than 10 or 15% were lower in other studies for this exposure. The second benchmark (70 ppb) represents an exposure level below the lowest exposures that have reported both statistically significant FEV₁ decrements and increased respiratory symptoms (reported at 73 ppb, Schelegle et al 2009) or statistically significant increases in airway resistance and responsiveness (reported at 80 ppb, Horstman et al., 1990). The lowest benchmark (60 ppb) represents still lower exposure, and a level for which findings from controlled human exposure studies of largely healthy subjects have included: Statistically significant decrements in lung function (with mean decrements ranging from 1.7% to 3.5% across the four studies with average exposures of 60 to 63 ppb ), but not respiratory symptoms; and, a statistically significant increase in a biomarker of airway inflammatory response relative to filtered air exposures in one study (Kim et al, 2011).

In turning to the exposure/risk analysis results, the Administrator considers the evidence represented by these benchmarks noting that due to differences among individuals in responsiveness, not all people experiencing such exposures experience a response, such as a lung function decrement, as illustrated by the percentages cited above. Further, among those that experience a response, not all will experience an adverse effect. Accordingly, the Administrator notes that not all people estimated to experience an exposure of 7-hour duration while at elevated exertion above even the highest benchmark would be expected to experience an adverse effect, even members of at-risk populations. With these considerations in mind, he notes that while single occurrences could be adverse for some people, particularly for the higher benchmark concentration where the evidence base is stronger, the potential for adverse response increases with repeated occurrences (as cautioned by the CASAC). In so doing, he also notes that while the exposure/risk analyses provide estimates of exposures of the at-risk population to concentrations of potential concern, they do not provide information on how many of such populations will have an adverse health outcome. Accordingly, in considering the exposure/risk analysis results, while giving due consideration to occurrences of one or more days with an exposure at or above a benchmark, particularly the higher benchmarks, he judges multiple occurrences to be of greater concern than single occurrences.

In this context, the Administrator considers the exposure risk estimates, focusing first on the results for the highest benchmark concentration (80 ppb), which represents an exposure well established to elicit an array of responses in sensitive individuals among study groups of largely healthy adult subjects, exposed while at elevated exertion. Similar to judgments of past Administrators, the current Administrator judges these effects in combination and severity to represent adverse effects for individuals in the population group studied, and to pose risk of adverse effects for individuals in at-risk populations, most particularly people with asthma, as noted above. Accordingly, he judges that the primary standard should provide protection from such exposures. In considering the exposure/risk estimates, he focuses on the results for children, and children with asthma, given the higher frequency of exposures of potential concern for children compared to adults, in terms of percent of the population groups. The exposure/risk estimates indicate more than 99.9% to 100% of children and children with asthma, on average across the three years, to be protected from one or more occasions of exposure at or above this level; the estimate is 99.9% of children with asthma and of all children for the highest year and study area (Table 2). Further, no children in the simulated populations (zero percent) are estimated to be exposed more than once (two or more occasions) in the 3-year simulation to 7-hr concentrations, while at elevated exertion, at or above 80 ppb (Table 2). These estimates indicate strong protection against exposures of at-risk populations that have been demonstrated to elicit a wide array of respiratory responses in multiple studies.

The Administrator next considers the results for the second benchmark concentration (70 ppb), which is just below the lowest exposure concentration (73 ppb) for which a study has reported a combination of a statistically significant increase in respiratory symptoms and statistically significant lung function decrements in sensitive individuals in a study group of largely healthy adult subjects, exposed while at elevated exertion (Schelegle et al., 2009). Recognizing the lack of evidence for people with asthma from studies at 80 ppb and 73 ppb, as well as the emphasis in the ATS statement on the vulnerability of people with compromised respiratory function, such as people with asthma, the Administrator judges it appropriate that the standard protect against exposure, particularly multiple occurrences of exposure, to somewhat lower levels. In so doing, he notes that the exposure/risk estimates indicate more than 99% of children with asthma, and of all children, to be protected from one or more occasions in a year, on average, of 7-hour exposures to concentrations at or above 70 ppb, while at elevated exertion (Table 2). The estimate is 99% of children with asthma for the highest year and study area (Table 2). Further, he notes that 99.9% of these groups are estimated to be protected from two or more such occasions, and 100% from still more occasions. These estimates also indicate strong protection of at-risk populations against exposures similar to those demonstrated to elicit lung function decrements and increased respiratory symptoms in healthy subjects, a response described as adverse by the ATS.

In consideration of the exposure/risk results for the lowest benchmark (60 ppb), the Administrator notes that the lung function decrements in controlled human exposure studies of largely healthy adult subjects exposed while at elevated exertion to concentrations of 60 ppb, although statistically significant, are much reduced from that observed in the next higher studied concentration (73 ppb), both at the mean and individual level, and are not reported to be associated with increased respiratory symptoms in healthy subjects. In light of these results and the transient nature of the responses, the Administrator does not judge these responses to represent adverse effects for generally healthy individuals. However, he further considers these findings specifically with regard to protection of at-risk populations, such as people with asthma. In so doing, he notes that such data are lacking for at-risk groups, such as people with asthma, and considers the evidence and comments from the CASAC regarding the need to consider endpoints of particular importance for this population group, such as risk of asthma exacerbation and prolonged inflammation. He takes note of comments from the CASAC (and also noted in the ATS statement) that small lung function decrements in this at-risk group may contribute to a risk of asthma exacerbation, an outcome described by the CASAC as “arguably the most important potential adverse effect” of O₃ exposure for a child with asthma. Thus, he judges it important for the standard to provide protection that reduces such risks. However, he recognizes gaps in our ability to predict risk of such events at the low concentrations such as those represented by the lowest benchmark in the exposure/risk analysis. With regard to the inflammatory response he notes the evidence, discussed in section II.B.2 above, indicating the role of repeated occurrences of inflammation in contributing to severity of response. Thus, he finds repeated occurrences of exposure events of potential concern to pose greater risk than single events, leading him to place greater weight to exposure/risk estimates for multiple occurrences.

In light of the uncertainties associated with the lack of controlled human exposure data for people with asthma, particularly with regard to the extent to which the lower exposure concentrations studied in generally healthy adults might be expected to elicit asthmatic responses in this at-risk population, the Administrator notes that the CASAC also recognized this, describing the gap in clinical studies to be a “key knowledge gap” important to considerations of margin of safety for the standard. The Administrator further notes that the CAA requirement that primary standards provide an adequate margin of safety was intended to address uncertainties associated with inconclusive scientific and technical information available at the time of standard setting. This approach is consistent with the requirements of the NAAQS provisions of the CAA and with how the EPA and the courts have historically interpreted the Act (summarized in section I.A. above). These provisions require the Administrator to establish primary standards that, in the judgment of the Administrator, are requisite to protect public health with an adequate margin of safety. In so doing, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose. The Act does not require that primary standards be set at a zero-risk level, but rather at a level that avoids unacceptable risks to public health, including the health of sensitive groups.

Thus, in this context, and given that the 70 ppb benchmark represents an exposure level somewhat below the lowest exposure concentration for which both statistically significant lung function decrements and increased respiratory symptoms have been reported in largely healthy adult subjects, the Administrator considers the exposure/risk estimates for the third benchmark of 60 ppb to be informative most particularly to his judgments on an adequate margin of safety. In that context, the Administrator turns to the third benchmark concentration (60 ppb). In so doing, he takes note that these estimates indicate more than 96% to more than 99% of children with asthma to be protected from more than one occasion in a year (two or more), on average, of 7-hour exposures to concentrations at or above this level, while at elevated exertion (Table 2). Additionally, the analysis estimates more than 90% of all children, on average across the three years, to be protected from one or more occasions of exposure at or above this level. The Administrator finds this to indicate an appropriate degree of protection from such exposures.

The Administrator additionally takes note of the new finding in this review of evidence of a likely to be causal relationship between O₃ and metabolic effects. In so doing, he notes the lack of evidence that would suggest such effects to be associated with exposures likely to occur with air quality conditions meeting the current standard, as discussed in section II.A.2.c above. Thus, he judges the current standard to provide protection from effects other than respiratory effects, for which the evidence is less certain. Accordingly, the Administrator concludes that the standard does not need to be revised to provide additional protection from such effects.

In reflecting on all of the information currently available, the Administrator considers the extent to which the currently available information might indicate support for a less stringent standard. He recognizes the advice from the CASAC, which generally indicates support for retaining the current standard without revision or for revision to a more stringent level based on additional consideration of the margin of safety for at-risk populations. He notes that the CASAC advice did not convey support for a less stringent standard. He additionally considers the current exposure and risk estimates for the air quality scenario for a design value just above the level of the current standard (at 75 ppb), in comparison to the scenario for the current standard, as summarized in section II.A.3 above. In so doing, he finds the markedly increased estimates of exposures to the higher benchmarks under air quality for a higher standard level to be of concern and indicative of less than the requisite protection (Table 2). Thus, in light of the considerations raised here, including the need for an adequate margin of safety, the Administrator judges that a less stringent standard would not be appropriate.

The Administrator additionally considers whether it would be appropriate to consider a more stringent standard that might be expected to result in reduced O₃ exposures. As an initial matter, he considers the advice from the CASAC. With regard to the CASAC advice, while part of the Committee concluded the evidence supported retaining the current standard without revision, another part of the Committee reiterated advice from the prior CASAC, which while including the current standard level among the range of recommended standard levels, also provided policy advice to set the standard at a lower level. In considering this advice now in this review, as it was raised by part of the current CASAC, the Administrator notes the slight differences of the current exposure and risk estimates from the 2014 HREA estimates for the lowest benchmark, which were those considered by the prior CASAC (Table 4). For example, while the 2014 HREA estimated 3.3 to 10.2% of children, on average, to experience one or more days with an exposures at or above 60 ppb (and as many as 18.9% in a single year), the comparable estimates for the current analyses are lower, particularly at the upper end (3.2 to 8.2% and 10.6%). While the estimates for two or more days with occurrences at or above 60 ppb, on average across the assessment period, are more similar between the two assessments, the current estimate for the single highest year is much lower (9.2 versus 4.3%). The Administrator additionally recognizes the PA finding that the factors contributing to these differences, which includes the use of air quality data reflecting concentrations much closer to the now-current standard than was the case in the 2015 review, also contribute to a reduced uncertainty in the current estimates, as summarized in section II.A.3 above (PA, sections 3.4 and 3.5). Thus, he notes that the current exposure analysis estimates indicate the current standard to provide appreciable protection against multiple days with a maximum exposure at or above 60 ppb. In the context of his consideration of the adequacy of protection provided by the standard and of the CAA requirement that the standard protect public health, including the health of at-risk populations, with an adequate margin of safety, the Administrator concludes, in light of all of the considerations raised here, that the current standard provides appropriate protection, and that a more stringent standard would be more than requisite to protect public health.

In light of all of the above, including advice from the CASAC, the Administrator finds the current exposure and risk analysis results to describe appropriately strong protection of at-risk populations from exposures associated with O₃-related health effects. Therefore, based on his consideration of the evidence and exposure/risk information, including that related to the lowest exposures studied in controlled human exposure studies, and the associated uncertainties, the Administrator judges that the current standard provides the requisite protection of public health, including an adequate margin of safety, and thus should be retained, without revision. Accordingly, he concludes that a more stringent standard is not needed to provide requisite protection and that the current standard provides the requisite protection of public health under the Act. With regard to key aspects of the specific elements of the standard, the Administrator recognizes the support in the current evidence base for O₃ as the indicator for photochemical oxidants. In so doing, he notes the ISA conclusion that O₃ is the most abundant of the photochemical oxidants in the atmosphere and the one most clearly linked to human health effects. He additionally recognizes the control exerted by the 8-hour averaging time on associated exposures of importance for O₃-related health effects. Lastly, with regard to form and level of the standard, the Administrator takes note of the exposure and risk results as discussed above and the level of protection that they indicate the elements of the current standard to provide. Beyond his recognition of this support in the available information for the elements of the current standard, the Administrator has considered the elements collectively in evaluating the health protection afforded by the current standard. For all of the reasons discussed above, the Administrator concludes that the current primary O₃ standard (in all of its elements) is requisite to protect public health with an adequate margin of safety, including the health of at-risk populations, and thus should be retained, without revision.

C. Decision on the Primary Standard

For the reasons discussed above and taking into account information and assessments presented in the ISA and PA, the advice from the CASAC, and consideration of public comments, the Administrator concludes that the current primary O₃ standard is requisite to protect public health with an adequate margin of safety, including the health of at-risk populations, and is retaining the current standard without revision.

III. Rationale for Decision on the Secondary Standard

This section presents the rationale for the Administrator's decision to retain the current secondary O₃ standard. This rationale is based on the scientific information presented in the ISA, on welfare effects associated with photochemical oxidants including O₃ and pertaining to the presence of these pollutants in ambient air. As summarized in section I.D above, the ISA was developed based on a thorough review of the latest scientific information generally published between January 2011 and March 2018, as well as more recent studies identified during peer review or by public comments on the draft ISA integrated with the information and conclusions from previous assessments (ISA, section IS.1.2 and Appendix 10, section 10.2). The Administrator's rationale also takes into account: (1) The PA evaluation of the policy-relevant information in the ISA and presentation of quantitative analyses of air quality, exposure, and risk; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the ISA and PA at public meetings, and in the CASAC's letters to the Administrator; (3) public comments on the proposed decision; and also (4) the August 2019 decision of the D.C. Circuit remanding the secondary standard established in the last review to the EPA for further justification or reconsideration. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019).

Within this section, introductory and background information is presented in section III.A. Section III.A.1 summarizes the 2015 establishment of the existing standard, as background for this review. Sections III.A.2 and III.A.3 provide overviews of the currently available welfare effects evidence and current air quality and environmental exposure information, respectively. Section III.B summarizes the basis for the proposed decision (III.B.1), including CASAC advice, discusses public comments on the proposed decision (III.B.2), and presents the Administrator's considerations, conclusions and decision in this review of the secondary standard (III.B.3). The decision is summarized in section III.C.

A. Introduction

As in prior reviews, the general approach to reviewing the current secondary standard is based, most fundamentally, on using the Agency's assessments of the current scientific evidence and associated quantitative analyses to inform the Administrator's judgment regarding a secondary standard for photochemical oxidants that is requisite to protect the public welfare from known or anticipated adverse effects associated with the pollutant's presence in the ambient air. The EPA's assessments are primarily documented in the ISA and PA, both of which have received CASAC review and public comment (84 FR 50836, September 26, 2019; 84 FR 58711, November 1, 2019; 84 FR 58713, November 1, 2019; 85 FR 21849, April 20, 2020; 85 FR 31182, May 22, 2020). In bridging the gap between the scientific assessments of the ISA and the judgments required of the Administrator in his decisions on the current standard, the PA evaluates policy implications of the assessment of the current evidence in the ISA and the quantitative air quality, exposure and risk analyses and information documented in the PA. In evaluating the public welfare protection afforded by the current standard, the four basic elements of the NAAQS (indicator, averaging time, level, and form) are considered collectively.

The final decision on the adequacy of the current secondary standard is a public welfare policy judgment to be made by the Administrator. In reaching conclusions on the standard, the decision draws on the scientific information and analyses about welfare effects, environmental exposure and risks, and associated public welfare significance, as well as judgments about how to consider the range and magnitude of uncertainties that are inherent in the scientific evidence and analyses. This approach is based on the recognition that the available evidence generally reflects a continuum that includes ambient air exposures at which scientists generally agree that effects are likely to occur through lower levels at which the likelihood and magnitude of responses become increasingly uncertain. This approach is consistent with the requirements of the provisions of the Clean Air Act related to the review of NAAQS and with how the EPA and the courts have historically interpreted the Act. These provisions require the Administrator to establish secondary standards that, in the judgment of the Administrator, are requisite to protect the public welfare from known or anticipated adverse effects associated with the presence of the pollutant in the ambient air. In so doing, the Administrator seeks to establish standards that are neither more nor less stringent than necessary for this purpose. The Act does not require that standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect the public welfare from known or anticipated adverse effects.

This decision on the secondary O₃ standard also considers the August 2019 decision by the D.C. Circuit and issues raised by the court in its remand of the 2015 standard to the EPA such that the decision in this review incorporates the EPA's response to the court's remand. The opinion issued by the court concluded, in relevant part, that EPA had not provided a sufficient rationale for aspects of its 2015 decision on the secondary standard. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir. 2019). Accordingly, the court remanded that standard to EPA for further justification or reconsideration, particularly in relation to its decision to focus on a 3-year average for consideration of the cumulative exposure for vegetation, in terms of W126, identified as providing requisite public welfare protection, and its decision to not identify a specific level of air quality related to visible foliar injury. Thus, in addition to considering the currently available welfare effects evidence and quantitative air quality, exposure and risk information, the decision described here, and the associated conclusions and judgments, also consider the court's remand. In consideration of the court remand, for example, certain analyses in this review are expanded compared with those conducted in the last review, issues raised in the remand have been discussed, and additional explanation of rationales for conclusions on these points is provided in this review.

1. Background on the Current Standard

As a result of the last O₃ review, completed in 2015, the level of the secondary standard was revised to 0.070 ppm, in conjunction with retaining the indicator, averaging time and form. This revision, establishing the current standard, was based on the scientific evidence and technical analyses available at that time, as well as the Administrator's judgments regarding the available welfare effects evidence, the appropriate degree of public welfare protection for the revised standard, and available air quality information on seasonal cumulative exposures that may be allowed by such a standard (80 FR 65292, October 26, 2015). In establishing this standard, the Administrator considered the extensive welfare effects evidence base compiled from more than fifty years of extensive research on the phytotoxic effects of O₃, conducted both in and outside of the U.S., that documents the impacts of O₃ on plants and their associated ecosystems (U.S. EPA, 1978, 1986, 1996, 2006, 2013). As was established in prior reviews, O₃ can interfere with carbon gain (photosynthesis) and allocation of carbon within the plant, making fewer carbohydrates available for plant growth, reproduction, and/or yield (U.S. EPA, 1996b, pp. 5-28 and 5-29). The 2015 decision drew upon: (1) The available scientific evidence assessed in the 2013 ISA; (2) assessments in the 2014 PA of the most policy-relevant information in the 2013 ISA regarding evidence of adverse effects of O₃ to vegetation and ecosystems, information on biologically-relevant exposure metrics, 2014 welfare REA (WREA) analyses of air quality, exposure, and ecological risks and associated ecosystem services, and staff analyses of relationships between levels of a W126-based exposure index and potential alternative standard levels in combination with the form and averaging time of the existing standard; (3) additional air quality analyses of the W126 index and design values based on the form and averaging time of the existing standard; (4) CASAC advice and recommendations; and (5) public comments received during the development of these documents and on the 2014 proposal (80 FR 65292, October 26, 2015). In addition to reviewing the most recent scientific information as required by the CAA, the 2015 rulemaking also incorporated the EPA's response to the judicial remand of the 2008 secondary O₃ standard in Mississippi v. EPA, 744 F.3d 1334 (D.C. Cir. 2013) and, in light of the court's decision in that case, explained the Administrator's conclusions as to the level of air quality judged to provide the requisite protection of public welfare from known or anticipated adverse effects.

Across the different types of studies, the strongest evidence for effects from O₃ exposure on vegetation was from controlled exposure studies of many species of vegetation (2013 ISA, p. 1-15). Primary consideration in the decision was given to the studies of O₃ exposures that reduced growth in tree seedlings from which E-R functions of seasonal relative biomass loss (RBL) have been established (80 FR 65385-86, 65389-90, October 26, 2015). The Administrator considered the effects of O₃ on tree seedling growth, as suggested by the CASAC, as a surrogate or proxy for the broader array of vegetation-related effects of O₃, ranging from effects on sensitive species to broader ecosystem-level effects (80 FR 65369, 65406, October 26, 2015). The metric used for quantifying effects on tree seedling growth in the review was RBL, with the evidence base providing robust and established E-R functions for seedlings of 11 tree species (80 FR 65391-92, October 26, 2015; 2014 PA, Appendix 5C). The Administrator used this metric in her judgments on O₃ effects on the public welfare. In this context, exposure was evaluated in terms of the W126 cumulative seasonal exposure index, an index supported by the evidence in the 2013 ISA for this purpose and that was consistent with advice from the CASAC (2013 ISA, section 9.5.3, p. 9-99; 80 FR 65375, October 26, 2015).

The 2015 decision was a public welfare policy judgment made by the Administrator, that drew upon the available scientific evidence for O₃-attributable welfare effects and on quantitative analyses of exposures and public welfare risks, as well as judgments about the appropriate weight to place on the range of uncertainties inherent in the evidence and analyses. Included in this decision were judgments on the weight to place on the evidence of specific vegetation-related effects estimated to result across a range of cumulative seasonal concentration-weighted O₃ exposures; on the weight to give associated uncertainties, including uncertainties of predicted environmental responses (based on experimental study data); variability in occurrence of the specific effects in areas of the U.S., especially in areas of particular public welfare significance; and on the extent to which such effects in such areas may be considered adverse to public welfare. For example, in considering the public welfare protection provided by the then-existing standard, the Administrator gave primary consideration to an analysis of cumulative seasonal exposures in or near Class I areas, which are lands that Congress set aside for specific uses intended to provide benefits to the public welfare, including lands that are to be protected so as to conserve the scenic value and the natural vegetation and wildlife within such areas, and to leave them unimpaired for the enjoyment of future generations. The decision additionally recognized that states, tribes and public interest groups also set aside areas that are intended to provide similar benefits to the public welfare for residents on those lands, as well as for visitors to those areas (80 FR 65390, October 26, 2015). In recognizing that her judgments regarding effects that are adverse to the public welfare consider the intended use of the natural resources and ecosystems affected, the Administrator utilized the RBL as a quantitative tool within a larger framework of considerations pertaining to the public welfare significance of O₃ effects (80 FR 65389, October 26, 2015; 73 FR 16496, March 27, 2008).

In the Administrator's consideration of the adequacy of public welfare protection afforded by the existing standard, she gave particular attention to the air quality analysis for Class I areas that estimated cumulative exposures, in terms of 3-year average W126 index values, at and above 19 ppm-hrs, to have occurred under the standard in nearly a dozen areas distributed across two NOAA climatic regions of the U.S (80 FR 65385-86, October 26, 2015). The Administrator took note of these occurrences of exposures in Class I areas during periods when the existing standard was met, for which the associated estimates of growth effects across the species with E-R functions extend above a magnitude considered to be “unacceptably high” by the CASAC (80 FR 65385-65386, 65389-65390, October 26, 2015). Based on this analysis and the considerations summarized above, including consideration of CASAC advice and public comment, the Administrator concluded that the protection afforded by the then-existing standard was not sufficient and that the standard needed to be revised to provide additional protection from known and anticipated adverse effects to public welfare, related to effects on sensitive vegetation and ecosystems, most particularly those occurring in Class I areas, and also in other areas set aside by states, tribes and public interest groups to provide similar benefits to the public welfare. In so doing, she further noted that a revised standard would provide increased protection for other growth-related effects, including relative yield loss (RYL) of crops, reduced carbon storage, and types of effects for which it is more difficult to determine public welfare significance, as well as other welfare effects of O₃, such as visible foliar injury (80 FR 65390, October 26, 2015).

In light of the judicial remand of the 2008 secondary O₃ standard referenced above, the 2015 decision on selection of a revised secondary standard first considered the available evidence and quantitative analyses in the context of an approach for considering and identifying public welfare objectives for the revised standard (80 FR 65403-65408, October 26, 2015). In light of the extensive evidence base of O₃ effects on vegetation and associated terrestrial ecosystems, the Administrator focused on protection against adverse public welfare effects of O₃-related effects on vegetation, giving particular attention to such effects in natural ecosystems, such as those in areas with protection designated by Congress, and areas similarly set aside by states, tribes and public interest groups, with the intention of providing benefits to the public welfare for current and future generations.

In reaching a conclusion on the amount of public welfare protection from the presence of O₃ in ambient air that is appropriate to be afforded by a revised secondary standard, the Administrator gave particular consideration to the following: (1) The nature and degree of effects of O₃ on vegetation, including her judgments as to what constitutes an adverse effect to the public welfare; (2) the strengths and limitations of the available and relevant information; (3) comments from the public on the Administrator's proposed decision, including comments related to identification of a target level of protection; and (4) the CASAC's views regarding the strength of the evidence and its adequacy to inform judgments on public welfare protection. The Administrator recognized that such judgments should neither overstate nor understate the strengths and limitations of the evidence and information nor the appropriate inferences to be drawn as to risks to public welfare, and that the choice of the appropriate level of protection is a public welfare policy judgment entrusted to the Administrator under the CAA taking into account both the available evidence and the uncertainties (80 FR 65404-05, October 26, 2015).

With regard to the extensive evidence of welfare effects of O₃, including visible foliar injury and crop RYL, the RBL information available for seedlings of a set of 11 tree species was judged to be more useful (particularly in a role as surrogate for the broader array of vegetation-related effects) in informing judgments regarding the nature and severity of effects associated with different air quality conditions and associated public welfare significance (80 FR 65405-06, October 26, 2015). With regard to visible foliar injury, while the Administrator recognized the potential for this effect to affect the public welfare in the context of affecting value ascribed to natural forests, particularly those afforded special government protection, she also recognized limitations in the available information that might inform consideration of potential public welfare impacts related to this vegetation effect noting the significant challenges in judging the specific extent and severity at which such effects should be considered adverse to public welfare (80 FR 65407, October 26, 2015). Similarly, while O₃-related growth effects on agricultural and commodity crops had been extensively studied and robust E-R functions developed for a number of species, the Administrator found this information less useful in informing her judgments regarding an appropriate level of public welfare protection (80 FR 65405, October 26, 2015). Thus, and in light of the extensive evidence base in this regard, the Administrator focused on the information related to trees and growth impacts in identifying the public welfare objectives for the revised secondary standard.

Accordingly, consideration of the appropriate public welfare protection objective for a revised standard focused on the estimates of tree seedling growth impacts (in terms of RBL) for a range of W126 index values, developed from the E-R functions for 11 tree species (80 FR 65391-92, Table 4, October 26, 2015). The Administrator also incorporated into her considerations the broader evidence base associated with forest tree seedling biomass loss, including other less quantifiable effects of potentially greater public welfare significance. That is, in drawing on these RBL estimates, the Administrator was not simply making judgments about a specific magnitude of growth effect in seedlings that would be acceptable or unacceptable in the natural environment. Rather, though mindful of associated uncertainties, the Administrator used the RBL estimates as a surrogate or proxy for consideration of the broader array of related vegetation-related effects of potential public welfare significance, which included effects on individual species and extending to ecosystem-level effects (80 FR 65406, October 26, 2015). This broader array of vegetation-related effects included those for which public welfare implications are more significant but for which the tools for quantitative estimates were more uncertain.

In using the RBL estimates as a proxy, the Administrator focused her attention on a revised standard that would generally limit cumulative exposures to those for which the median RBL estimate for seedlings of the 11 species with robust and established E-R functions would be somewhat below 6% (80 FR 65406-07, October 26, 2015). In so doing, she noted that the median RBL estimate was 6% for a cumulative seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4, October 26, 2015). Given the information on median RBL at different W126 exposure levels, using a 3-year cumulative exposure index for assessing vegetation effects, the potential for single-season effects of concern, and CASAC comments on the appropriateness of a lower value for a 3-year average W126 index, the Administrator concluded it was appropriate to identify a standard that would restrict cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-year W126 index, in nearly all instances (80 FR 65407, October 26, 2015). Based on such information, available at that time, to inform consideration of vegetation effects and their potential adversity to public welfare, the Administrator additionally judged that the RBL estimates associated with marginally higher exposures in isolated, rare instances were not indicative of effects that would be adverse to the public welfare, particularly in light of variability in the array of environmental factors that can influence O₃ effects in different systems and uncertainties associated with estimates of effects associated with this magnitude of cumulative exposure in the natural environment (80 FR 65407, October 26, 2015).

Using these objectives, the Administrator's decision regarding a revised standard was based on extensive air quality analyses that included the most recently available data (monitoring year 2013) and extended back more than a decade (80 FR 65408, October 26, 2015; Wells, 2015). These analyses evaluated the cumulative seasonal exposure levels in locations meeting different alternative levels for a standard of the existing form and averaging time. Based on these analyses, the Administrator judged that the desired level of public welfare protection, considered in terms of cumulative exposure (quantified as the W126 index), could be achieved by a standard with a revised level in combination with the existing form and averaging time (80 FR 65408, October 26, 2015).

In the most recent period of air quality data (2011-2013), across the more than 800 monitor locations meeting the existing standard (with its level of 75 ppb), the 3-year average W126 index values were above 17 ppm-hrs in 25 sites distributed across different NOAA climatic regions, and above 19 ppm-hrs at nearly half of these sites, with some well above (Wells, 2015). In comparison, among the more than 500 sites meeting an alternative standard of 70 ppb across 46 of the 50 states, there were no occurrences of a W126 value above 17 ppm-hrs and fewer than five occurrences that equaled 17 ppm-hrs (Wells, 2015 and associated dataset [document identifier, EPA-HQ-OAR-2008-0699-4325]). For the full air quality dataset (extending back to 2001), among the nearly 4000 instances where a monitoring site met a standard level of 70 ppb, the Administrator noted that there was only “a handful of isolated occurrences” of 3-year W126 index values above 17 ppm-hrs, “all but one of which were below 19 ppm-hrs” (80 FR 65409, October 26, 2015). The Administrator concluded that that single value of 19.1 ppm-hrs (just equaling 19, when rounded), observed at a monitor for the 3-year period of 2006-2008, was reasonably regarded as an extremely rare and isolated occurrence, and, as such, it was unclear whether it would recur, particularly as areas across the U.S. took further steps to reduce O₃ to meet revised primary and secondary standards. Further, based on all of the then available information, as noted above, the Administrator did not judge RBL estimates associated with marginally higher exposures in isolated, rare instances to be indicative of adverse effects to the public welfare. The Administrator concluded that a standard with a level of 70 ppb and the existing form and averaging time would be expected to limit cumulative exposures, in terms of a 3-year average W126 exposure index, to values at or below 17 ppm-hrs, in nearly all instances, and accordingly, to eliminate or virtually eliminate cumulative exposures associated with a median RBL of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a proxy in judging effects to public welfare, the Administrator judged that such a standard with a level of 70 ppb would provide the requisite protection from adverse effects to public welfare by limiting cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-year W126 index, in nearly all instances, and decided to revise the standard level to 70 ppb.

In summary, the Administrator judged that the revised standard would protect natural forests in Class I and other similarly protected areas against an array of adverse vegetation effects, most notably including those related to effects on growth and productivity in sensitive tree species. The Administrator additionally judged that the revised standard would be sufficient to protect public welfare from known or anticipated adverse effects. This judgment by the Administrator appropriately recognized that the CAA does not require that standards be set at a zero-risk level, but rather at a level that reduces risk sufficiently so as to protect the public welfare from known or anticipated adverse effects. Thus, based on the conclusions drawn from the air quality analyses which demonstrated a strong, positive relationship between the 8-hour and W126 metrics and the findings that indicated the significant amount of control provided by the fourth-high metric, the evidence base of O₃ effects on vegetation and her public welfare policy judgments, as well as public comments and CASAC advice, the Administrator decided to retain the existing form and averaging time and revise the level to 0.070 ppm, judging that such a standard would provide the requisite protection to the public welfare from any known or anticipated adverse effects associated with the presence of O₃ in ambient air (80 FR 65409-10, October 26, 2015).

2. Overview of Welfare Effects Information

The information summarized here is an overview of the scientific assessment of the welfare effects evidence available in this review; this assessment is documented in the ISA and its policy implications are further discussed in the PA. As in past reviews, the welfare effects evidence evaluated in the ISA for O₃ and related photochemical oxidants is focused on O₃ (ISA, p. IS-3). Ozone is the most prevalent photochemical oxidant present in the atmosphere and the one for which there is a very large, well-established evidence base of its health and welfare effects (ISA, p. IS-3). Thus, the current welfare effects evidence and the Agency's review of the evidence, including the evidence newly available in this review, continues to focus on O₃. The subsections below briefly summarize the following aspects of the evidence: the nature of O₃-related welfare effects, the potential public welfare implications, and exposure concentrations associated with effects.

a. Nature of Effects

The welfare effects evidence base available in the current review includes more than sixty years of extensive research on the phytotoxic effects of O₃ and subsequent effects on associated ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013 ISA, 2020 ISA). As described in past reviews, O₃ can interfere with carbon gain (photosynthesis) and allocation of carbon within the plant, making fewer carbohydrates available for plant growth, reproduction, and/or yield (2013 ISA, p. 1-10; 1996 AQCD, pp. 5-28 and 5-29). As described in the 2013 ISA, the strongest evidence for effects from O₃ exposure on vegetation is from controlled exposure studies, which “have clearly shown that exposure to O₃ is causally linked to visible foliar injury, decreased photosynthesis, changes in reproduction, and decreased growth” in many species of vegetation (2013 ISA, p. 1-15). Such effects at the plant scale can also be linked to an array of effects at larger spatial scales (and higher levels of biological organization), with the evidence available in the last review indicating that “O₃ exposures can affect ecosystem productivity, crop yield, water cycling, and ecosystem community composition” (2013 ISA, p. 1-15, Chapter 9, section 9.4). Beyond its effects on plants, the 2013 ISA also recognized O₃ in the troposphere as a major greenhouse gas (ranking behind carbon dioxide and methane in importance), with associated radiative forcing and effects on climate, and recognized the accompanying “large uncertainties in the magnitude of the radiative forcing estimate . . . making the impact of tropospheric O₃ on climate more uncertain than the effect of the longer-lived greenhouse gases” (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).

The evidence newly available in this review supports, sharpens and expands somewhat on the conclusions reached in the last review (ISA, Appendices 8 and 9). Consistent with the evidence in the last review, the currently available evidence describes an array of O₃ effects on vegetation and related ecosystem effects, as well as the role of O₃ in radiative forcing and subsequent climate-related effects. The ISA concludes there to be causal relationships between O₃ and visible foliar injury, reduced vegetation growth and reduced plant reproduction, as well as reduced yield and quality of agricultural crops, reduced productivity in terrestrial ecosystems, alteration of terrestrial community composition, and alteration of belowground biogeochemical cycles (ISA, section IS.5). The current ISA also concludes there likely to be a causal relationship between O₃ and alteration of ecosystem water cycling, reduced carbon sequestration in terrestrial ecosystems, and with increased tree mortality (ISA, section IS.5). Additionally, evidence newly available in this review augments more limited previously available evidence related to insect interactions with vegetation, contributing to the ISA conclusion that the evidence is sufficient to infer that there are likely to be causal relationships between O₃ exposure and alteration of plant-insect signaling (ISA, Appendix 8, section 8.7) and of insect herbivore growth and reproduction (ISA, Appendix 8, section 8.6). Thus, conclusions reached in the last review continue to be supported by the current evidence base and conclusions are also reached in a few new areas based on the now expanded evidence.

As in the last review, the strongest evidence and the associated findings of causal or likely causal relationships with O₃ in ambient air, and the quantitative characterizations of relationships between O₃ exposure and occurrence and magnitude of effects are for vegetation effects. Visible foliar injury has long been used as a bioindicator of O₃ exposure, although it is not always a reliable indicator of other negative effects on vegetation (ISA, sections IS.5.1.2 and 8.2). Effects of O₃ on physiology of individual plants at the cellular level, such as through photosynthesis and carbon allocation, can impact plant growth and reproduction (ISA, section IS.5.1.2). The scales of these effects range from the individual plant scale to the ecosystem scale, with potential for impacts on the public welfare (as discussed in section III.A.2.b below). The effects of O₃ on plants and plant populations have implications for ecosystems. Effects at the ecosystem scale include reduced terrestrial productivity and carbon storage, and altered terrestrial community composition, as well as impacts on ecosystem functions, such as belowground biogeochemical cycles and ecosystem water cycling (ISA, Appendix 8, sections 8.11 and 8.9).

Ozone welfare effects also extend beyond effects on vegetation and associated biota due to it being a major greenhouse gas and radiative forcing agent. The current evidence, augmented since the 2013 ISA, continues to support a causal relationship between the global abundance of O₃ in the troposphere and radiative forcing, and a likely causal relationship between the global abundance of O₃ in the troposphere and effects on temperature, precipitation, and related climate variables (ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013). Uncertainty in the magnitude of radiative forcing estimated to be attributed to tropospheric O₃ contributes to the relatively greater uncertainty associated with climate effects of tropospheric O₃ compared to such effects of the well mixed greenhouse gases, such as carbon dioxide and methane (ISA, section IS.6.2.2).

Lastly, the evidence regarding tropospheric O₃ and UV-B shielding (shielding of ultraviolet radiation at wavelengths of 280 to 320 nanometers) was evaluated in the 2013 ISA and determined to be inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The current ISA concludes there to be no new evidence since the 2013 ISA relevant to the question of UV-B shielding by tropospheric O₃ (ISA, IS.1.2.1 and Appendix 9, section 9.1.3.4).

b. Public Welfare Implications

The secondary standard is to “specify a level of air quality the attainment and maintenance of which in the judgment of the Administrator . . . is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant in the ambient air” (CAA, section 109(b)(2)). As recognized in prior reviews of secondary standards, the secondary standard is not meant to protect against all known or anticipated O₃-related welfare effects, but rather those that are judged to be adverse to the public welfare, and a bright line determination of adversity is not required in judging what is requisite (78 FR 3212, January 15, 2013; 80 FR 65376, October 26, 2015; see also 73 FR 16496, March 27, 2008). The significance of each type of welfare effect with regard to potential effects on the public welfare depends on the type and severity of effects, as well as the extent of such effects on the affected environmental entity, and on the societal use of the affected entity and the entity's significance to the public welfare. Such factors have been considered in the context of judgments and conclusions made in some prior reviews regarding public welfare effects. For example, judgments regarding public welfare significance in the last two O₃ NAAQS decisions gave particular attention to O₃ effects in areas with special federal protections (such as Class I areas), and lands set aside by states, tribes and public interest groups to provide similar benefits to the public welfare (73 FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015). In the 2015 review, the EPA recognized the “clear public interest in and value of maintaining these areas in a condition that does not impair their intended use and the fact that many of these lands contain O₃-sensitive species” (73 FR 16496, March 27, 2008).

Judgments regarding effects on the public welfare can depend on the intended use for, or service (and value) of, the affected vegetation, ecological receptors, ecosystems and resources and the importance of that use to the public welfare (73 FR 16496, March 27, 2008; 80 FR 65377, October 26, 2015). Uses or services provided by areas that have been afforded special protection can flow in part or entirely from the vegetation that grows there. Ecosystem services range from those directly related to the natural functioning of the ecosystem to ecosystem uses for human recreation or profit, such as through the production of lumber or fuel (Costanza et al., 2017; ISA, section IS.5.1). Services of aesthetic value and outdoor recreation depend, at least in part, on the perceived scenic beauty of the environment. Additionally, public surveys have indicated that Americans rank as very important the existence of resources, the option or availability of the resource and the ability to bequest or pass it on to future generations (Cordell et al., 2008).

The different types of O₃ effects on vegetation recognized in section III.A.2.a above differ with regard to aspects important to judging their public welfare significance. For example, in the case of effects on crop yield, such judgments may consider aspects such as the heavy management of agriculture in the U.S., while judgments for other categories of effects may generally relate to considerations regarding natural areas, including specifically those areas that are not managed for harvest. In this context, it may be important to consider that O₃ effects on tree growth and reproduction could, depending on severity, extent and other factors, lead to effects on a larger scale including reduced productivity, altered forest and forest community (plant, insect and microbe) composition, reduced carbon storage and altered ecosystem water cycling (ISA, section IS.5.1.8.1; 2013 ISA, Figure 9-1, sections 9.4.1.1 and 9.4.1.2). For example, the composition of vegetation or of terrestrial community composition can be affected through O₃ effects on growth and reproductive success of sensitive species in the community, with the extent of compositional changes dependent on factors such as competitive interactions (ISA, section IS.5.1.8.1; 2013 ISA, sections 9.4.3 and 9.4.3.1). Impacts on some of these characteristics (e.g., forest or forest community composition) may be considered of greater public welfare significance when occurring in Class I or other protected areas, due to value for particular services that the public places on such areas.

Agriculture and silviculture provide ecosystem services with clear public welfare benefits. With regard to agriculture-related effects of O₃, however, there are complexities in this consideration related to areas and plant species that are heavily managed to obtain a particular output (such as commodity crops or commercial timber production). In light of this, the degree to which O₃ impacts on agriculturally important vegetation would impair the intended use at a level that might be judged adverse to the public welfare has been less clear (80 FR 65379, October 26, 2015; 73 FR 16497, March 27, 2008). While having sufficient crop yields is of high public welfare value, important commodity crops are typically heavily managed to produce optimum yields. Moreover, based on the economic theory of supply and demand, increases in crop yields would be expected to result in lower prices for affected crops and their associated goods, which would primarily benefit consumers. Analyses in past reviews have described how these competing impacts on producers and consumers complicate consideration of these effects in terms of potential adversity to the public welfare (2014 WREA, sections 5.3.2 and 5.7).

Other ecosystem services valued by people that can be affected by reduced tree growth, productivity and associated forest effects include aesthetic value; provision of food, fiber, timber, other forest products, habitat, and recreational opportunities; climate and water regulation; erosion control; air pollution removal, and desired fire regimes (PA, Figure 4-2; ISA, section IS.5.1; 2013 ISA, sections 9.4.1.1 and 9.4.1.2). In considering such services in past reviews, the Agency has given particular attention to effects in natural ecosystems, indicating that a protective standard, based on consideration of effects in natural ecosystems in areas afforded special protection, would also “provide a level of protection for other vegetation that is used by the public and potentially affected by O₃ including timber, produce grown for consumption and horticultural plants used for landscaping” (80 FR 65403, October 26, 2015). For example, locations potentially vulnerable to O₃-related impacts might include forested lands, both public and private, where trees are grown for timber production. Forests in urbanized areas also provide a number of services that are important to the public in those areas, such as air pollution removal, cooling, and beautification. There are also many other tree species, such as various ornamental and agricultural species (e.g., Christmas trees, fruit and nut trees), that provide ecosystem services that may be judged important to the public welfare.

With its effect on the physical appearance of plants, visible foliar injury has the potential to be significant to the public welfare, depending on its severity and spatial extent, by impacting aesthetic or scenic values and outdoor recreation in Class I and other similarly protected areas valued by the public. To assess evidence of injury to plants in forested areas on national and regional scales, the U.S. Forest Service (USFS) conducted surveys of the occurrence and severity of visible foliar injury on sensitive (bioindicator) species at biomonitoring sites across most of the U.S., beginning in 1994 (in eastern U.S.) and extending through 2011 (Smith et al., 2003; Coulston et al., 2003). At these sites (biosites), a national protocol, including verification and quality assurance procedures and a scoring system, was implemented. The resultant biosite index (BI) scores may be described with regard to one of several categories ranging from little or no foliar injury to severe injury (e.g., Smith et al., 2003; Campbell et al., 2007; Smith et al., 2007; Smith, 2012). However, the available information does not yet address or describe the relationships expected to exist between some level of injury severity (e.g., little, low/light, moderate or severe) and/or spatial extent affected and scenic or aesthetic values. This gap impedes consideration of the public welfare implications of different injury severities, and accordingly judgments on the potential for public welfare significance. That notwithstanding, while minor spotting on a few leaves of a plant may easily be concluded to be of little public welfare significance, some level of severity and widespread occurrence of visible foliar injury, particularly if occurring in specially protected areas, where the public can be expected to place value (e.g., for recreational uses), might reasonably be concluded to impact the public welfare.

The tropospheric O₃-related effects of radiative forcing and subsequent effects on temperature, precipitation and related climate also have important public welfare implications, although their quantitative evaluation in response to O₃ concentrations in the U.S. is complicated by “[c]urrent limitations in climate modeling tools, variation across models, and the need for more comprehensive observational data on these effects” (ISA, section IS.6.2.2). An ecosystem service provided by forested lands is carbon sequestration or storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA, section 2.6.2.1 and p. 9-37) , which has an extremely valuable role in counteracting the impact of greenhouse gases on radiative forcing and related climate effects on the public welfare. Accordingly, the service of carbon storage can be of paramount importance to the public welfare no matter in what location the trees are growing or what their intended current or future use (e.g., 2013 ISA, section 9.4.1.2). This benefit exists as long as the trees are growing, regardless of what additional functions and services it provides.

Categories of effects newly identified as likely causally related to O₃ in ambient air, such as alteration of plant-insect signaling and insect herbivore growth and reproduction, also have potential public welfare implications (e.g., given the role of the plant-insect signaling process in pollination and seed dispersal). Uncertainties and limitations in the current evidence (e.g., summarized in sections III.B.3.c and III.D.1 of the proposal) preclude an assessment of the extent and magnitude of O₃ effects on these endpoints, which thus also precludes an evaluation of the potential for associated public welfare implications.

In summary, several considerations are recognized as important to judgments on the public welfare significance of the array of welfare effects of different O₃ exposure conditions. These include uncertainties and limitations associated with the magnitude of key welfare effects that might be concluded to be adverse to ecosystems and associated services. Additionally, the presence of O₃-sensitive tree species may contribute to a vulnerability of numerous locations to public welfare impacts from O₃ related to tree growth, productivity and carbon storage and their associated ecosystems and services. Other important considerations include the exposure circumstances that may elicit effects and the potential for the significance of the effects to vary in specific situations due to differences in sensitivity of the exposed species, the severity and associated significance of the observed or predicted O₃-induced effect, the role that the species plays in the ecosystem, the intended use of the affected species and its associated ecosystem and services, the presence of other co-occurring predisposing or mitigating factors, and associated uncertainties and limitations.

c. Exposures Associated With Effects

The welfare effects identified in section III.A.2.a above vary widely with regard to the extent and level of detail of the available information that describes the O₃ exposure circumstances that may elicit the effects. The information on exposure metric and E-R relationships for effects related to vegetation growth is long-standing, having been first described in the 1997 review, while such information is much less established for visible foliar injury. The evidence base for other categories of effects is also lacking in information that might support characterization of potential impacts of changes in O₃ concentrations.

(i) Growth-Related Effects

The long-standing body of vegetation effects evidence includes a wealth of information on aspects of O₃ exposure that influence its effects on plant growth and yield, and that has been described in the scientific assessments across the last several decades (1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of factors have been investigated, and a number of mathematical approaches have been developed for summarizing O₃ exposure for the purpose of assessing effects on vegetation, including several that cumulate exposures over some specified period while weighting higher more than lower concentrations (2013 ISA, sections 9.5.2 and 9.5.3; ISA, Appendix 8, section 8.2.2.2). Over this period, the EPA's scientific assessments have focused on the use of a cumulative, seasonal concentration-weighted index when considering the growth-related effects evidence and when analyzing exposures for purposes of reaching conclusions on the secondary standard. Such metrics have included SUM06, in the past, and more recently (since the 2008 review), the focus has been on the W126-based, seasonal metric, termed the “W126 index” (ISA, section IS.3.2, Appendix 8, sections 8.1 and 8.13).

Quantifying exposure using cumulative, concentration-weighted indices of exposure, such as the W126 index, has been found to improve the explanatory power of E-R models for growth and yield over using indices based only on mean and peak exposure values (ISA, section IS.5.1.9, p. IS-79; 2013 ISA, section 2.6.6.1, p. 2-44). The most well-analyzed datasets in such evaluations are two detailed datasets established two decades ago, one for seedlings of 11 tree species and one for 10 crops (e.g., Lee and Hogsett, 1996, Hogsett et al., 1997). These datasets, which include species-specific seedling growth and crop yield response information across multiple seasonal cumulative exposures, were used to develop robust quantitative E-R functions to predict growth reduction relative to a zero-O₃ setting (RBL) in seedlings of the tree species and similarly, E-R functions for predicting RYL for a set of 10 common crops (ISA, Appendix 8, section 8.13.2; 2013 ISA, section 9.6.2).

The tree seedling E-R functions were derived from data for multiple studies documenting effects on tree seedling growth under a variety of O₃ exposures and growing conditions. Importantly the data included hourly concentrations recorded across the duration of the exposure, which allowed for derivation of various metrics that were analyzed for association with reduced growth (2013 ISA, section 9.6.2; Lee and Hogsett, 1996). In producing E-R functions of consistent duration across the experiments, the E-R functions were derived first based on the exposure duration of the experiment and then normalized to 3-month (seasonal) periods (see Lee and Hogsett, 1996, section I.3; PA, Appendix 4A). The species-specific composite E-R functions developed from the experiment-specific functions indicate the wide variation in growth sensitivity of the studied tree species at the seedling stage (PA, Appendix 4A, section 4A.1.1).

Since the initial set of tree seedling growth studies were completed, several additional studies, focused on aspen, have been published based on the Aspen FACE experiment in a planted forest in Wisconsin; the findings were consistent with earlier open top chamber (OTC) studies (ISA, Appendix 8, section 8.13.2). Newly available studies that investigated growth effects of O₃ exposures are also consistent with the existing evidence base, and generally involve particular aspects of the effect rather than expanding the conditions under which plant species, particularly tree species, have been assessed (ISA, section IS.5.1.2). These publications include a compilation of previously available studies on plant biomass response to O₃; the compilation reports linear regressions conducted on the associated varying datasets. Based on these regressions, this study describes distributions of sensitivity to O₃ effects on biomass across many tree and grassland species, including 17 species native to the U.S. and 65 introduced species (ISA, Appendix 8, section 8.13.2; van Goethem et al., 2013). Additional information is needed to more completely describe O₃ exposure response relationships for these species in the U.S.

(ii) Visible Foliar Injury

Current evidence “continues to show a consistent association between visible injury and ozone exposure,” while also recognizing the role of modifying factors such as soil moisture and time of day (ISA, section IS.5.1.1). The ISA summarizes several recently available studies that continue to document that O₃ elicits visible foliar injury in many plant species. As in the prior review, the evidence in the current review, while documenting that elevated O₃ conditions in ambient air generally results in visible foliar injury in sensitive species (when in a predisposing environment), does not include a quantitative description of the relationship of incidence or severity of visible foliar injury in natural areas of the U.S. with specific metrics of seasonal O₃ exposure.

Although studies of the incidence of visible foliar injury in national forests, wildlife refuges, and similar areas have often used cumulative indices (e.g., SUM06) to investigate variations in incidence of foliar injury, studies also suggest an additional role for metrics focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et al., 1996; Smith, 2012). Other studies have indicated this uncertainty regarding the influential metric(s), e.g., by recognizing the need for research to help develop a “better linkage between air levels and visible injury” (Campbell et al., 2007). Some studies of visible foliar injury incidence data have investigated such a role for peak concentrations quantified by an O₃ exposure index that is a count of hourly concentrations (e.g., in a growing season) above a threshold concentration of 100 ppb, N100 (e.g., Smith, 2012; Smith et al., 2012). For example, a study describing injury patterns over 16 years at USFS biosites in 24 states in the Northeast and North Central regions, in the context of the SUM06 index and N100 metrics, suggested that there may be a threshold exposure needed for injury to occur, and that the number of hours of elevated O₃ concentrations during the growing season (such as what is captured by a metric like N100) may be more important than cumulative exposure in determining the occurrence of foliar injury (Smith, 2012). This finding is consistent with statistical analyses of seven years of visible foliar injury data from a wildlife refuge in the mid-Atlantic area (Davis and Orendovici, 2006).

The established significant role of higher or peak O₃ concentrations, as well as pattern of their occurrence, in plant responses has also been noted in prior ISAs or AQCDs. The evidence has included studies that use indices to summarize the incidence of injury on bioindicator species present at specific monitored sites, as well as experimental studies that assess varying O₃ treatments on cultured stands of different tree species (2013 ISA, section 9.5.3.1; 2006 AQCD, p. AX9-169; Oksanen and Holopainen, 2001; Yun and Laurence, 1999). In identifying support for such O₃ metrics with regard to foliar injury as the response, the 2013 ISA and 2006 AQCD both cite studies that support the “important role that peak concentrations, as well as the pattern of occurrence, plays in plant response to O₃” (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-169).

A recent study (by Wang et al. [2012]) involved a statistical modeling analysis on a subset of the years of USFS BI data that were described in Smith (2012). This analysis tested a number of models for their ability to predict the presence of visible foliar injury (a nonzero biosite score), regardless of severity, and generally found that the type of O₃ exposure metric (e.g., SUM06 versus N100) made only a small difference, although the models that included both a cumulative index (SUM06) and N100 had a just slightly better fit (Wang et al., 2012). Based on their investigation of 15 different models, using differing combinations of several types of potential predictors, the study authors concluded that they were not able to identify environmental conditions under which they “could reliably expect plants to be damaged” (Wang et al., 2012). This is indicative of the current state of knowledge, in which there remains a lack of established quantitative functions describing E-R relationships that would allow prediction of visible foliar injury severity and incidence under varying air quality and other environmental conditions.

The information related to O₃ exposures associated with visible foliar injury of varying severity available in this review also includes quantitative presentations of the dataset (developed by the EPA in the last review) of USFS BI scores, collected during the years 2006 through 2010 at locations in 37 states. In developing this dataset, the BI scores were combined with estimates of soil moisture and estimates of seasonal cumulative O₃ exposure in terms of W126 index (PA, Appendix 4C). This dataset includes more than 5,000 records of which more than 80 percent have a BI score of zero (indicating a lack of visible foliar injury). While the estimated W126 index assigned to records in this dataset ranges from zero to somewhat above 50 ppm-hrs, more than a third of all the records (and also of records with BI scores above zero or five) are at sites with W126 index estimates below 7 ppm-hrs. In an extension of analyses developed in the last review, the presentation in the PA describes the BI scores for the records in this dataset in relation to the W126 index estimate for each record, using “bins” of increasing W126 index values. The PA presentation utilizes the BI score breakpoints in the scheme used by the USFS to categorize severity. This presentation indicates that, across the W126 bins, there is variation in both the incidence of particular magnitude BI scores and in the average score per bin. In general, however, the greatest incidence of records with BI scores above zero, five, or higher—and the highest average BI score—occurs with the highest W126 bin, i.e., the bin for W126 index estimates greater than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).

Overall, the dataset described in the PA generally indicates the risk of injury, and particularly injury considered at least light, moderate or severe, to be higher at the highest W126 index values, with appreciable variability in the data for the lower bins (PA, Appendix 4C). This appears to be consistent with the conclusions of the detailed quantitative analysis studies, summarized above, that the pattern is stronger at higher O₃ concentrations. A number of factors may contribute to the observed variability in BI scores and lack of a clear pattern with W126 index bin; among other factors, these may include uncertainties in assignment of W126 estimates and soil moisture categories to biosite locations, variability in biological response among the sensitive species monitored, and the potential role of other aspects of O₃ air quality not captured by the W126 index. Thus, the dataset has limitations affecting associated conclusions, and uncertainty remains regarding the tools for and the appropriate metric (or metrics) for quantifying O₃ exposures, as well as perhaps for quantifying soil moisture conditions, with regard to their influence on extent and/or severity of injury in sensitive species in natural areas, as quantified via BI scores (Davis and Orendovici, 2006, Smith et al., 2012; Wang et al., 2012).

(iii) Other Effects

With regard to radiative forcing and subsequent climate effects associated with the global tropospheric abundance of O₃, the newly available evidence in this review does not provide more detailed quantitative information regarding response to O₃ concentrations at the national scale. Rather, it is noted that “the heterogeneous distribution of ozone in the troposphere complicates the direct attribution of spatial patterns of temperature change to ozone induced [radiative forcing]” and there are “ozone climate feedbacks that further alter the relationship between ozone [radiative forcing] and temperature (and other climate variables) in complex ways” (ISA, Appendix 9, section 9.3.1, p. 9-19). Further, “precisely quantifying the change in surface temperature (and other climate variables) due to tropospheric ozone changes requires complex climate simulations that include all relevant feedbacks and interactions” (ISA, section 9.3.3, p. 9-22). Yet, there are limitations in current climate modeling capabilities for O₃; an important one is representation of important urban- or regional-scale physical and chemical processes, such as O₃ enhancement in high-temperature urban situations or O₃ chemistry in city centers where NO_X is abundant. Such limitations impede our ability to quantify the impact of incremental changes in O₃ concentrations in the U.S. on radiative forcing and subsequent climate effects.

With regard to tree mortality, the evidence available in the last several reviews included field studies of pollution gradients that concluded O₃ damage to be an important contributor to tree mortality although “several confounding factors such as drought, insect outbreak and forest management” were identified as potential contributors (2013 ISA, p. 9-81, section 9.4.7.1). Among the newly available studies, there is only limited experimental evidence that isolates the effect of O₃ on tree mortality and might be informative regarding O₃ concentrations of interest in the review, and evidence is lacking regarding exposure conditions closer to those occurring under the current standard and any contribution to tree mortality.

With regard to alteration of herbivore growth and reproduction, although “[t]here are multiple studies demonstrating ozone effects on fecundity and growth in insects that feed on ozone-exposed vegetation”, “no consistent directionality of response is observed across studies and uncertainties remain in regard to different plant consumption methods across species and the exposure conditions associated with particular severities of effects” (ISA, pp. ES-18). The evidence for alteration of plant-insect signaling draws on new research yielding clear evidence of O₃ modification of volatile plant signaling compounds and behavioral responses of insects to the modified chemical signals (ISA, section IS.6.2.1). The evidence includes a relatively small number of plant species and plant-insect associations and is limited to short controlled exposures, posing limitations for consideration of the potential for associated impacts to be elicited by air quality conditions that meet the current standard (ISA, section IS.6.2.1 and Appendix 8, section 8.7).

For categories of vegetation-related effects that were recognized in past reviews, other than growth and visible foliar injury (e.g., reduced plant reproduction, reduced productivity in terrestrial ecosystems, alteration of terrestrial community composition and alteration of below-ground biogeochemical cycles), the newly available evidence includes a variety of studies that quantify exposure of varying duration in various countries using a variety of metrics (ISA, Appendix 8, sections 8.4, 8.8 and 8.10). The ISA also describes publications that analyze and summarize previously published studies. For example, a meta-analysis of reproduction studies categorized the reported O₃ exposures into bins of differing magnitude, grouping differing concentration metrics and exposure durations together, and performed statistical analyses investigating associations with an O₃-related effect (ISA, Appendix 8, section 8.4.1). While such studies continue to support conclusions of O₃ ecological hazards, they do not improve capabilities for characterizing the likelihood of such effects under patterns of environmental O₃ concentrations occuring with air quality conditions that meet the current standard (e.g., factors such as variation in exposure assessments and limitations in response information preclude detailed analysis for such conditions), as discussed further in the PA.

As at the time of the last review, growth impacts, most specifically as evaluated by RBL for tree seedlings and RYL for crops, remain the type of vegetation-related effects for which we have the best understanding of exposure conditions likely to elicit them. Accordingly, as was the case in the last review, the quantitative analyses of exposures occurring under air quality that meets the current standard, summarized below, are focused primarily on the W126 index, given its established relationship with growth effects.

3. Overview of Air Quality and Exposure Information

The air quality and exposure analyses developed in this review, like those in the last review, are of two types: (1) W126-based cumulative exposure estimates in Class I areas; and (2) analyses of W126-based exposures and their relationship with the current standard for all U.S. monitoring locations (PA, Appendix 4D). We recognize relatively lower uncertainty associated with the use of these types of analyses (compared to the national or regional-scale modeling analyses performed in the last review) to inform a characterization of cumulative O₃ exposure (in terms of the W126 index) associated with air quality just meeting the current standard (IRP, section 5.2.2). As in the last review, the lower uncertainty of these air quality monitoring-based analyses contributes to their value in informing the current review.

The analyses conducted in this review focus on design values (3-year average annual fourth-highest 8-hour daily maximum concentration, also termed the “4th max metric”) and W126 index values (in terms of the 3-year average) for the recent 2016 to 2018 period and across the historical record back to 2000 (PA, Section 4.3). These analyses are based primarily on the hourly air monitoring data that were reported to EPA from O₃ monitoring sites nationwide and in or near Class 1 areas.

a. Influence of Form and Averaging Time of Current Standard on Environmental Exposure

The findings of the quantitative analyses in this review of relationships between air quality in terms of the form and averaging time of the current standard and environmental exposures in terms of the W126 index are similar to those based on the data available during the last review (PA, Appendix 4D, section 4D.2.2). As previously, the current analysis of data spanning 19 years and including seventeen 3-year periods documented a positive nonlinear relationship between cumulative seasonal exposure (quantified using the W126 index) and design values (based on the form and averaging time of the current standard). In the current analysis, which revealed the variability in the annual W126 index values across a 3-year period to be relatively low, the positive nonlinear relationship is shown for both the average W126 index across the 3-year design value period and for W126 index values for individual years within the period (PA, Figure 4-7; Appendix 4D, section 4D.3.1.2). That is, W126 index values (in a single year or averaged across years) are lower at monitoring sites with lower design values. This is seen both for design values above the standard and across lower design values, indicating the effectiveness of the averaging time and form of the current standard at controlling W126-based cumulative exposures.

Further, analysis of the relationship between trends or long-term changes in design value and long-term changes in W126 index shows there to be a positive, linear relationship at monitoring sites across the U.S. (PA, Appendix 4D, section 4D.3.2.3). The existence of this relationship means that a change in the design value at a monitoring site was generally accompanied by a similar change in the W126 index. The relationship varies across the NOAA climate regions, with the greatest change in W126 index per unit change in design value observed in the Southwest and West regions, the regions which had the highest W126 index values at sites meeting the current standard (PA, Appendix 4D, Figures 4D-6 and 4D-14, Table 4D-12). Thus, this analysis indicates that going forward, as design values are reduced in areas that are presently not meeting the current standard, the W126 index in those areas would also be expected to decline and the greatest improvement in W126 index per unit decrease in design value would be expected in the Southwest and West regions (PA, Appendix 4D, section 4D.3.2.3 and 4D.5). The overall trend showing reductions in the W126 index concurrent with reductions in the design value metric for the current standard is positive whether the W126 index is expressed in terms of the average across the 3-year design value period or the annual value (PA, Appendix 4D, section 4D.3.2.3).

The available air quality information also indicates that the current standard's form and averaging time exerts control on other vegetation exposures of potential concern, such as days with particularly high O₃ concentrations that may contribute to visible foliar injury. The current form and averaging time, by their very definition, limit occurrences of such concentrations. This is demonstrated by reductions in daily maximum 8-hour concentrations, as well as in the frequency of elevated 1-hour concentrations, including concentrations at or above 100 ppb, with decreasing design values (PA, Figure 2-11, Appendix 2A, section 2A.2). As the form and averaging time of the secondary standard have not changed since 1997, the analyses have been able to assess the amount of control exerted by these aspects of the standard, in combination with reductions in the standard level (i.e., from 0.08 ppm in 1997 to 0.075 ppm in 2008 to 0.070 ppm in 2015), on cumulative seasonal exposures in terms of W126 index, and on the magnitude of short-term peak concentrations. These analyses indicate that the long-term reductions in the design values, presumably associated with implementation of the revised standards, were accompanied by reductions in W126 index, as well as in short-term peak concentrations.

b. Environmental Exposures in Terms of W126 Index

The analyses summarized here describe the nature and magnitude of vegetation exposures associated with conditions meeting the current standard at sites across the U.S., particularly in specially protected areas, such as Class I areas. Given the evidence indicating the W126 index to be strongly related to growth effects and its use in the E-R functions for tree seedling RBL (as summarized in section III.A.2.c above), exposure is quantified using the W126 metric. These analyses include a particular focus on monitoring sites in or near Class I areas, in light of the greater public welfare significance of many O₃ related impacts in such areas, as described in section III.A.2.b above, and consider both recent air quality (2016-2018) and the air quality record since 2000 (PA, Appendix 4D). As was the case in the last review, the currently available quantitative information continues to indicate appreciable control of seasonal W126 index-based cumulative exposure at all sites with air quality meeting the current standard.

Among sites nationwide meeting the current standard in the recent period of 2016 to 2018, there are none with a W126 index, based on the 3-year average, above 19 ppm-hrs, and just one with such a value above 17 ppm-hrs (Table 4). Additionally, the full historical dataset includes no occurrences of a 3-year average W126 index above 19 ppm-hrs for sites meeting the current standard, and just eight occurrences of a W126 index above 17 ppm-hrs (less than 0.1% of the dataset), with the highest such occurrence just equaling 19 ppm-hrs (Table 4; PA, Appendix 4D, section 4D.3.2.1).

With regard to Class I areas, the updated air quality analyses include data from sites in or near 65 Class I areas. The findings for these sites, which are distributed across all nine NOAA climate regions in the contiguous U.S., as well as Alaska and Hawaii, mirror the findings for the analysis of all U.S. sites in the dataset. Among the Class I area sites meeting the current standard (i.e., having a design value at or below 70 ppb) in the most recent period of 2016 to 2018, there are none with a W126 index (averaged over the design value period) above 17 ppm-hrs (Table 4). The historical dataset includes just seven occurrences (all dating from the 2000-2010 period) of a Class I area site meeting the current standard and having a 3-year average W126 index above 17 ppm-hrs, and no such occurrences above 19 ppm-hrs (Table 4).

The W126 index values at sites that do not meet the current standard are much higher, with values at such sites ranging as high as approximately 60 ppm-hrs (PA, Appendix 4D, Figure 4D-3). Among all sites across the U.S. that do not meet the current standard in the 2016 to 2018 period, more than a quarter have average W126 index values above 19 ppm-hrs and a third exceed 17 ppm-hrs (Table 4). A similar situation exists for Class I area sites (Table 4). For example, out of the 11 Class I area sites with design values above 70 ppb during the most recent period, eight sites had a 3-year average W126 index above 19 ppm-hrs (with a maximum value of 47 ppm-hrs) and for nine, it was above 17 ppm-hrs (Table 4; PA, Appendix 4D, Table 4D-17).

B. Conclusions on the Secondary Standard

In drawing conclusions on the adequacy of the current secondary standard, in view of the advances in scientific knowledge and additional information now available, the Administrator has considered the currently available welfare effects evidence and air quality and ecological exposure information. He additionally has considered the evidence base, information, and policy judgments that were the foundation of the last review, to the extent they remain relevant in light of the currently available information. The Administrator has taken into account both evidence-based and air quality and exposure-based considerations discussed in the PA, as well as advice from the CASAC and public comments. Evidence-based considerations draw upon the EPA's assessment and integrated synthesis of the scientific evidence as presented in the ISA, with a focus on policy-relevant considerations as discussed in the PA (summarized in sections III.B and III.D.1 of the proposal and section III.A.2 above). The air quality and exposure-based considerations draw from the results of the quantitative air quality analyses presented and considered in the PA (as summarized in section III.C of the proposal and section III.A.3 above). The Administrator additionally considered the August 2019 decision of the D.C. Circuit remanding the 2015 secondary standard for further justification or reconsideration.

The consideration of the evidence and air quality/exposure information in the PA informed the Administrator's proposed conclusions and judgments in this review, and his associated proposed decision. Section III.B.1 below briefly summarizes the basis for the Administrator's proposed decision, drawing from section III.D of the proposal. Section III.B.1.a provides a brief overview of key aspects of the policy evaluations presented in the PA, and the advice and recommendations of the CASAC are summarized in III.B.1.b. An overview of the Administrator's proposed conclusions is presented in section III.B.1.c. Public comments on the proposed decision are addressed below in sections III.B.2 and the Administrator's conclusions and decision in this review regarding the adequacy of the current secondary standard and whether any revisions are appropriate are described in section III.B.3.

1. Basis for Proposed Decision

a. Policy-Relevant Evaluations in the PA

The main focus of the policy-relevant considerations in the PA is consideration of the question: Does the currently available scientific evidence- and air quality and environmental exposure-based information support or call into question the adequacy of the protection afforded by the current secondary O₃ standard? The PA response to this overarching question takes into account discussions that address the specific policy-relevant questions for this review, focusing first on consideration of the evidence, as evaluated in the ISA, including that newly available in this review. The PA also considers the quantitative information available in this review that relates O₃ environmental exposures to vegetation responses (presented in Appendices 4A and 4C of the PA) and the air quality analyses that investigate relationships between air quality that meets the current standard and cumulative and peak exposures (presented in detail in Appendix 4D of the PA). The PA additionally discusses the key aspects of the evidence and exposure/risk estimates that were emphasized in establishing the current standard, and key aspects of the 2019 court remand on the standard. In so doing, the PA also considers associated public welfare policy judgments and judgments about the uncertainties inherent in the scientific evidence and quantitative analyses that are integral to the Administrator's consideration of whether the currently available information supports or calls into question the adequacy of the current secondary O₃ standard (PA, section 3.5).

Key policy-relevant considerations identified by the PA included the following. The new information available is consistent with that available in the last review for the principal effects for which the evidence is strongest (e.g., growth, reproduction, and related larger-scale effects, as well as, visible foliar injury) and for key aspects of the decision in that review. The currently available information does not provide established quantitative relationships and tools for estimating incidence and severity of visible foliar injury in protected areas across the U.S. or provide information linking extent and severity of injury to aesthetic values that might be useful for considering public welfare implications. Further, the currently available evidence for forested locations across the U.S., such as studies of USFS biosites, does not indicate widespread incidence of significant visible foliar injury. Additionally, the evidence regarding RBL and air quality in areas meeting the current standard does not appear to call into question the adequacy of protection. For other vegetation-related effects that the ISA newly concludes likely to be causally related to O₃, the new information does not provide us an indication of the extent to which such effects might be anticipated to occur in areas that meet the current standard of a significance reasonably judged significant to public welfare. Thus, the PA does not find the current information for these newly identified categories to call into question the adequacy of the current standard. Similarly, the current information regarding O₃ contribution to radiative forcing or effects on temperature, precipitation and related climate variables is not strengthened from that available in the last review, including with regard to uncertainties that limit quantitative evaluations. Based on such considerations, discussed in detail in the PA, it concludes that the currently available evidence and quantitative exposure/risk information does not call into question the adequacy of the current secondary standard such that it is appropriate to consider retaining the current standard without revision. In so doing, it recognized that, as is the case in NAAQS reviews in general, the extent to which the Administrator judges the current secondary O₃ standard to be adequate will depend on a variety of factors, including science policy judgments and public welfare policy judgments.

b. CASAC Advice in This Review

In comments on the draft PA, the CASAC concurred with the PA conclusions, stating that it “finds, in agreement with the EPA, that the available evidence does not reasonably call into question the adequacy of the current secondary ozone standard and concurs that it should be retained” (Cox, 2020a, p. 1). The CASAC additionally stated that it “commends the EPA for the thorough discussion and rationale for the secondary standard” (Cox, 2020a, p. 2). The CASAC also provided comments particular to the consideration of climate and growth-related effects.

With regard to O₃ effects on climate, the CASAC recommended quantitative uncertainty and variability analyses, with associated discussion (Cox, 2020a, p. 2 and Consensus Responses to Charge Questions p. 22). With regard to growth-related effects and consideration of the evidence in quantitative exposure analyses, it stated that the W126 index “appears reasonable and scientifically sound,” “particularly [as] related to growth effects” (Cox, 2020a, Consensus Responses to Charge Questions p. 16). Additionally, with regard to the prior Administrator's expression of greater confidence in judgments related to public welfare impacts based on a seasonal W126 index estimated by a three-year average and accordingly relying on that metric the CASAC expressed the view that this “appears of reasonable thought and scientifically sound” (Cox, 2020a, Consensus Responses to Charge Questions p. 19). Further, the CASAC stated that “RBL appears to be appropriately considered as a surrogate for an array of adverse welfare effects and based on consideration of ecosystem services and potential for impact to the public as well as conceptual relationships between vegetation growth effects and ecosystem scale effects” and that it agrees “that biomass loss, as reported in RBL, is a scientifically-sound surrogate of a variety of adverse effects that could be exerted to public welfare,” concurring that this approach is not called into question by the current evidence which continues to support “the use of tree seedling RBL as a proxy for the broader array of vegetation related effects, most particularly those related to growth that could be impacted by ozone” (Cox, 2020a, Consensus Responses to Charge Questions p. 21). The CASAC additionally concurred that the strategy of a secondary standard that generally limits 3-year average W126 index values somewhat below those associated with a 6% RBL in the median species is “scientifically reasonable” and that, accordingly, a W126 index target value of 17 ppm-hrs for generally restricting cumulative exposures “is still effective in particularly protecting the public welfare in light of vegetation impacts from ozone” (Cox, 2020a, Consensus Responses to Charge Questions p. 21).

With regard to the court's remand of the 2015 secondary standard to the EPA for further justification or reconsideration (“particularly in relation to its decision to focus on a 3-year average for consideration of the cumulative exposure, in terms of W126, identified as providing requisite public welfare protection, and its decision to not identify a specific level of air quality related to visible foliar injury”), while the CASAC stated that it was not clear whether the draft PA had fully addressed this concern (Cox, 2020a, Consensus Responses to Charge Questions p. 21), it described there to be a solid scientific foundation for the current secondary standard and also commented on areas related to the remand. With regard to support in the information available in the current review for the focus on the 3-year average W126 index in assessing different patterns of air quality using median tree seedling RBL, in addition to the comments summarized above, the CASAC concluded, in considering the approach used in the last review, that reliance on the 3-year average and associated judgments in doing so “appears of reasonable thought and scientifically sound” (Cox, 2020a, Consensus Responses to Charge Questions p. 19). Further, while recognizing the existence of established E-R functions that relate cumulative seasonal exposure of varying magnitudes to various incremental reductions in expected tree seedling growth (in terms of RBL) and in expected crop yield, the CASAC letter also noted that while decades of research also recognizes visible foliar injury as an effect of O₃, “uncertainties continue to hamper efforts to quantitatively characterize the relationship of its occurrence and relative severity with ozone exposures” (Cox, 2020a, Consensus Responses to Charge Questions p. 20). In summary, the CASAC stated that the approach described in the draft PA to considering the evidence for welfare effects “is laid out very clearly, thoroughly discussed and documented, and provided a solid scientific underpinning for the EPA conclusion leaving the current secondary standard in place” (Cox, 2020a, Consensus Responses to Charge Questions p. 22).

c. Administrator's Proposed Conclusions

In reaching conclusions on the adequacy and appropriateness of protection provided by the current secondary standard and his proposed decision to retain the standard, the Administrator carefully considered: (1) The assessment of the available welfare effects evidence and conclusions contained in the ISA, with supporting details in the 2013 ISA and past AQCDs; (2) the evaluation of policy-relevant aspects of the evidence and quantitative analyses in the PA; (3) the advice and recommendations from the CASAC; (4) the August 2019 decision of the D.C. Circuit remanding the secondary standard established in the last review to the EPA for further justification or reconsideration; and (5) public comments that had been received up to that point (85 FR 49830, August 14, 2020). In considering the evidence base on welfare effects associated with exposure to photochemical oxidants, including O₃, in ambient air, he noted the newly available evidence, and the extent to which it alters key scientific conclusions from the last review. He additionally considered the quantitative analyses developed in this review, and their associated limitations and uncertainties, with regard to what they indicate regarding the protection provided by the current standard. Key aspects of the evidence and air quality and exposure information emphasized in establishing the current standard were also considered. Further, he considered uncertainties in the evidence and quantitative information as a part of public welfare policy judgments that are essential and integral to his decision on the adequacy of protection provided by the standard. In considering the CASAC advice, he noted the CASAC characterization of the “thorough discussion and rationale for the secondary standard” presented in the PA (Cox, 2020a, p. 2), and also considered the Committee's overall agreement that the currently available evidence does not call into question the adequacy of the current standard and that it should be retained (Cox, 2020a, p. 1).

As an initial matter, the Administrator recognized the continued support in the current evidence for O₃ as the indicator for photochemical oxidants, noting that no newly available evidence has been identified in this review on the importance of photochemical oxidants other than O₃ with regard to abundance in ambient air and potential for welfare effects. For such reasons, described with more specificity in the ISA and PA and summarized in the proposal, he proposed to conclude it to be appropriate to retain O₃ as the indicator for the secondary NAAQS for photochemical oxidants and he focused on the current information for O₃.

With regard to the currently available welfare effects evidence, the Administrator recognized that, consistent with the evidence in the last review, the currently available evidence describes an array of effects on vegetation and related ecosystem effects causally or likely to be causally related to O₃ in ambient air, as well as the causal relationship of tropospheric O₃ in radiative forcing and subsequent likely causally related effects on temperature, precipitation and related climate variables. The evidence for three additional categories of effects was newly determined in this review to be sufficient to infer likely causal relationships with O₃. However, the Administrator did not find the evidence for these effects to be informative to his proposed decision in review of the standard. For example, the Administrator noted the PA did not find the current evidence to indicate air quality under the current standard to cause increased tree mortality, and, accordingly, he found it appropriate to focus on more sensitive effects, such as tree seedling growth, in his review of the standard. With regard to the two insect-related categories of effects with new ISA determinations (alteration of plant-insect signaling and alteration of insect herbivore growth and reproduction), the Administrator noted the associated uncertainties in the evidence that preclude a full understanding of key aspects of the effects and indicate there to be insufficient information to judge the current standard inadequate based on these effects as described in the proposal.

In considering the evidence documenting tropospheric O₃ as a greenhouse gas causally related to radiative forcing, and likely causally related to subsequent effects on variables such as temperature and precipitation, the Administrator took note of the limitations and uncertainties in the evidence base that affect characterization of the extent of any relationships between O₃ concentrations in ambient air in the U.S. and climate-related effects. He found this to preclude quantitative characterization of climate responses to changes in O₃ concentrations in ambient air at regional (versus global) scales. This lack of quantitative tools precluding important analyses and the resulting uncertainty led the Administrator to conclude there to be insufficient information available for these effects in the current review to support judging the existing standard inadequate or to identify an appropriate revision.

With regard to visible foliar injury, the Administrator recognized that, depending on its severity and spatial extent, as well as the location(s) and intended use(s), the impact of visible foliar injury on the physical appearance of plants has the potential to be significant to the public welfare. For example, depending on its extent and severity, its occurrence in specially protected natural areas may affect aesthetic and recreational values, such as the aesthetic value of scenic vistas in protected natural areas (e.g., national parks and wilderness areas). While recognizing there to be a paucity of information that relates incidence or severity of injury on vegetation in public lands to impacts on the public welfare (e.g., related to recreational services), the Administrator noted the USFS BI scoring scheme, and proposed to judge that occurrence of the lower categories of BI scores does not pose concern for the public welfare, but that findings of BI scores categorized as “moderate to severe” injury by the USFS scheme would be an indication of visible foliar injury occurrence that, depending on extend and severity, may raise public welfare concerns.

While recognizing that important uncertainties remain in the understanding of the O₃ exposure conditions that will elicit visible foliar injury of varying severity and extent in natural areas, the Administrator took note of the evidence indicating a general association of injury incidence and severity with cumulative exposure metrics, including the W126 index, and also an influence of peak concentrations, as well as the quantitative analyses in the PA of USFS biosite data and of air quality monitoring data. In the PA analysis of biosite scores, the incidence of nonzero BI scores, and particularly of relatively higher scores, such as those indicative of “moderate to severe” injury in the USFS scheme, appear to markedly increase only with W126 index values above 25 ppm-hrs. The Administrator noted that such a magnitude of W126 index (either as a 3-year average or in a single year) is not seen to occur at monitoring locations in or near Class I areas where the current standard is met (and such a W126 index, in a single year, has occurred only once in 19 years of monitoring data at sites across the U.S.), and that values above 17 or 19 ppm-hrs are rare (PA, Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3; 85 FR 49911, August 14, 2020). The Administrator further took note of the PA consideration of the USFS publications that identify an influence of peak concentrations on BI scores (beyond an influence of cumulative exposure) and the PA observation of the appreciable control of peak concentrations exerted by the form and averaging time of the current standard, as evidenced by the air quality analyses which document reductions in 1-hour daily maximum concentrations with declining design values. Based on these considerations, the Administrator agreed with the PA finding that the current standard provides control of air quality conditions that contribute to increased BI scores and to scores of a magnitude indicative of “moderate to severe” foliar injury. Based on his consideration of PA findings that areas that meet the current standard are unlikely to have BI scores reasonably considered to be impacts of public welfare significance, the Administrator further proposed to conclude that the current standard provides sufficient protection of natural areas, including particularly protected areas such as Class I areas, from O₃ concentrations in the ambient air that might be expected to elicit visible foliar injury of such an incidence and severity as would reasonably be judged adverse to the public welfare.

With regard to the welfare effects of reduced plant growth or yield, the Administrator recognized that the evidence base continues to indicate growth-related effects as sensitive welfare effects, with the potential for ecosystem-scale ramifications. While recognizing associated uncertainties, the Administrator took note of the PA conclusion and CASAC advice that the approach taken in the last review of using estimates of O₃ impacts on tree seedling growth (in terms of RBL) as a surrogate for comparable information on other species and lifestages, as well as a proxy or surrogate for other vegetation-related effects, including larger-scale effects, continues to appear to be a reasonable judgment in the current review (85 FR 49910, August 14, 2020; PA, section 4.5.3). These estimates were medians based on the established E-R functions for 11 tree species. In light of this and the lack of an alternative metric or approach being indicated by the current evidence, the Administrator found it appropriate to adopt this approach in the current review.

The Administrator additionally took note of considerations in the PA regarding aspects of the derivation of the tree seedling E-R functions that he found informative to his consideration of issues discussed in the court's remand of the 2015 secondary standard with respect to use of a 3-year average W126. In this context, the Administrator considered whether aspects of this evidence support making judgments using the E-R functions with W126 index derived as an average across multiple years. He noted that such averaging would have some conceptual similarity to the assumptions underlying the adjustment made to develop seasonal W126 E-R functions from exposures that extended over multiple seasons (or less than a single season). The Administrator also noted uncertainties in regard to estimated RBL at lower cumulative exposure levels, given the more limited data and fewer findings of statistical significance supporting the functions at the relatively lower cumulative exposure levels most commonly associated with the current standard (e.g., at or below 17 ppm-hrs). The Administrator additionally took note of the PA summary of different comparisons that had been performed in the 2013 ISA and the current ISA of RBL estimated via the aspen E-R function using either a cumulative average multi-year W126 index (2013 ISA) or a single-year W126 index (current ISA) with RBL estimates derived directly from aspen growth information in a multi-year O₃ exposure study. In this context, he noted the PA finding that consideration of these two different comparisons illustrate the variability inherent in the magnitude of growth impacts of O₃ and in the quantitative relationship of O₃ exposure and RBL, while also providing general agreement of predictions (based on either metric) with observations. In light of these considerations, the Administrator recognized that such factors as identified in the proposal, including the currently available evidence and its recognized limitations, variability and uncertainties, support a conclusion that it is reasonable to use a seasonal RBL averaged over multiple years, such as a 3-year average (85 FR 49910, August 14, 2020). The Administrator additionally took note of the CASAC advice reaffirming the EPA's focus on a 3-year average W126, concluding such a focus to be reasonable and scientifically sound. In light of these considerations, the Administrator found there to be support for use of an average seasonal W126 index derived from multiple years (with their representation of variability in environmental factors), concluding the use of such averaging to provide an appropriate representation of the evidence and attention to considerations summarized above. In so doing, he found that a reliance on single year W126 estimates for reaching judgments with regard to magnitude of O₃-related RBL and associated judgments of public welfare protection would ascribe a greater specificity and certainty to such estimates than supported by the current evidence. Thus, he proposed to conclude that it is appropriate to use a seasonal W126 averaged over a 3-year period, which is the design value period for the current standard, to estimate median RBL using the established E-R functions for purposes in this review of considering the public welfare protection provided by the standard.

In reaching his proposed conclusions and judgments related to the use of RBL as a surrogate for the broad array of vegetation-related effects, the Administrator recognized a number of important public welfare policy judgments. The Administrator proposed to conclude that the current evidence base and available information (qualitative and quantitative) continue to support consideration of the potential for O₃-related vegetation impacts in terms of the RBL estimates from established E-R functions as a quantitative tool within a larger framework of considerations pertaining to the public welfare significance of O₃ effects. He judged the framework to include consideration of effects that are associated with effects on vegetation, and particularly those that conceptually relate to growth, and that are causally or likely causally related to O₃ in ambient air, yet for which there are greater uncertainties affecting estimates of impacts on public welfare. In his consideration of the adequacy of protection provided by the current standard, the Administrator also noted judgments of the prior Administrator in considering the public welfare significance of small magnitude estimates of RBL and associated unquantified potential for larger-scale related effects. In light of CASAC advice and based on the current evidence as evaluated in the PA, the Administrator proposed to conclude that the approach or framework initially described with the 2015 decision, with its focus on controlling air quality such that cumulative exposures at or above 19 ppm-hrs, in terms of a 3-year average W126 index, are isolated and rare, is appropriate for a secondary standard that provides the requisite public welfare protection and proposed to use such an approach in this review (85 FR 49911, August 14, 2020).

With this approach and protection target in mind, the Administrator considered the analyses of air quality at sites across the U.S., particularly including those sites in or near Class I areas. In virtually all design value periods and all locations at which the current standard was met (i.e., in more than 99.9% of such instances) across the 19 years of the data analyzed, the 3-year average W126 metric was at or below 17 ppm-hrs. Further, in all such design value periods and locations the 3-year average W126 index was at or below 19 ppm-hrs. The Administrator additionally considered the protection provided by the current standard from the occurrence of O₃ exposures within a single year with potentially damaging consequences, such as a significantly increased incidence of areas with visible foliar injury that might be judged moderate to severe. In so doing, he noted the PA findings that incidence of sites with BI scores above 15 (termed “moderate to severe injury” by the USFS categorization scheme) markedly increases with W126 index estimates above 25 ppm-hrs, and the scarcity of single-year W126 index values above 25 ppm-hrs at sites that meet the current standard, with just a single occurrence across all U.S. sites with design values meeting the current standard in the 19-year historical dataset dating back to 2000 (PA, section 4.4 and Appendix 4D). In light of the evidence indicating that peak short-term concentrations (e.g., of durations as short as one hour) may also play a role in the occurrence of visible foliar injury, the Administrator additionally recognized the control of peak 1-hour concentrations provided by the form and averaging time of the current standard and noted there to be less than one day per site with a maximum hourly concentration at or above 100 ppb (PA, Appendix 2A, section 2A.2). In consideration of these findings, the Administrator proposed to judge that the current standard provides adequate protection from air quality conditions with the potential to be adverse to the public welfare (85 FR 49912, August 14, 2020).

In reaching his proposed decision, the Administrator gave primary attention to the principal effects of O₃ as recognized in the current ISA, the 2013 ISA and past AQCDs, and for which the evidence is strongest (e.g., growth, reproduction, and related larger-scale effects, as well as visible foliar injury). With respect to the currently available information related to O₃-related visible foliar injury, the Administrator considered air quality analyses that may be informative with regard to air quality conditions associated with appreciably increased incidence and severity of BI scores at USFS biomonitoring sites, noting that this information does not indicate a potential for public welfare impacts of concern under air quality conditions that meet the current standard. In light of these and other considerations discussed more completely in the proposal, and with particular attention to Class I and other areas afforded special protection, the Administrator proposed to conclude that the evidence regarding visible foliar injury and air quality in areas meeting the current standard indicates that the current standard provides adequate protection for this effect.

The Administrator additionally considered O₃ effects on crop yield, taking note of the long-standing evidence, qualitative and quantitative, of the reducing effect of O₃ on the yield of many crops, as summarized in the PA and current ISA and characterized in detail in past reviews (e.g., 2013 ISA, 2006 AQCD, 1997 AQCD, 2014 WREA). In so doing, he recognized that not every effect on crop yield will be adverse to public welfare and in the case of crop yield effects in particular there are a number of complexities related to the heavy management of many crops to obtain a particular output for commercial purposes, and to other factors, that contribute uncertainty to predictions of potential O₃-related public welfare impacts, as summarized in sections III.B.2 and III.D.1 of the proposal (PA, sections 4.5.1.3 and 4.5.3). Thus, in judging the extent to which the median RYL estimated for the W126 index values generally occurring in areas meeting the current standard would be expected to be of public welfare significance, he recognized the potential for a much larger influence of extensive management of such crops, and also considered other factors recognized in the PA and proposal, including similarities in median estimates of RYL and RBL (PA, sections 4.5.1.3 and 4.5.3). With this context, the information for crop yield effects did not lead the Administrator to identify this endpoint as requiring separate consideration or to provide a more appropriate focus for the standard than RBL, in its role as a proxy or surrogate for the broader array of vegetation-related effects, as discussed above. Rather, in light of these considerations, he proposed to judge that a decision based on RBL as a proxy for other vegetation-related effects will provide adequate protection against crop related effects. In light of the current information and considerations discussed more completely in the proposal, the Administrator further proposed to conclude that the evidence regarding RBL, and its use as a proxy or surrogate for the broader array of vegetation-related effects, in combination with air quality in areas meeting the current standard, provide adequate protection for these effects (85 FR 49912, August 14, 2020).

In reaching his proposed conclusion on the current standard, the Administrator also considered the extent to which the current information may provide support for an alternative standard, proposing to conclude that the appreciably greater occurrence of higher levels of cumulative exposure, in terms of the W126 index, as well as an appreciably greater occurrence of peak concentrations (both hourly and 8-hour average concentrations) in areas that do not meet the current standard (e.g., areas meeting a higher standard level), would not provide the appropriate protection of public welfare in light of the potential for adverse effects on the public welfare. The Administrator also considered an alternative based solely on the W126 metric, as was considered in the last review, based on such a concentration-weighted, cumulative exposure metric having been identified as quantifying exposure in a way that relates to reduced plant growth (ISA, Appendix 8, section 8.13.1). While recognizing a role for W126 index in quantifying exposure to develop estimates of RBL that the Administrator considers appropriately used as a proxy or surrogate for the broader array of vegetation-related effects, he notes that the evidence indicates there to be aspects of O₃ air quality not captured by measures of cumulative exposure like W126 index that may pose a risk of harm to the public welfare (e.g., risk of visible foliar injury related to peak concentrations). Thus, in light of the information available in this review, the Administrator proposed to conclude that such an alternative standard in terms of a W126 index would be less likely to provide sufficient protection against such occurrences and accordingly would not provide the requisite control of aspects of air quality that pose risk to the public welfare.

In summary, the Administrator recognized that his proposed decision on the public welfare protection afforded by the current secondary O₃ standard from identified O₃-related welfare effects, and from their potential to present adverse effects to the public welfare, is based in part on judgments regarding uncertainties and limitations in the available information, such as those identified above. In this context, he considered what the available evidence and quantitative information indicated with regard to the protection provided from the array of O₃ welfare effects, finding it to not indicate the current standard to allow air quality conditions with implications of concern for the public welfare. He additionally took note of the advice from the CASAC in this review. Based on all of the above considerations, described in more detail in the proposal, including his consideration of the currently available evidence and quantitative exposure/risk information, the Administrator proposed to conclude that the current secondary standard provides the requisite protection against known or anticipated effects to the public welfare, and thus that the current standard should be retained, without revision.

3. Comments on the Proposed Decision

Over 50,000 individuals and organizations indicated their views in public comments on the proposed decision. Most of these are associated with mass mail campaigns or petitions. Approximately 40 separate submissions were also received from individuals, and 75 from organizations and groups of organizations; 40 elected officials also submitted comments. Among the organizations commenting were state and local agencies and organizations of state agencies, organizations of health professionals and scientists, environmental and health protection advocacy organizations, industry organizations and regulatory policy-focused organizations. The comments on the proposed decision to retain the current secondary standard are addressed here. Those in support of the proposed decision are addressed in section III.B.2.a and those in disagreement are addressed in section III.B.2.b. Comments related to aspects of the process followed in this review of the O₃ NAAQS (described in section I.D above), as well as comments related to other legal, procedural or administrative issues, and those related to issues not germane to this review are addressed in the separate Response to Comments document.

a. Comments in Support of Proposed Decision

Of the comments supporting the Administrator's proposed decision to retain the current secondary standard without revision, all generally state that the record supports the proposed decision, and note the CASAC conclusion that the current evidence is generally consistent with that available in the last review, and the CASAC conclusion that the evidence does not call into question the adequacy of the current standard and should be retained. In support of their views, some of these commenters state that new evidence is lacking that might call into question the objective for the standard to generally protect against cumulative exposures associated with median RBL estimates above 6%. They additionally state that the proposed decision appropriately addresses the Murray Energy remand issues. Further, these commenters conclude that the available evidence with regard to areas meeting the current standard does not call into question the adequacy of protection provided by the current standard from the array of vegetation effects, including in Class I areas. Lastly, these commenters find the EPA's proposed judgments regarding the uncertainties associated with predicting responses of climate-related effects to changes in O₃ concentrations across the U.S., as well as the limitations in the availability of tools for such purposes, to be appropriate and well supported. The EPA agrees with these comments.

Some of these comments also express the view that welfare benefits of a more restrictive O₃ standard are highly uncertain, while such a standard would likely cause socioeconomic impacts that the EPA should consider and find to outweigh the uncertain benefits. While as discussed in section III.B.3 below, the Administrator does not find a more stringent secondary standard requisite to protect the public welfare, he does not consider economic impacts of alternate standards in reaching this judgment. As summarized in section I.A. above, in setting primary and secondary standards that are “requisite” to protect public health and welfare, respectively, as provided in section 109(b), the EPA may not consider the costs of implementing the standards. See generally Whitman v. American Trucking Ass'ns, 531 U.S. 457, 465-472, 475-76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant considerations in the promulgation of national ambient air quality standards.” See American Petroleum Institute v. Costle, 665 F.2d 1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C. Cir. 2019). Arguments such as the views on socioeconomic impacts expressed by these commenters have been rejected by the courts, as summarized in section I.A above, including in the recent Murray Energy decision, with the reasoning that consideration of such impacts was precluded by Whitman's holding that the “plain text of the Act unambiguously bars cost considerations from the NAAQS-setting process” (Murray Energy Corp. v. EPA, 936 F.3d at 621, quoting Whitman [531 U.S. at 471]).

b. Comments in Disagreement With Proposed Decision

Among those submitting comments that disagreed with the proposed decision to retain the current secondary standard, or that raised concerns with the basis for the decision, most of these commenters expressed concerns regarding the process for reviewing the criteria and standards and state that the proposal must be withdrawn, and a new review conducted. Most of these commenters also disagree with the EPA's proposed conclusion that the current standard, with its current averaging time and form, provides the requisite public welfare protection from known or anticipated adverse public welfare effects associated with the array of O₃-related effects, and generally state that the standard should be revised to be in terms of a single-year W126 index. Among the claims made in describing the basis for their view, these commenters claim that EPA failed to describe the basis for its proposed conclusion; to explain why a standard using the W126 index was not proposed, consistent with 2014 advice from the former CASAC, and to address the issues raised by court remand of the 2015 standard. Some commenters expressing the view that the standard should be revised also express the view that an additional standard should be established to protect from O₃ effects on climate.

With regard to the process by which this review has been conducted, we disagree with the commenters that claim that it is arbitrary and capricious or that it does not comport with legislative requirements. The review process, summarized in section I.D, implemented a number of features, some of which have been employed in past reviews and others which have not, and several which represent efficiencies in consideration of the statutorily required time frame for completion of the review. The comments received that raise concerns regarding specific aspects of the process are addressed in the separate Response to Comments document. As indicated there, the EPA disagrees with these comments. The EPA finds the review to have been lawfully conducted and the process reasonably explained. Accordingly, the EPA is not withdrawing the proposal and restarting the review.

(i) Metric for Standard

The premise of many of the comments expressing disagreement with the proposed decision is that the secondary standard must be a “biologically relevant” metric, which they identify to be the W126 index. Similarly, some commenters assert that EPA cannot lawfully or rationally set a secondary standard using the metric of the current standard, which is also the metric used for the primary standard, claiming that this contradicts EPA's recognition of the relevance of the W126 index as an exposure metric for assessing the level of protection from welfare effects, such as RBL. These commenters also claim that this approach arbitrarily disregards the recommendations of the prior CASAC, and, in doing so, imply that EPA must establish a W126 based standard because of prior CASAC advice.

We disagree with these commenters. The Clean Air Act includes no requirements with respect to what metrics should be used to establish the secondary standards. As is clear from the text of Section 109(b)(2) of the CAA, the critical test for NAAQS is whether they achieve the requisite protection. In so doing, it is not uncommon for the form and averaging time of a NAAQS to differ from exposure metrics most relevant to assessment of particular effects. These exposure metrics are based on the health or welfare effects evidence for the specific pollutant and commonly, in assessments for primary standards, on established exposure-response relationships or health-based benchmarks (doses or exposures of concern) for effects associated with specific exposure circumstances. Evidence for this is found in the common use, in assessments conducted for NAAQS reviews, of exposure metrics that differ in a variety of ways from the ambient air concentration metrics of those standards. Across reviews for the various NAAQS pollutants over the years, the EPA has used a variety of exposure metrics to evaluate the protection afforded by the standards (see examples identified at 80 FR 65399-65400, October 26, 2015). Further, a single standard may provide protection from multiple different effects, the protection for which may be assessed using different exposure metrics. One standard may also provide protection from multiple pathways of exposure. Both the primary and secondary Pb standards provide examples of this. While these standards are expressed in terms of the concentration of lead in particles suspended in air, different exposure metrics have been used to evaluate the protection provided by the Pb standards. The salient exposure metric for assessment of protection provided by the primary standard has been blood Pb, while for the secondary standard, concentrations of lead in soil, surface water and sediment are pertinent, and have been evaluated to assess the potential for welfare effects related to lead deposition from air (73 FR 67009, November 12, 2008). In somewhat similar manner, the exposure metric used to evaluate health impacts in the primary sulfur dioxide standard review includes a 5-minute exposure concentration. In contrast, the health-based standard for this pollutant is the average across three years of the 99th percentile of 1-hour daily maximum concentration of sulfur dioxide in ambient air (75 FR 35520, June 22, 2010; 84 FR 9866, March 18, 2019).

We disagree with the comment that a secondary standard with the same form and averaging time as the primary standard does not comply with the CAA. The CAA does not require that the secondary standard be established in a specific form or averaging time. The Act, at Section 109(b)(2), provides only that any secondary NAAQS “shall specify a level of air quality the attainment and maintenance of which in the judgment of the Administrator, based on [the air quality] criteria, is requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant in the ambient air. . . . [S]econdary standards may be revised in the same manner as promulgated.” The EPA interprets this provision to leave it considerable discretion to determine whether a particular form and averaging time are appropriate, in combination with the other aspects of the standard (level and indicator), for specifying the air quality that provides the requisite protection, and to determine whether, once a standard has been established in a particular form, that form must be revised. Moreover, nothing in the Act or the relevant case law precludes the EPA from establishing a secondary standard equivalent to the primary standard in some or all respects, as long as the Agency has engaged in reasoned decision-making.

Thus, we note that particular metrics may logically, reasonably, and for technically or scientifically sound reasons, be used in assessing exposures of concern or characterizing risk. The purpose, and use, of exposure metrics is different from the purpose, and use, of metrics for the standard, and as a result the metrics may differ from one use to the other. Exposure metrics are used to assess the likely occurrence and/or frequency and extent of effects under different air quality conditions, while the air quality standards are intended to control air quality to the extent requisite to protect from the occurrence of public health or welfare effects judged to be adverse. In this review of the O₃ secondary standard, the EPA agrees that based on evidence summarized in section III.A above, metrics such as the W126 index are appropriate for assessing exposures of concern for vegetation, characterizing risk to public welfare, and evaluating what air quality conditions might provide the appropriate degree of public welfare protection. We disagree, however, that the secondary standard must be established using those same metrics. Rather, when the Administrator judges that a standard using a different metric provides the requisite protection, in light of his consideration of all the elements of the standard together, he may reasonably establish or retain such a standard.

With regard to the commenter's emphasis on recommendations from the CASAC on the form of the secondary standard, the EPA generally agrees with the importance of giving such recommendations careful consideration. However, it is not necessary for EPA to address in this review each statement a prior CASAC made in a prior review. In addition, if a recommendation of a prior CASAC is raised in a subsequent review (e.g., in public comments or as a focus in court decision being addressed), it is reasonable for the Agency to consider it in the context both of the current review and of consideration of all the other now available scientific, technical and policy-relevant information, including advice from the current CASAC. We note that in this review of the secondary standard, the current CASAC, based on its review of the information and analyses available in the current review, concurs with retention of a secondary standard with a metric that differs from commonly used vegetation exposure metrics, such as the W126 index (Cox, 2020a). We further note, under the relevant provisions of the CAA and case law interpreting them, the Administrator is never bound by the CASAC's conclusions but rather may depart from them when he has provided an explanation of the reasons for such differences. While the EPA does not interpret the requirements of CAA sections 307(d)(3) and 307(d)(6)(A) to apply to every recommendation it has received from a prior CASAC, even assuming there are some circumstances in which EPA were required to comply with the requirements of CAA section 307(d)(3) and (6)(A) with respect to particular recommendations from a prior CASAC, these same principles would apply. Thus, the Administrator would not be bound to follow those recommendations, but rather could depart from them when he had explained his reasons for doing so. Accordingly, in reaching conclusions on the revised secondary standard in this review, the Administrator has given careful consideration to the current CASAC advice in this review and to issues raised by the prior CASAC that are subject to the Murray Energy remand. When he has differed from those CASAC recommendations, the reasons and judgments that led to a different conclusion are explained, as summarized in this section and in section III.B.3 below. Consistent with his consideration of all significant issues raised in public comments, the Administrator has also considered the issues raised by commenters that have also been raised by a prior CASAC, together with the Agency's responses to those comments, as summarized in this section and in section III.B.3 below.

The current air quality analyses demonstrate the successfulness of the current form and averaging time in controlling cumulative exposures, in terms of W126. These extensive air quality analyses, presented in the PA and summarized in the proposal, are based on data collected across the U.S. over a time span of nearly 20 years (85 FR 49892-49895, 49903-49904, August 14, 2020). One of these analyses describes the positive, linear relationship between long-term changes in the O₃ design value and long-term changes in the W126 index at monitoring sites across the U.S. This positive, linear relationship exists for the O₃ design value with both a 3-year average and single-year W126 index (PA, Appendix 4D, Figure 4D-11). The existence of this relationship means that a change (e.g., reduction) in the design value at a monitoring site was generally accompanied by a similar change (e.g., reduction) in the W126 index, both in the 3-year average and in the single-year values. As the form and averaging time of the secondary standard have not changed since 1997, the analyses performed have been able to assess the amount of control exerted by these aspects of the standard, in combination with reductions in the level (i.e., from 80 ppb in 1997 to 75 ppb in 2008 to 70 ppb in 2015) on cumulative seasonal exposures in terms of W126 index. The analyses have found that the reductions in design value, presumably associated with implementation of the revised standards, have been accompanied by reductions in cumulative seasonal exposures in terms of W126 index (PA, section 4.4.1). Further, while the formulation of the W126 metric gives more weight to higher concentrations (in the context of its focus on cumulative exposure), it is much less effective at curbing elevated hourly concentrations (that can be important in altering plant growth and yield) than the current design value metric, as discussed in section III.B.2.b(ii) below.

In expressing the view that the secondary standard should be in terms of a W126 index, some commenters describe the EPA's statements regarding the protection from cumulative exposures that is provided by the current form and averaging time to be “incidental” and “happenstance,” which leads them to claim the EPA's findings of protection to be arbitrary. In support of their view, the commenters quote a statement of the prior CASAC cautioning against interpreting the W126 index levels in the W126 index scenario created for the 2014 WREA, by first adjusting air quality to meet the then-existing fourth maximum standard of 75 ppb, to be representative of implementation of a W126 index standard. The issue described by the prior CASAC related to the application to all monitoring sites of the precursor reduction necessary for the highest monitoring site in a region to just meet the scenario target; the prior CASAC's concern was that actual implementation of the target as a standard would not necessarily yield such reductions. We disagree with the commenters that this is relevant to the air quality analysis in the current review, in which we simply observe the W126 index values that exist in reality at sites that have met the existing secondary standard. Contrary to the context for the prior CASAC's caution, the analysis in the current review is not showing the results of a theoretical scenario created by modeling theoretical precursor reductions estimated for attaining a particular W126-based or fourth high standard. Rather, we are observing what the W126-based cumulative exposure is at ambient air monitoring sites that meet the current secondary standard. Thus, regardless of the labels assigned by the commenter to the findings of the air quality analyses in the current review, these analyses clearly document the success of the existing standard (with its fourth maximum form and 8-hour averaging time) in controlling exposure in terms of the W126 index.

Thus, in light of this evidence, the EPA disagrees with the commenters who express the view that to provide the requisite protection the secondary standard must be a W126 index standard. In assessing the air quality necessary to provide the requisite degree of protection, particularly for growth and related vegetation and ecosystem effects, the Agency has recognized the importance of cumulative exposures, but also the significance of higher peak exposures (as summarized in section III.B.2.b(ii) below) that can be characterized through other metrics (e.g., N100). As a result, in assessing the protection provided by the current standard, the Agency has focused on the W126 index, expressed in terms of the average of three consecutive years (in light of considerations discussed below), as a metric for cumulative exposure, but has also considered the frequency and magnitude of elevated single-year W126 index values, and of elevated hourly O₃ concentrations (as discussed further below).

(ii) Protection Against Unusually Damaging Years

In the last review, the Administrator relied on the 70 ppb standard (as the fourth highest daily maximum 8-hour average concentration averaged over three consecutive years) to achieve a level of air quality that would restrict cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-year average W126 value, in nearly all instances. The Murray Energy court found in relevant part that the EPA had not explained why that level of protection was requisite, in light of certain comments from the CASAC in 2014 recommending that EPA base a standard on a one-year W126 metric, in part to limit exposures in single unusually damaging years. In responding to the remand, we are explaining in this document that the EPA is looking to prevent the damaging effects of O₃ on tree growth as a proxy for public welfare effects related to the broad array of O₃'s vegetation-related effects conceptually related to growth effects, including ecosystem-level effects (as discussed in section III.B.2.b(v) below). In this review, in assessing the air quality requisite to prevent adverse effects on public welfare from these effects, the EPA is not relying solely on maintaining a particular 3-year W126 value. Rather, we are considering air quality patterns that are associated with meeting the current standard, including control of peak hourly concentrations, and the exposures that would be expected under the current standard, including in terms of W126 values, particularly those averaged over a 3-year period. The EPA is explaining the grounds for our conclusion that use of the 3-year average W126 index is a reasonable basis for assessing protection from RBL, but also that the Administrator is using other exposure information in reaching the conclusion that retention of the existing standard (with its form and averaging time of the fourth highest annual daily maximum 8-hour average concentration, averaged over three years) provides the needed protection of RBL, including from what the Murray Energy court noted that the prior CASAC termed “unusually damaging years.”

In disagreeing with the EPA's proposed decision, some commenters object to the EPA's use of a 3-year average W126 index in assessing different patterns of air quality using median tree seedling RBL as a surrogate for an array of vegetation-related effects, particularly those related to growth and productivity. In so doing, these commenters variously claim that this use of a 3-year average W126 index (rather than a single-year W126 index) is inconsistent with recommendations from the prior CASAC, does not address the court remand on this point, and that it is inadequate to protect vegetation from high years or years with hourly O₃ concentrations that can be most important in eliciting adverse effects.

The EPA disagrees with these commenters and notes that it has taken such concerns, as well as the court's remand, into account in the final decision. In evaluating the air quality conditions allowed by the current standard, the EPA has focused on the W126 metric averaged over 3 years as the most appropriate measure of cumulative exposure for consideration of adverse effects on public welfare, but EPA has also considered other relevant exposure information, including higher exposures that might be expected to occur in an “unusually damaging year.” The Administrator's decision on the adequacy of protection provided by the current standards is based on the full scope of exposure information he has considered.

The EPA concludes that the 3-year average W126 index is a reasonable metric for assessing the level of protection provided by the current standard from cumulative seasonal exposures related to RBL, while noting that our evaluation for the protection provided by the current standard has also been informed by our consideration of other metrics (as described further below). In reaching this conclusion, we have taken into account the available evidence base and air quality analyses, with a focus on two types of considerations, as well as consideration of the context for RBL as a proxy for an array of other vegetation effects (discussed in section III.B.2.b(v) below). The first of the two consideration types concerns the E-R functions and their use with a 3-year average W126 index, and the second concerns the control by the W126 index metric of exposures that might be termed “unusually damaging.” With regard to the first, we find our use of the 3-year average W126 index appropriate in light of the approach used in deriving the E-R functions from the underlying data (from exposures of varying durations, including of multiple years), and the evidence available for evaluating these functions across multiyear exposures. Additionally, with regard to the second consideration, we recognize limitations associated with a reliance solely on W126 index as a metric to control exposures that might be termed “unusually damaging.” For example, two different air quality patterns for which the associated W126 index is the same may have very different incidence of elevated O₃ concentrations, and accordingly pose different risks to vegetation. As discussed below, however, the occurrence of such concentrations (and any associated risk of damage) are controlled by the current secondary standard.

In light of this evidence, and recognizing the role for both peak and cumulative exposures in eliciting growth and related vegetation and ecosystem effects, the EPA concludes that focusing solely on W126 index (either in terms of a single year or 3-year average) in considering the public welfare protection provided by the current standard would not be considering all the relevant scientific information. To the extent that the prior CASAC advised that the EPA should focus solely on single-year W126 index values in evaluating the protection provided by the secondary standard, the EPA disagrees that this would provide the needed protection, for the reasons explained more fully below. In this regard, we additionally note that the current CASAC concluded that focusing on three-year average W126 index values in considering the public welfare protection offered by the secondary standard “appears of reasonable thought and scientifically sound” (Cox, 2020a, p. 19).

With regard to the established tree seedling E-R functions, we note there are aspects of the datasets and methodology on which the E-R functions are based which provide support for a 3-year average approach. As summarized in section III.A.2.c(i) above, in deriving the E-R functions from studies of durations that varied from shorter than 90 days to multiple years or growing seasons, the results were normalized to the duration of a single 90-day seasonal period (PA, section 4.5.1.2 and Appendix 4A, pp. 4A-28 to 4A-29 and footnote 17). Inherent in this approach is an assumption that the growth impacts relate generally to the cumulative O₃ exposure across the multiple growing seasons, i.e., with little additional influence related to any year to year differences in the exposures. As discussed in the proposal, the use of a 3-year average in assessing RBL using the established tree seedling E-R functions is compatible with the normalization step taken to derive functions for a seasonal 90-day period from the underlying data with its varying exposure durations (85 FR 49901, August 14, 2020).

This concept of the importance of cumulative multiyear O₃ exposure to multiyear impacts, and its representation as an average, is also reflected in the evaluation of the predicted growth impacts compared to observations from the multiyear study of O₃ impacts on aspen by King et al (2005), as presented in the 2013 and 2020 ISAs and summarized in the PA (PA, Section 4.5.1.2). The ISAs considered the 6-year experimental dataset of O₃ exposures and aspen growth effects with regard to correspondence of E-R function predictions with study observations (2020 ISA, Appendix 8, section 8.13.2 and Figure 8-17; 2013 ISA, section 9.6.3.2, Table 9-15, Figure 9-20). The analysis in the 2013 ISA compared observed reductions in growth for each of the six years to those predicted by applying the established E-R function for Aspen to cumulative multi-year average W126 index values (2013 ISA, section 9.6.3.2). The evaluation in the 2020 ISA applied the E-R functions to the single-year W126 index for each year rather than the cumulative multi-year W126 (2020 ISA, Appendix 8, Figure 8-17), with this approach indicating a somewhat less tight fit to the experimental observations (2020 ISA, Appendix 8, p. 8-192), Both ISAs reach similar conclusions regarding general support for the E-R functions across a multiyear study of trees in naturalistic settings (ISA, Appendix 8, section 8.13.3 and p. 8-192; 2013 ISA, p. 9-135).

Based on all of the above considerations, the EPA finds the evidence to support a 3-year average W126 index for use in assessing the level of protection provided by the current standard from cumulative seasonal exposures related to RBL of concern based on the established E-R functions. As discussed in section III.B.3 below, the EPA additionally finds the 3-year average metric to be reasonable in the context of the use of RBL as a proxy to represent an array of vegetation-related effects. In the discussion immediately below, we additionally and specifically address the issue of protection from “unusually damaging years” of vegetation exposure.

With regard to the comment that cited a recommendation from the prior CASAC on protection of vegetation against “unusually damaging years” and the part of the court remand referencing that CASAC recommendation, we have considered the CASAC discussion using this term, in the context of the court remand. Use of this term by the prior CASAC occurs in the 2014 letter on the second draft PA in the 2015 review (Frey, 2014b). Most prominently, the prior CASAC defined as damage “injury effects that reach sufficient magnitude as to reduce or impair the intended use or value of the plant to the public, and thus are adverse to public welfare” (Frey, 2014b, p. 9). The prior CASAC additionally provided advice with regard to surrogate metrics for judging such “damage,” e.g., use of RBL for judging effects on trees and their related functions and ecosystem services, use of crop RYL for judging public welfare effects of crop effects (Frey, 2014b, p. 10). We also note that the context for the prior CASAC's use of the phrase “unusually damaging years” is in considering the form and averaging time for a revised secondary standard in terms of a W126 index (Frey, 2014b, p. 13), which as discussed below is relatively less controlling of high-concentration years, rather than in the context of the current secondary standard and its fourth highest daily maximum 8-hour metric.

While the prior CASAC did not provide any specificity or details as to the exposure circumstances and damage intended by its more general phrasing, nor did it cite to specific evidence in scientific publications, we agree with the general concept that particular air quality patterns in a year may pose particular risk of vegetation damage, in terms of both or either growth-related effects or visible foliar injury (discussed in section III.B.2(iii) below). Across past O₃ NAAQS reviews, the air quality criteria for vegetation effects have emphasized the risk posed to vegetation from higher hourly average O₃ concentrations (e.g., “[h]igher concentrations appear to be more important than lower concentrations in eliciting a response” [ISA, p. 8-180]; “higher hourly concentrations have greater effects on vegetation than lower concentrations” [2013 ISA, p. 91-4] “studies published since the 2006 O₃ AQCD do not change earlier conclusions, including the importance of peak concentrations, . . . in altering plant growth and yield” [2013 ISA, p. 9-117]). In fact, the EPA has recognized the W126 index for E-R models for growth and yield (in the current and prior ISA and prior AQCD) in part due to its preferential weighting of higher concentrations (ISA, p. 8-130).

We note, however, that while the W126 index weights higher hourly concentrations, it cannot, given its definition as an index that sums three months of weighted hourly concentrations into a single value, always differentiate between air quality patterns with high peak concentrations and those without such concentrations. This is illustrated by the following two hypothetical examples. In the first example, two air quality monitors have a similar pattern of generally lower average hourly concentrations, but differ in the occurrence of higher concentrations (e.g., hourly concentrations at or above 100 ppb). The W126 index describing these two monitors would differ. In the second example, one monitor has appreciably more hourly concentrations above 100 ppb compared to a second monitor; but the second monitor has higher average hourly concentrations than the first. In the second example, the two monitors may have the same W126 index, even though the air quality patterns observed at those monitors are quite different, particularly with regard to the higher concentrations, which have been recognized to be important in eliciting responses (as noted above).

Thus, the EPA disagrees with a view implied by many of the commenters (who object to the EPA's proposed decision) that the sole focus for assessing public welfare protection, related to vegetation damage, and air quality control provided by the secondary standard should be on the W126 index. This view ignores both the limitations of the W126 index itself in distinguishing among different patterns of hourly O₃ concentrations and the fact that the current secondary standard has, by virtue of its form, a metric that does. With regard to these limitations of the W126 index, as described above, two different locations or years may have different patterns of hourly concentrations but the same W126 index value. This was recognized in the study by Lefohn et al. (1997), which observed the appreciable differences between the prevalence of hourly concentrations at or above 100 ppb in exposures on which the E-R functions are based and those common in ambient air.

This potential for such a difference in peak concentrations between two different locations with the same W126 index was noted by one commenter who objected to the EPA's focus on a 3-year average W126 index in assessing RBL and advocated use of a single-year W126 index. This commenter stated that the same 3-year average could be maintained in two different locations in which the annual exposure may differ due to “variability of the higher hourly average concentrations associated with vegetation effects.” In emphasizing the higher hourly average concentrations associated with effects, the commenter cited the support provided by the evidence for the San Bernardino National Forest, described in the 2013 ISA and prior CDs (e.g., 2013 ISA, section 9.5.3.1). We agree with this point and additionally note that this point also applies to two locations with the same single-year W126 index, given its definition (as noted above).

Given the mathematics inherent in calculation of the W126 index, while the metric is useful for comparing cumulative exposures, it can conceal peak concentrations that can be of concern (as described above). More specifically, one year or location could have few, or even no, hourly concentrations above 100 ppb and the second could have many such concentrations; yet each of the two years or locations could have the identical W126 index (e.g., equal to 25 or 17 or 10 ppm-hrs, or some other value). However, as can be seen by the historical ambient air monitoring dataset of O₃ concentrations, the form of the current standard limits the occurrence of such elevated concentrations, e.g., at or above 100 ppb (PA, Appendix 2A, section 2A.2; Wells, 2020).

Analyses of hourly concentrations for different air quality scenarios developed in consideration of the remand and such comments (and documented in a technical memorandum to the docket) show the form and averaging time of the existing standard to be much more effective than the W126 index in limiting the number of hours with O₃ concentrations at or above 100 ppb (N100) and in limiting the number of days with any such hours (Wells, 2020). For example, during the recent design value period (2016-2018), across all sites that met the current standard, few sites had any hours at or above 100 ppb in a year (6% in the highest year, Wells, 2020, Table 2). Among the sites with any such hours, the vast majority had fewer than five such hours (99.5% in the highest year, Wells, 2020, Table 2), with none having more than ten such hours, and no site having more than three days in any one year with any such concentrations (Wells, 2020, Figures 4 and 5). In comparison, sites with an annual W126 index below 15 ppm-hrs recorded nearly 40 hourly concentrations at or above 100 ppb, and as many as seven days with such a concentration (Wells, 2020, e.g., Figures 10 and 11). A similar pattern is seen using the historical dataset extending back to 2000. This historical dataset also shows the appreciable reductions in peak concentrations (via either the N100 or D100 metric) that have been achieved in the U.S. as air quality has improved under O₃ standards of the existing form and averaging time (Wells, 2020, Figures 12 and 13). Thus, based on the findings of both the analyses in the PA (PA, Appendix 2A) and the additional analyses (Wells, 2020), the EPA disagrees with the commenter that the proposed decision ignores the importance of elevated hourly O₃ concentrations in eliciting effects on vegetation. Rather, the proposed decision, and final decision to retain the existing standard, which controls peak concentrations and also cumulative seasonal exposure in terms of W126 index, explicitly considers this importance and address it in a way that is more effective than a standard expressed in terms of the W126 index would be, even based on a single-year W126 well below 17 ppm-hrs (as shown in the additional air quality analyses [Wells, 2020]).

In summary, we find that a 3-year average is appropriate for use in assessing protection for RBL based on the established tree seedling E-R functions, in light of the discussion above, while also finding it important to consider additional aspects of O₃ air quality, that influence vegetation exposures of potential concern, in reaching conclusions about the adequacy of the current standard. We disagree with the commenters and the prior CASAC that focus on a single year W126 index is needed to protect against years with O₃ concentrations with the potential to be “unusually damaging,” Rather, as described here, the metric of the current standard provides strong protection against elevated hourly concentrations that might contribute to “unusually damaging” years with the potential to be adverse to the public welfare, as well as providing protection against effects of cumulative exposures seen in experimental studies. Accordingly, we disagree with those commenters that express the view that the current standard does not provide such protection.

(iii) Visible Foliar Injury

In support of their disagreement with the EPA's proposed decision, some commenters express the view that the EPA's proposed conclusion that the current standard provides sufficient protection from an incidence and severity of visible foliar injury that would reasonably be judged adverse to the public welfare is unlawful. These commenters variously claim that EPA analyses are flawed, arbitrary, and ignore conclusions and judgments of the prior CASAC; cite some studies that they state indicate a threshold for foliar injury lower than 25 or 17 ppm-hrs; claim that the EPA must, yet does not, identify a level of injury that is adverse; state that the EPA does not explain its use of USFS biosite scores in this regard, and state that the EPA does not adequately address the Murray Energy remand related to these effects. With regard to the latter, the Agency intends this decision, associated analyses conducted for this review in consideration of issues raised by the court remand, and the discussions herein to constitute its response to the Murray Energy remand on these effects.

With regard to EPA's analyses of the current information on O₃-related visible foliar injury, some commenters claim that the EPA needs to and has not adequately explained why it disagrees with the conclusions and judgments of the prior CASAC in comments on the 2014 draft PA regarding a W126 index value of 10 ppm-hrs. As an initial matter, we note that in discussing this topic, these commenters conflate the prior CASAC's scientific evidence-based recommendations on the secondary standard with its judgments of scientific information in the context of its policy recommendations. In its letter on the draft PA, the prior CASAC explicitly separates into two separate paragraphs its scientific judgment based recommendations to the Administrator on the standard from its additional policy recommendations, with this statement regarding visible foliar injury occurring in the second paragraph (that addresses policy recommendations) (Frey, 2014b, p. iii). Thus, we reasonably interpreted the statement by the prior CASAC as simply indicating a consideration of the prior CASAC in reaching its decision on the recommended range of levels, stated multiple times in the same letter and including levels higher than 10 ppm-hrs, that the Committee thought might be useful (e.g., as a “policy recommendation”) to the Administrator in exercising the discretion granted him under the Act for specifying a secondary standard (Frey, 2014b, p. iii). The prior CASAC statement regarding a W126 index value of 10 ppm-hrs, is related to visible foliar injury at biosites, and, more specifically, is based on its consideration of an EPA cumulative analysis of a biomonitoring dataset presented in the 2013 draft WREA. This analysis, the dataset for which is further described in Appendix 3C of the PA for the current review, does not show, as implied by the 2014 CASAC comments, that, in considering sites with W126 index values from highest to lowest, there is no reduction in prevalence of sites with visible foliar injury above a W126 index of 10 ppm-hrs (i.e., there are not differences in the occurrence of injury across higher values). The 2014 WREA analysis could not and was not addressing this issue.

The 2014 WREA analysis is a cumulative analysis of the proportion of records with nonzero BI scores; each point graphed in the analysis includes the records for the same and lower W126 index values. Not only is the analysis silent with regard to severity of injury, but it also does not compare the incidence of visible foliar injury for records of differing W126 index values. Rather, each point in the cumulative frequency figure represents all the records included in the group (thus far), which increase by one with each new point (moving through dataset). Where the record added to the group has the same W126 index value as the prior included record, the point is at the same location along the x-axis, but at a slightly higher location along the y-axis (if it has a nonzero BI), thus contributing to an increase in the proportion of sites (the metric assessed on the y-axis). Thus, where there are many records with quite similar W126 index values, the points do not appreciably move along the x-axis, yet when they have a nonzero BI score, they are placed higher along the y-axis (as each represents another nonzero record in the dataset, thus increasing the proportion of records). At such a location along the x-axis, an inflection occurs (i.e., a location along the x-axis for which each additional record had the same or quite similar W126 index as the prior record such that the point is at a similar location on the x-axis but contributes to increasing values along the y-axis). As the addition of each new record makes the dataset larger, such increases (or decreases for zero BI records) become progressively smaller (along the y-axis), making such changes or inflections less pronounced at higher W126 index values. Accordingly, given the much greater representation in the dataset of relatively lower W126 index records (some two thirds of the dataset has W126 index values at/below 11 ppm-hrs), the prominent inflection point noted by the prior CASAC on the cumulative frequency graph occurs around 11 ppm-hrs, and the figure from the 2014 WREA shows only small changes in the height of the line with increasing W126 index. This does not mean that records with higher W126 index values have no greater occurrence of foliar injury than values below 11 ppm-hrs; in fact, they do, most particularly the records with W126 index values above 25 ppm-hrs (PA, Figure 4-5). Thus, we disagree with the prior CASAC statement that W126 index values below 10 ppm-hrs are required for any reduction in visible foliar injury and with the suggestion that the WREA cumulative analysis supports such a conclusion. Given that the statement by the prior CASAC did not provide any information to indicate another basis for its statement and because the 2014 WREA analysis cannot and does not address this issue, we conclude that the prior CASAC's statement lacks scientific support. Based on this conclusion, the Administrator does not find this statement from the prior CASAC informative to his consideration of the adequacy of the protection provided by the current standard for adverse public welfare effects related to visible foliar injury (discussed in section III.B.3 below).

Unlike the 2014 WREA cumulative frequency analysis, the presentations in the PA for this review allow for comparison of injury incidence, and severity, at distinctly different exposures. As can be seen by graphs of the distribution of nonzero BI scores for bins of increasing W126 index estimates, the greatest representation of nonzero BI scores occurs in the bin with the highest W126 index estimates, which for the normal soil moisture category is above 25 ppm-hrs (PA, Figure 4-5). In disagreeing with the EPA's observations from this analysis, these commenters express the view that the higher percentage at the higher W126 index level is not meaningful because there are fewer records for the higher W126 index levels. While we agree that there are fewer records in the higher W126 index bins, as noted above, we disagree that there are too few records in those bins to support some interpretation for some soil moisture categories (such as the normal or dry categories), although for other soil moisture categories (i.e., wet), the small sample size does limit interpretation. Sample size in each bin was considered in the PA analysis and was recognized as placing a limitation on interpretation of patterns for the wet soil moisture category. Contrary to these commenters' view that EPA provides no reason for giving little focus to the higher W126 index bins for the wet soil moisture category, the PA explains that interpretations of patterns across the higher W126 bins are limited for the wet soil moisture category, noting that the number of records in each of the W126 bins above 13 ppm-hrs comprise less than 1% of the records available for that soil moisture category (PA, Appendix 4C, section 4C.6). Thus, we agree with these commenters that sample size is an important consideration in reaching conclusions from this dataset, and, contrary to the commenters' assertion of providing no valid reasons with regard to the EPA's lesser emphasis on the wet soil moisture category, the proposal stated that the PA observations focused primarily on the records for the normal or dry soil moisture categories explicitly in recognition of those categories having adequate sample size which the bins above 13 ppm-hrs did not for the wet soil moisture category (85 FR 49890, August 14, 2020). While the dataset includes an extremely small number of records in the wet soil moisture category that fall into the higher W126 index bins (just 18 distributed across the three W126 index bins above 13 ppm-hrs), there are more than 550 records categorized as normal soil moisture distributed across all five bins for W126 index above 13 ppm-hrs, more than 40 in each bin (PA, Appendix 4C, Table 4C-4). To the extent that the commenters are suggesting that the EPA is disregarding data for sites categorized as wet soil moisture, we disagree. In recognition of the role of soil moisture in contributing to a condition “necessary for visible foliar injury to occur,” the PA analysis presents BI scores separated into groups based on categorization related to soil moisture (ISA, Appendix 8, p. 8-13; 85 FR 49881; PA, pp. 4-40 to 4-41). The EPA thus considered the available evidence for all of the soil moisture categories, but with regard to any patterns evidenced for the higher W126 index bins (above 13 ppm-hrs), the EPA reasonably explained its focus on two of the three categories (the normal or dry soil moisture categories), and lesser attention to the third category (wet soil moisture) due to the extremely small number of records in that category that fall into the higher W126 index bins.

Further, in addition to incidence of sites with any injury, the PA presentations indicate that the severity of injury is also highest in records for the highest W126 index values, appreciably higher that it is in all of the lower W126 index bins. For normal soil moisture category, the median BI score across the nonzero records in the highest W126 bin (greater than 25 ppm-hrs) is just over 10 (with an average over 15), compared to well below 5 (averages below 7) for each of the lower W126 bins (PA, Figure 4-5, Appendix 4C, Table 4C-5). Both of these observations are consistent with an E-R relationship of O₃ with visible foliar injury, while the variability observed across the full dataset, in addition to perhaps indicating limitations in some aspects of the dataset (e.g., categorization by soil moisture, among others [PA, Appendix 4C, section 4C.5]), no doubt also indicates the role of other factors that have not been completely accounted for. Given the evidence from controlled experiments documented across many years, the lack of noticeable change in incidence or severity across lower W126 index values may, as recognized in the PA, relate to a number of factors, including uncertainties in the assignment of W126 index estimates to the biosite locations and the soil moisture categorization of sites, as well as potential for differences in individual plant responses in controlled experiments from plant communities in natural environmental settings. Although such factors may contribute to an unclear pattern at lower exposures, precluding reaching conclusions regarding O₃-related response across the lower W126 index bins, the observed response for the highest bin clearly indicates an O₃-related response for W126 index values above 25 ppm-hrs.

Some commenters question the significance EPA ascribes to its observation that the BI scores are appreciably higher for records in the highest W126 index bin, cryptically characterizing the observation as describing a “derivative of a derivative.” Yet, this observation is simply focused on the response (e.g., incidence of BI score greater than 0 or 5 or 15) exhibited across the range of exposure levels evaluated. The EPA makes this observation in assessing the dataset as to whether an E-R relationship is exhibited and if so, at what part of the exposure range is there a noticeable increase in response. This assessment, in combination with related evidence, then informs the Agency's conclusions regarding O₃ exposure circumstances that influence BI scores, as well as levels of W126 for which such an influence is indicated. The commenters quote the prior CASAC as characterizing the 2014 WREA analysis as “a change in the E-R slope,” but, as discussed in detail above, the 2014 WREA figure is presenting a cumulative frequency analysis, which, by its design, does not show “a change in the E-R slope.” Such an analysis, because responses are not compared among distinct and discrete exposures, as explained above, is not well described as an exposure-response assessment (i.e., an analysis of responses occurring across a range of different exposures). This is in contrast to the current PA presentation of BI scores across bins of increasing W126 index, which presents the occurrence of responses, quantified by magnitude of BI score, associated with multiple different exposures (presented as bins). Thus, the EPA finds the current analyses in the PA, and not the cumulative frequency analysis in the 2014 WREA, to be informative to the consideration of relationships between extent of visible foliar injury and W126 index, and finds the 2014 WREA analysis to be mistakenly interpreted by the commenters.

Further some commenters, who object to the Administrator's proposed focus on BI scores above 15 for his consideration of visible foliar injury that may be adverse to the public welfare, additionally suggest that EPA should give weight to all nonzero BI scores in considering the appropriate protection against this effect for the standard. As an initial matter, contrary to the implication of the commenters that any amount of visible foliar injury is adverse to the affected plant, we note the long-standing conclusions that visible foliar injury “is not always a reliable indicator of other negative effects on vegetation,” such as growth and reproduction, and the “significance of ozone injury at the leaf and whole-plant levels depends on how much of the total leaf area of the plant has been affected, as well as the plant's age, size, developmental stage, and degree of functional redundancy, among the existing leaf area” (ISA, p. 8-24; 2013 ISA, section 9.4.2). Further, we disagree with the further implication of these commenters that any occurrence of a nonzero BI score in the PA dataset can be used to identify O₃ exposure conditions that are adverse to the public welfare. As discussed in section III.A.2.b above, a number of factors influence the public welfare implications of visible foliar injury, and as discussed further below, the Administrator has taken these into account in his decision making regarding the protection from such effects that should be afforded by the secondary standard.

These commenters additionally claim that the USFS dataset indicates a clear relationship between the W126 metric and foliar injury. While we agree that the dataset provides some support for the conclusion of a greater incidence of nonzero BI scores and higher scores for the highest W126 bin, a change in response is not evident across the full range of W126 index levels (for records of similar soil moisture category), thus suggesting a limitation of the dataset in its ability to describe the E-R relationship of BI scores with W126 index. As discussed in the PA, limitations in the dataset (e.g., with regard to assignment of W126 index estimates to biosite records and the approach for accounting for the role of soil moisture) may be contributing to the lack of a clearly delineated E-R relationship of injury occurrence and BI score with W126 index across a range of W126 index values, such that a clear shape for a relationship between these variables is not evident with this dataset, and may be contributing to uncertainties in this regard. It is with the increase in W126 for the last bin (>25 ppm-hrs) that the accompanying noticeable increase in response provides increased confidence in that response (BI scores) being related to a particular magnitude of the O₃ metric. It is this consideration which leads to the emphasis that EPA's conclusions from this analysis place on W126 index above 25 ppm-hrs, albeit with a recognition of some associated uncertainty.

Regarding the Administrator's judgment of the extent and severity of visible foliar injury that may be adverse to the public welfare, some commenters state that the EPA must, and has not, considered the full USFS dataset, including records for which the BI scores are below 5, and they express the view that the USFS data indicate injury (i.e., a nonzero BI score) to be occurring at W126 index values as low as 3 ppm-hrs. In so doing, they note the occurrence of scores above 15 in the lowest bin (W126 index below 7 ppm-hrs). These commenters note that a third of all records with a BI above 15 are in the lowest W126 index bin (W126 <7 ppm-hrs) and more than 500 records with nonzero BI are in higher bins, seemingly intending this as support of their view that the EPA should identify a W126 of 7 ppm-hrs as a target level for visible foliar protection. However, this line of logic seems to ignore the fact that this bin also has over a third of the records with a BI above zero (PA, Table 4C-4), a fact which would seem contrary to these commenters' position that 7 ppm-hrs would protect against such scores. All three of these observations are likely due to the fact that this bin contains 42% of all records and the most records of any bin, by far (PA, Appendix 4C, Table 4C-4). Accordingly, the more important observation with regard to the extent of conclusions supported by the dataset on the role of W126 index in influencing BI scores is that the proportion of records in the lowest W126 bin that have scores above 15, 5 or 0 is appreciably less that in the highest W126 index bin (PA, Appendix 4C, Table 4C-6). The fact that there is not a clear pattern of increasing proportion across the intervening (and full set of) bins indicates there to be factors unaccounted-for in this dataset with regard to the O₃ exposure circumstances and the environmental circumstances that together elicit increased scores in vegetated areas.

In considering the PA analyses of the biosite dataset in light of these comments, we first note that, as described in the PA, the USFS dataset includes a broad assortment of BI scores, extending down to zero, occurring across the range of W126 estimates applied to the records (PA, Appendix 4C, Figure 4C-3). Contrary to the statement by these commenters, the EPA has considered the full dataset. The PA documents the various ways in which this is done, and the proposal discusses key observations from this dataset to inform the Administrator's judgment on adversity to public welfare (PA, section 4.3.3.2, 4.5.1.2 and Appendix 4C; 85 FR 49889-90, 49903, August 14, 2020). For example, the lack of clear BI score response to W126 across the range of lower values is consistent with findings of published studies of the USFS biomonitoring data which find that W126 index alone may not be sufficient to characterize the O₃ conditions contributing to injury levels that may be of interest (e.g., Smith et al., 2012; Smith, 2012; 85 FR 49888-49889, August 14, 2020). Similar to the discussion above, these studies suggest a role for the occurrence of elevated hourly concentrations and a focus solely on W126 index may miss this. This consideration of the larger evidence base for visible foliar injury and associated USFS biomonitoring findings is important to judging the findings of analyses of the BI dataset and their informativeness to the Administrator's needs in judging public welfare adversity. Based on a detailed evaluation of the currently available record regarding such data, the EPA recognizes the need to consider factors beyond just W126 index in considering O₃ conditions most influential in the incidence and extent of visible foliar injury.

With regard to lower “thresholds,” the commenters simply cite a set of studies that describe visible foliar injury observations in bioindicator species and for which estimates of W126 index for a prior time period are below 25 ppm-hrs. The first group of these studies focus on naturally occurring plants in locations during which the current standard (with its level of 70 ppb) is not met. As discussed above, the current standard limits the occurrence of elevated concentrations which, as discussed above, is suggested to be important in the occurrence of visible foliar injury in sites of the USFS biosite monitoring program, and such elevated concentrations are much more prevalent in areas that do not meet the current standard (e.g., PA, Appendix 4A, section 2A.2; Wells [2020]). Thus, this group of studies do not provide sufficient information to characterize the O₃ exposure circumstances that may be eliciting the observed responses. Nor are they informative with regard to consideration of the incidence and extent or severity of injury that may occur under air quality conditions allowed by the current standard. Two other examples raised by commenters (but without complete study citations), appear to relate to leaf injury assessed in potted plants either outdoors but watered daily or maintained in greenhouse conditions. The injury assessed is at the individual plant level, making implications with regard to natural vegetation communities unclear, and the extent to which either finding in artificial conditions might represent such plant responses in natural environmental conditions is unknown. These commenters additionally note what they describe as “threshold values” reported in a National Park Service publication (Kohut, 2020). This publication includes three “injury thresholds” in terms of three assessment metrics, with one being a 3-month W126 index and a second in terms of SUM06. For each metric, three ranges of “thresholds” are presented (for different purposes). The ranges for SUM06 come from a 1996 workshop report (Heck and Cowling, 1997). The ranges for W126 index are based on a W126 index conversion of the SUM06 ranges. One of the ranges is labeled as pertaining to foliar injury as a response, yet, the publication cited does not provide data on foliar injury in relation to that range, nor do publications cited by the former publication. As we can best discern based on cited and related publications, it appears to at the lower end relate to a benchmark derived for growth effects (10% RBL) in the highly sensitive species, black cherry, rather than visible foliar injury (Kohut, 2007b; Lefohn et al., 1997; 80 FR 65378, October 26, 2015). Thus, contrary to the commenters' assertion, the range for W126 index (labeled as pertaining to foliar injury) does not appear to provide a threshold based on evidence for visible foliar injury.

Some commenters (citing page 4C-18 of the PA), express confusion over how EPA can state there to be an incomplete understanding of the relationships influencing severity of visible foliar injury while also using the USFS scores to inform the Administrator's judgments regarding conditions that may be adverse to the public welfare. We see no contradiction in this. Rather, it is this recognition of an incomplete understanding, including the recognition of uncertainty in “specific aspects of [the influences of environmental/genetic factors] on the relationship between O₃ exposures, the most appropriate exposure metrics, and the occurrence or severity of visible foliar injury” (PA, Appendix 4C, p. 4C-18), that leads the EPA to place greatest weight on the most clear findings from the USFS data. With regard to the PA presentation, with its recognized uncertainties and limitations, such a finding is the obviously increased prevalence and severity of visible foliar injury for records with W126 index estimates above 25 ppm-hrs.

Further, in considering public welfare implications of O₃ related visible foliar injury, the EPA continues to recognize that the occurrence of visible foliar injury has the potential to be adverse to the public welfare (e.g., as summarized in section III.A.2.b above and section III.B.2 of the proposal). However, as noted in the proposal, the EPA does not find that any small discoloring on a single leaf of a plant (which might yield a quite low, nonzero BI score in the USFS system) is reasonably considered adverse to the public welfare. Thus, findings such as those raised by commenters of injury on individual plants in controlled conditions, while providing support to the conclusion of a causal relationship between O₃ exposure and visible foliar injury (ISA, Appendix 8, Table 8-3), are less informative to the Administrator's judgment on adequacy of the protection provided by the current standard from adverse effects to the public welfare. Rather, the USFS biosite monitoring data provide information that is more useful for such a judgment because this monitoring program, as summarized in section III.A.2.b above (and III.B.3.b of the proposal), and the scale of its objectives which focus on natural settings in the U.S. and forests as opposed to individual plants is better suited for the Administrator's consideration with regard to the public welfare protection afforded by the current standard. In this context, as described in section III.B.3 below, the Administrator judges that very low BI scores, such as those less than 5, described by the USFS scheme as “little or no foliar injury” do not pose concern for the public welfare.

Lastly, we disagree with the comment that the Act requires the EPA to specify “a level” of injury that is adverse. The Court of Appeals for the D.C. Circuit has held that “the Agency may sometimes need to articulate the level of threat to the population it considers tolerable; but there is no separate methodological requirement under § 109 that the Administrator establish a measure of the risk to safety it considers adequate to protect public health every time it establishes a standard pursuant to § 109.” See Nat. Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973 (D.C. Cir. 1990), opinion vacated in part on other grounds, 921 F.2d 326 (D.C. Cir. 1991). The same principle applies for consideration of the protection of public welfare in the context of establishing or reviewing secondary standards. The court later confirmed that it “expressly rejected the notion that the Agency must `establish a measure of the risk to safety it considers adequate to protect public health every time it establishes a [NAAQS].” See ATA III, 283 F.3d at 369 (D.C. Cir. 2002) (quoting Natural Res. Def. Council, Inc. v. EPA, 902 F.2d 962, 973 [D.C. Cir. 1990]). As is recognized by the courts and by EPA and CASAC across NAAQS reviews, the judgment of the Administrator, in addition to being based on the scientific evidence, depends on a variety of factors, including science policy judgments and public welfare policy judgments. As noted by the case law and also in section III.B.2.b(iv) below, the EPA is not required under the Act to identify individual levels of adversity or set separate standards for every type of effect that may be caused by a pollutant in ambient air, as long as it has engaged in reasoned decision making in determining that a particular standard provides the requisite protection. Thus, it is common for one NAAQS to provide protection for multiple effects, with the most sensitive effect influencing the stringency of the standard and accordingly leading to protection that is adequate for other, less sensitive effects. Given the significant uncertainties which are present in every NAAQS review, it is enough for the Administrator to set standards that specify a level of air quality that will be “tolerable,” (NRDC, 902 F.2d at 973), and “qualitatively to describe the standard governing its selection of particular NAAQS” (ATA III, 283 F.3d at 369). In reviewing each standard, the EPA gives due consideration to each of the effects that are relevant for that standard in considering whether the standard provides adequate protection from the type, magnitude or extent of such effects known or anticipated to be adverse to the public welfare. In the case of visible foliar injury, as discussed in section III.B.3 below, the Administrator has considered the available scientific evidence, with associated uncertainties and limitations, in reaching his decision that the current secondary standard provides adequate public welfare protection for this effect.

(iv) Crop Yield Effects

Some commenters object to the proposed conclusions with regard to the protection provided by the existing secondary standard from adverse effects on the public welfare related to O₃ effects on crop yield, expressing the view that the EPA must specify “a level” to protect the public welfare against crop yield reductions and that not doing so is unlawful and arbitrary. These commenters' additionally object to the Administrator's proposed judgment that a decision based on RBL as a proxy for other vegetation-related effects will also provide adequate protection against crop related effects, indicating their view that EPA does not adequately explain the basis for this judgment. These commenters additionally claim that the prior CASAC described 5.1% RYL as constituting an adverse welfare effect and express the view that the EPA arbitrarily and unlawfully does not “give effect to” the prior CASAC's recommendation.

We disagree with the implication of these commenters that, in judging adequacy of protection provided by the current standard for a particular effect, it is per se unlawful to conclude that the air quality achieved by the current standard provides adequate protection for that particular effect, even if the greater attention in reviewing the current standard is on another effect. The EPA is not precluded from reaching such a conclusion as long as the Agency has engaged in reasoned decision-making in doing so. In reaching his proposed conclusions regarding the extent to which the current standard provides appropriate protection from O₃ effects on crop yield that may be adverse to the public welfare, as in his conclusions described in section III.B.3 below, the Administrator recognizes the long-standing evidence of O₃ effects on crop yield and the established E-R functions for which RYL estimates for the median crop species are presented in the PA (PA, Appendix 3A). He also considers factors that might be important to his judgments related to the requisite protection for a secondary standard that protects against adverse effects to the public welfare. In this context he judges that the median RYL estimated for air quality that achieves his RBL-related objectives for the current standard does not constitute an adverse effect on public welfare and thus concludes that the current standard also provides adequate protection for crop yield-related effects. Given that the decision on adequacy of protection is a judgment of the Administrator and that the Clean Air Act does not require a particular approach for reaching such judgments, we disagree with the commenters to the extent that they suggest that it is per se unlawful for the Administrator to use such an approach. The circumstances for his use of this approach include particular aspects of the information available on O₃-related crop yield effects and other factors important to judgments on public welfare effects related to crop yield effects.

In reaching his decision in this review, as described in section III.B.3 below, the Administrator has also considered public comments on these issues, including that regarding a prior CASAC statement. The comment regarding the prior CASAC appears to draw on a judgment of the prior CASAC that a median RYL of 5% “represents an adverse impact” (Frey, 2014b, p. 14). The prior CASAC provided no clear scientific foundation for this judgment. While we infer this judgment to draw on discussion at a 1996 workshop, neither the prior CASAC nor the workshop summary provides any explicit rationale for identification of 5% (with regard to RYL), or any description of a connection of an estimated 5% RYL to broader impacts of a specific magnitude or type, or to judgments on significance of a 5% RYL to the public welfare. Thus the EPA disagrees with the commenters regarding the weight to give the prior CASAC statement and, as described below, respectfully disagrees with the prior CASAC on this statement.

In reaching his judgment regarding whether the current standard provides the requisite public welfare protection, as described in section III.B.3 below, the Administrator considers the extent to which a specific estimate of RYL may be indicative of adverse effects to the public welfare. In so doing, he notes that the secondary standard is not intended to protect against all known or anticipated O₃-related effects, but rather those that are judged to be adverse to the public welfare, and that a bright-line determination of adversity is not required in judging what is requisite. In his decision described below, the Administrator also notes that the determination of the extent of RYL estimated from experimental O₃ exposures that should be judged adverse to the public welfare is not clear, in light of the extensive management of agricultural crops that occurs to elicit optimum yields (e.g., through irrigation and usage of soil amendments, such as fertilizer). Further, in considering effects on the public welfare that may relate to agricultural markets, we note that detrimental impacts on crops, as well as beneficial impacts, can be unevenly distributed between producers and consumers, complicating conclusions with regard to relative adversity. In light of such considerations, the Administrator, while finding consideration of the RYL estimate for the median crop species informative to his judgment on the adequacy of the protection provided by the current standard for this effect, does not find such an RYL estimate of 5% to represent an adverse effect to the public welfare, as described more fully in section III.B.3 below. For these reasons, and for the reasons discussed earlier in this section, including those regarding advice from a prior CASAC, the EPA also disagrees with the commenters' assertion that the Administrator is arbitrarily and unlawfully failing to “give effect to” the prior CASAC's recommendation.

Further, we disagree with the comment that the Act requires the EPA to specify “a level” to protect the public welfare against crop yield reductions. As discussed in greater detail in section III.B.2.b(iii) above, the EPA is not required under the Act to set separate standards for every type of effect that may be caused by a pollutant in ambient air, as long as it has engaged in reasoned decision making in determining that a particular standard provides the requisite protection. Thus, it is common for one NAAQS to provide protection for multiple effects, with the most sensitive effect influencing the stringency of the standard and accordingly leading to protection that is adequate for other less sensitive effects. As discussed further in section III.B.2.b(iii) above, in reviewing each standard, the EPA gives due consideration to each effect relevant for that standard in considering whether the standard provides adequate protection from the type, magnitude or extent of such effects known or anticipated to be adverse to the public welfare. In the case of crop yield loss, as discussed in section III.B.3 below, the Administrator has considered the magnitude of RYL that may be associated with W126 index values that occur under the current standard and, based on the current information with regard to the RYL estimates, notes that these estimates are generally no higher than 5.1% and predominantly well below that. In so doing, he has also considered factors such as those raised above, and in light of all of these considerations, he judges that a RYL of 5.1% does not represent an adverse effect to the public welfare. Thus, the Administrator judges that the current standard provides adequate protection of the public welfare for crop yield loss related effects.

(v) RBL

In objecting to the EPA's proposed decision, some commenters disagree with the target level of protection identified based on use of RBL. In so doing, such commenters variously claim that a 3-year average of 17 ppm-hrs is “ill-suited” to protect against adverse impacts to the public welfare; that 6% RBL is too high to protect the public welfare; that use of a 3-year average instead of a single year W126 index is needed; and, that EPA must focus a target on exposures that would avoid 2% RBL, citing comments from the prior CASAC on the second draft PA in the 2015 review, and claiming that a focus on a W126 index of 7 ppm-hrs is needed for that. With regard to the EPA's use of 6% in considering the adequacy of protection related to RBL, these commenters recognize that Murray Energy rejected an argument that EPA's prior reliance on 6% (in the 2015 decision) was arbitrary based on the record in that case (Murray Energy, 936 F.3d at 615-16). In pressing their views, however, the commenters state that nothing in Murray Energy prevents EPA from revising its prior determination based on the scientific evidence and CASAC advice.

With respect to the latter point, the EPA agrees that the Administrator's decision in this review must take into account the currently available scientific evidence and advice from the CASAC, and that the Agency is not bound by the Administrator's conclusions in the prior review. As summarized in the proposal for the current review, in the proposal, the Administrator took the currently available scientific evidence and advice from the CASAC into account, while also choosing to consider the judgments and decision made by the prior Administrator in that Administrator's consideration of RBL related targets for cumulative seasonal exposure. He did so, in light of the welfare effects evidence and air quality information now available, as well as the advice from the current CASAC reflecting its concurrence that implementation of the prior Administrator's approach or framework is “still effective” in protecting the public welfare from vegetation effects of O₃ (Cox, 2020a, Consensus Responses to Charge Questions p. 21). As described in section III.B.3 below, after considering the public comments on this point, he is taking a similar approach in reaching his decision in this review.

With regard to the commenters' objection to the EPA's use of a 3-year average in assessing RBL, we note, as an initial matter, that the EPA's focus on a 3-year average of 17 ppm-hrs as a target level relates to an RBL estimate of 5.3%, a value that was also chosen in 2015 in recognition of the prior CASAC advice both with regard to 6% RBL and about considering a lower W126 index target for a 3-year average due to the prior CASAC's concern about “unusually damaging years.” In the current review, the CASAC has explicitly considered the EPA's interpretation of 6% in identifying a target of 17 ppm-hrs as a 3-year average, and expressed its view that this target “is still effective in particularly protecting the public welfare in light of vegetation impacts from ozone” (Cox, 2020a, Consensus Responses to Charge Questions p. 21). Accordingly, the EPA disagrees with the comments that 6% RBL and a 3-year average W126 index target of 17 ppm-hrs are too high to inform the Administrator's judgments on O₃ air quality that protects the public welfare; rather, the Administrator continues to find this useful in informing his judgments regarding the public welfare protection provided by the standard, together with a broader consideration of air quality patterns associated with meeting the current standard, such as control of peak hourly concentrations, as described in section III.B.3 below. Further, we refer to the discussion above of how the existing standard, with its current averaging time and form provides the protection from the occurrence of elevated hourly concentrations that may characterize what the prior CASAC described as “unusually damaging years.” As discussed above, the available air quality data demonstrate the strong protection provided by the current standard from elevated concentrations that may occur in some years. As noted above, these analyses indicate that while the current form and averaging time of the existing standard provides control of these concentrations and the associated peak exposures, reliance solely on a standard in the form of the W126 index based standard, as advocated by the commenters, even with a level as low as 7 ppm-hrs cannot be relied on to provide it.

In support of their view that the EPA must focus on avoiding 2% RBL with a W126 index of 7 ppm-hrs, these commenters provide little rationale beyond citing a comment by the prior CASAC made in the last review. In so doing, the commenters assert that because the prior CASAC had noted that 7 ppm-hrs was the only W126 index level for which the E-R functions yielded a RBL for the median tree species that was less than or equal to 2%, the EPA must protect against 2% RBL and adopt a W126 index level of 7 ppm-hrs. We disagree. As an initial matter, we note our discussion above regarding the EPA's consideration in this review of advice from a prior CASAC, including prior CASAC statements that are raised by commenters, such as those noted here. Further, in making the statement that the commenters' cite, the prior CASAC did not reach the same conclusion as the commenters with regard to the extent to which a revised secondary standard should limit cumulative exposures and associated estimates of RBL, such that the prior CASAC did not recommend that the EPA consider only W126 index levels associated with median RBL estimates at or below 2%. See Murray Energy, 936 F.36 at 615-16 (noting that “CASAC did not identify 2% growth loss as the only sufficiently protective level” but merely recommended “2% as the lower end of a range of permissible target levels” to be considered). In fact, seven of the nine W126 index levels in the range recommended by the prior CASAC (7 to 15 ppm-hrs [Frey, 2014b]) are associated with RBL estimates higher than 2% (PA, Appendix 4A). As a basis for their assertion that the secondary standard should protect against a median RBL of 2%, these commenters additionally oddly declare that after three years, a 2% RBL per year “becomes 6%.” There is no evidence in the record, and the commenter provides no evidence, that would support their declaration that without a tripling in exposure, the O₃-attributable reduction in annual growth (the RBL) would triple. Nor is there evidence that would support an alternative interpretation of the commenters' statement as a claim that a tree experiencing a 2% RBL per year is reduced in absolute biomass by 6% after three years.

Some commenters who disagree with the proposed decision also express the view that the EPA has “proposed” to use RBL functions for trees as a proxy for all vegetation effects. Based on this view, these commenters variously assert that the EPA is failing to comply with its obligation under the Clean Air Act that a secondary standard protect the public welfare from “any known or anticipated adverse effects”; that the EPA's approach is not the same as the prior CASAC's discussion of RBL as a surrogate; that the EPA is contravening its statutory obligation by using one adverse effect as a surrogate for another without showing that prevention of the former will prevent the latter; and that, based on the commenters' interpretation of a statement made by the prior CASAC, a standard that allows tree growth loss above 2% cannot protect against visible foliar injury. As an initial matter, we note that the citation provided by the commenters for their statement that the “EPA proposes” to use RBL functions as a proxy for the broad array of O₃ vegetation-related effects does not include such a “proposal.” Rather the commenters' citation points to the background section of the proposal which simply summarizes the concept of RBL as a proxy or surrogate which was employed in the last review and which was described by the prior CASAC (85 FR 49899, August 14, 2020). In describing use of RBL as a proxy or surrogate, the proposal (and the PA) use several phrases, ranging from “for consideration of the broader array of vegetation-related effects of potential public welfare significance, that included effects on growth of individual sensitive species and extended to ecosystem-level effects, such as community composition in natural forests, particularly in protected public lands, as well as forest productivity” (85 FR 49878, August 14, 2020), to shorter phrases, such as “for the broad array of vegetation related effects that extend to the ecosystem scale” (85 FR 49911, August 14, 2020).

We disagree with these commenters that the way the EPA uses RBL as a “proxy” or “surrogate” is contrary to law, and with their contention that the EPA uses one adverse effect as a surrogate for another without showing that prevention of the former will prevent the latter. As described in the Administrator's decision below, the most precise use of RBL as a surrogate or proxy is in the target level of protection for cumulative seasonal exposure (17 ppm-hrs as a 3-year average W126 index). This use relates specifically to public welfare effects related to O₃ effects on growth of individual sensitive species and related effects, including ecosystem-level effects, such as community composition in natural forests, particularly in protected public lands, as well as forest productivity (as discussed in the PA, section 4.5.1.2). In fact, the ISA describes (or relies on) conceptual relationships among such effects in considering causality determinations for ecosystem-scale effects such as altered terrestrial community composition and reduced productivity, as well as reduced carbon sequestration, in terrestrial ecosystems (ISA, Appendix 8, sections 8.8 and 8.10). Beyond these relationships of plant-level effects and ecosystem-level effects, RBL can be appropriately described as a scientifically valid surrogate of a variety of welfare effects based on consideration of ecosystem services and the potential for adverse impacts on public welfare, as well as conceptual relationships between vegetation growth-related effects (including carbon allocation) and ecosystem-scale effects (PA, pp. 4-75 and 4-76). Both the prior CASAC and the current CASAC recognized this (Frey, 2014b, pp. iii, 9-10; Cox, 2020a, Consensus Responses to Charge Questions pp. 18 and 21 ). As was discussed in the proposal, the information available in this review provides continued support for the use of tree seedling RBL as a proxy for the broad array of vegetation-related effects conceptually related to growth effects, a conclusion with which the CASAC agreed (85 FR 49899,49906, August 14, 20202).

As recognized in the proposal (and PA) there are two other vegetation effect categories with extensive evidence bases (which include analyses that assess the influence of cumulative seasonal exposure); these are crop yield loss and visible foliar injury. As discussed above, the consideration of protection provided by the current standard for the former goes beyond the target focused on RBL and includes aspects of the evidence specific to those effects. As described above and in section III.B.3 below, the EPA is concluding that the level of protection is adequate to protect the public welfare from effects related to crop yield loss. With regard to the latter, contrary to the commenter's assertion, the EPA is not claiming that protection focused on RBL provides protection for visible foliar injury. The EPA's consideration of visible foliar injury is described earlier in this section and in section III.B.3 below.

With regard to the two newly identified categories of insect-related effects, the Administrator finds there to be insufficient information to judge the current standard inadequate based on these effects, as discussed in section III.B.3 below. He does not claim that RBL provides a surrogate for these effects. However, he notes that the available information in the air quality criteria does not indicate a greater sensitivity of such effects as compared to O₃ effects on vegetation growth, and that he lacks sufficient information in the air quality criteria to identify requisite air quality for these effects.

(vi) W126 Index in Areas Meeting Current Standard

In objecting to the proposed decision, one group of commenters disagree with EPA's findings regarding the W126 index levels in areas that meet the current standard. In so doing, these commenters claim that the EPA is mistaken to claim that in virtually all design value periods and locations at which the current standard was met across the period covered by the historical dataset the 3-year W126 index was at or below 17 ppm-hrs because they variously assert there are either 25 or 21 such occurrences, and they further assert there to be either 50 occurrences of a single-year W126 index at or above 19 ppm-hrs or 52 occurrences of a single-year W126 index above 19 ppm-hrs. These counts are in disagreement with the air quality analyses documented in Appendix 4D of the PA. For example, out of 8,292 values across nearly 20 years for U.S. ambient air monitoring sites, distributed across all nine climate regions, with air quality that meets the current standard, there are just eight occurrences of a 3-year W126 index value above 17 ppm-hrs (PA, Appendix 4D, Tables 4D-10 and 4D-7). This means that 99.9% of the records (virtually all) were at or below 17 ppm-hrs. While the details of each step of the analyses in the PA are extensively documented, including data handling, rounding conventions and data acceptability criteria (PA, Appendix 4D, section4D.2), the lack of documentation provided by the commenters and their conflicting claims (indicated above) leave the EPA to hypothesize that the reason for the disagreements include differences with regard to these details, such as those regarding rounding conventions. As described in the PA, W126 values “were rounded to the nearest unit ppm-hr for applications requiring direct comparison to a W126 level,” a convention intended to provide consistency in the precision of the comparison as the W126 levels for comparison were also in whole numbers (PA, Appendix 4D, section 4D.2.2). With the rounding conventions applied in the PA, there are eight 3-year W126 index values greater than 17 ppm-hrs (i.e., equal to 18 or 19). It may be that the commenters counted unrounded 3-year W126 index values as low as 17.01 as being greater than 17 ppm-hrs, although the reason for them providing two conflicting counts is unclear. Similarly with regard to the counts for single-year W126 index values above 19 ppm-hrs, the commenters may have counted unrounded single-year index values as low as 19.01 ppm-hrs as being greater than 19 ppm-hrs. Thus, we find the commenters criticism of the EPA's characterization of the findings of the air quality analyses, as well as the commenters' counts, to be unfounded.

Some commenters claim EPA pays inadequate attention to the relatively few occurrences of single-year W126 index values at or above 19 ppm-hrs that have occurred at sites meeting the current standard since 2002 and that the standard must be set to avoid such occurrences. The EPA disagrees with these commenters, as described below, after carefully considering the relatively few occurrences of W126 index values at or above 19 ppm-hrs, including single-year values. In so doing, we have given particular focus on Class I areas, recognizing the attention given to such areas by the Administrator in judging the potential for effects adverse to the public welfare, a focus recognized by the CASAC and with which the prior CASAC explicitly concurred (Cox, 2020a; Frey, 2014b, p. 9).

Among the nearly 500 values for monitoring sites in or near Federal Class I areas across the U.S., during periods from 2000 through 2018 when the current standard was met, there are no occurrences of a 3-year average W126 index above 19 ppm-hrs (PA, Table 4-1). Across this same period in the same Class I locations, there are just 15 occurrences of single-year W126 index values above 19 ppm-hrs, all of which date prior to 2013 (PA, Appendix 4D, section 4D.3.2.4). All of these occurrences are below 25 ppm-hrs. Thus, in addition to their being relatively few occurrences of a single-year W126 above 19 ppm-hrs in/near a Class I area in the 19-year dataset, none of them (the most recent of which was in 2012) is higher than 25 ppm-hrs; in fact, the highest is 23 ppm-hrs (PA, Appendix 4D, section 4D.3.2.4).

We have also considered the full 19-year dataset for locations beyond those in or near Class I areas, noting that, at other sites across the U.S., occurrences of single-year W126 index above 19 ppm-hrs (22) were predominantly in urban areas, including Los Angeles, and the highest values were just equal to 25 ppm-hrs, or, in one instance, just equal to 26 ppm-hrs, when rounded (85 FR 49895, 49904, August 14, 2020; PA, sections 4.4 and 4.5, Appendix 4D). In considering the potential risk posed by these scattered occurrences, largely in urban areas, with none since 2012 in or near a Class I areas, we additionally consider the data on peak hourly concentrations also discussed above (Wells, 2020). Together, these data indicate the control provided by the current standard in areas that are of particular focus in protecting the public welfare, on the extent and frequency of occurrence of cumulative exposures in terms of the W126 index (and of peak concentrations) of a magnitude of potential concern. As discussed in section III.B.3 below, the Administrator does not find the air quality patterns allowed by the current standard, as indicated by these analyses, to pose a risk of adverse effects to the public welfare.

In their criticism of the EPA's air quality analyses, one commenter claims that the analyses are difficult to evaluate for California and other West region states and suggest that California sites brought into compliance with the existing standard would still have elevated W126 index values, similar to sites in the Southwest region. We disagree with the commenter's claim that the air quality analyses suggest that compliance with the existing standard would not reduce the W126 index values at California sites. In making their claim, the commenters cite Figures 4D-4 and 4D-5 of the PA. These figures, however, simply document W126 index at sites with various design values at one point in time (2016-2018). They do not describe analyses of trends over time, with changes in air quality. Yet, that very issue was the subject of separate regression analyses in the PA (PA, Appendix 4D, section 4D.3.2.3). These analyses show that the Southwest region, which had highest W126 index values at sites meeting the current standard, also exhibited the greatest improvement in the W126 metric values per unit decrease in their design value (slope of 0.93) over the nearly 20 year period analyzed. The pattern is very similar for the West region (with a slope of 0.80), with the exception of three sites (in downtown LA); however, the design values for these sites are above 100 ppb, making such projections quite uncertain (PA, Appendix 4D, section 4D.3.2.3).

(vii) Climate Effects

In support of their disagreement with the EPA's proposed decision, some commenters claim that EPA needs to establish a standard to protect from radiative forcing and related climate effects. In so doing, they stated that EPA cannot rely on uncertainty by retaining the existing standard and instead, given the uncertainties recognized in the ISA, which they suggest could mean current information underestimates O₃ climate related impacts, the Administrator should strengthen the existing standard or establish an additional standard. Some commenters additionally assert that the EPA has failed to address a recommendation from CASAC regarding a quantitative analysis, while also criticizing EPA conclusions regarding a carbon storage analysis in the last review. The EPA disagrees with the commenters that the available information is sufficient to identify such a standard that could be judged to provide the requisite protection under the Act, and notes that the commenters do not submit or describe such information; nor do the commenters identify a standard that they claim would provide such protection.

With regard to the CASAC recommendation cited by some commenters, we note in its review of the draft PA, the CASAC recommended changes to the PA to “more thoroughly address effects of ozone on climate change,” that would include some quantitation, such as estimates of climate change related to a change in O₃ (Cox 2020a, Consensus Responses to Charge Questions p. 22). In consideration of this advice, the final PA included additional discussion on the available information and tools related to such estimates. As discussed below, we conclude that this information, as documented in the ISA, does not provide a foundation with which to derive such estimates as might pertain to O₃ and public health and welfare considerations relevant to decisions on the NAAQS.

As recognized in the proposal and summarized in section III.A.2 above, there are a number of limitations and uncertainties that affect our ability to characterize the extent of any relationships between O₃ concentrations in ambient air in the U.S. and climate-related effects, thus precluding a quantitative characterization of climate responses to changes in O₃ concentrations in ambient air at regional (vs global) scales. While evidence supports a causal relationship between the global abundance of O₃ in the troposphere and radiative forcing, and a likely causal relationship between the global abundance of O₃ in the troposphere and effects on temperature, precipitation, and related climate variables (ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013), the non-uniform distribution of O₃ (spatially and temporally) makes the development of quantitative relationships between the magnitude of such effects and differing O₃ concentrations in the U.S. challenging (ISA, Appendix 9). Additionally, “the heterogeneous distribution of ozone in the troposphere complicates the direct attribution of spatial patterns of temperature change to ozone induced [radiative forcing]” and there are “ozone climate feedbacks that further alter the relationship between ozone [radiative forcing] and temperature (and other climate variables) in complex ways” (ISA, Appendix 9, section 9.3.1, p. 9-19). Thus, various uncertainties “render the precise magnitude of the overall effect of tropospheric ozone on climate more uncertain than that of the well-mixed GHGs” and “[c]urrent limitations in climate modeling tools, variation across models, and the need for more comprehensive observational data on these effects represent sources of uncertainty in quantifying the precise magnitude of climate responses to ozone changes, particularly at regional scales” (ISA, section IS.6.2.2, Appendix 9, section 9.3.3, p. 9-22). For example, current limitations in modeling tools include “uncertainties associated with simulating trends in upper tropospheric ozone concentrations” (ISA, section 9.3.1, p. 9-19), and uncertainties such as “the magnitude of [radiative forcing] estimated to be attributed to tropospheric ozone” (ISA, section 9.3.3, p. 9-22). Further, “precisely quantifying the change in surface temperature (and other climate variables) due to tropospheric ozone changes requires complex climate simulations that include all relevant feedbacks and interactions” (ISA, section 9.3.3, p. 9-22). For example, an important limitation in current climate modeling capabilities for O₃ is representation of important urban- or regional-scale physical and chemical processes, such as O₃ enhancement in high-temperature urban situations or O₃ chemistry in city centers where NOx is abundant. Because of such limitations we cannot quantify or judge the impact of incremental changes in O₃ concentrations in the U.S. on radiative forcing and subsequent climate effects.

Thus, as discussed in section III.B.3 below, the significant limitations and uncertainties summarized here together preclude identification of an O₃ standard that could be judged to provide requisite protection of the public welfare from adverse effects linked to O₃ influence on radiative forcing, and related climate effects. Contrary to the commenters' charge that the lack of a quantitative analysis of climate-related effects due to recognition of such limitations and uncertainties is unlawful and arbitrary, the information available in this review is insufficient to judge the existing standard inadequate or to identify an appropriate revision based on O₃-related climate effects. In the face of insufficient evidence for such conclusions, it might, on the contrary, be judged unlawful and arbitrary for the Agency to perform guesswork to assert a particular new standard provided requisite protection for this category of effects. The EPA agrees with the commenters that “perfect information” is not required. However, information that provides for assessment of how the current and potential alternative or additional standards would affect O₃-related climate impacts is lacking. As noted in the ISA, few studies have even attempted to estimate “climate response to changes in tropospheric ozone concentrations alone in the future atmosphere,” and effects of O₃ on radiative forcing and climate depend on many factors other than tropospheric ozone concentrations, including “changes in a suite of climate-sensitive factors, such as the water vapor content of the atmosphere” (ISA, p. 9-20; Myhre et al., 2013). Thus, as discussed in section III.B.3 below, while the Administrator recognizes that the evidence supports a relationship of tropospheric O₃ with climate effects, he judges the quantitative uncertainties to be too great to support identification of a standard specific to such effects.

4. Administrator's Conclusions

Based on the large body of evidence concerning the welfare effects, and potential for public welfare impacts, of exposure to O₃ in ambient air, and taking into consideration the attendant uncertainties and limitations of the evidence, the Administrator concludes that the current secondary O₃ standard provides the requisite protection against known or anticipated adverse effects to the public welfare, and should therefore be retained, without revision. In reaching these conclusions, the Administrator has carefully considered the assessment of the available welfare effects evidence and conclusions contained in the ISA, with supporting details in the 2013 ISA and past AQCDs; the evaluation of policy-relevant aspects of the evidence and quantitative analyses in the PA (summarized in sections III.A.2 and III.A.3 above); the advice and recommendations from the CASAC (summarized in section III.B.1.b above); and public comments (as discussed in section III.B.2 above and the separate Response to Comments document), as well as the August 2019 decision of the D.C. Circuit remanding the secondary standard established in the last review to the EPA for further justification or reconsideration.

In considering the currently available information in this review of the O₃ secondary standard, the Administrator recognizes the longstanding evidence base for vegetation-related effects, augmented in some aspects since the last review, described in section III.A.2.a above. The currently available evidence describes an array of effects on vegetation and related ecosystem effects causally or likely to be causally related to O₃ in ambient air, as well as the causal relationship of tropospheric O₃ in radiative forcing and subsequent likely causally related effects on temperature, precipitation and related climate variables. The Administrator also takes note of the quantitative analyses and policy evaluations documented in the PA that, with CASAC advice and consideration of public comment, inform the judgments required of him in reaching his decision on a secondary standard that provides the requisite protection under the Act.

As an initial matter, the Administrator recognizes the continued support in the current evidence for O₃ as the indicator for photochemical oxidants (as recognized in section III.B.1.c above). In so doing, he notes that no newly available evidence has been identified in this review regarding the importance of photochemical oxidants other than O₃ with regard to abundance in ambient air, and potential for welfare effects, and that, as stated in the current ISA, “the primary literature evaluating the health and ecological effects of photochemical oxidants includes ozone almost exclusively as an indicator of photochemical oxidants” (ISA, section IS.1.1). Thus, the Administrator recognizes that, as was the case for previous reviews, the evidence base for welfare effects of photochemical oxidants does not indicate an importance of any other photochemical oxidants. For these reasons, described with more specificity in the ISA and PA, he proposes to conclude it is appropriate to retain O₃ as the indicator for the secondary NAAQS for photochemical oxidants (85 FR 49896, August 14, 2020).

In his review of the existing secondary O₃ standard, in light of the evidence base and quantitative analyses available today, the Administrator has given particular attention to consideration of the issues raised by the August 2019 court remand, and related issues raised in public comment, as well as analyses that were conducted or updated in this review in consideration of the remand and related public comment. In so doing, he has also given careful consideration of the form and averaging time of the current standard and its ability to control the patterns of O₃ concentrations that contribute to environmental exposures of potential concern to the public welfare. Further, he has considered what is indicated by the information currently available with regard to exposure metrics, supported by the current evidence, for assessing potential risks posed to vegetation, and protection provided from such exposures. Additionally, with regard to visible foliar injury, he has considered the current evidence in the ISA in combination with quantitative information and policy evaluations in the PA, advice from the CASAC and public comment, in making judgments regarding adequacy of the protection provided by the current standard from adverse effects to the public welfare related to this effect. Before turning to these issues, discussed, in turn, below in the context of the EPA's understanding of the information now available in the current review, he addresses two endpoints newly identified in this review, as well as tropospheric O₃ effects related to climate.

With regard to the two insect-related categories of effects with new ISA determinations in this review, the Administrator takes note of the conclusions that the current evidence is sufficient to infer likely causal relationships of O₃ with alterations of plant-insect signaling and insect herbivore growth and reproduction (as summarized in section III.A.2.a above). He additionally recognizes the PA finding that uncertainties in the current evidence, as summarized in section III.A.2 above, preclude a full understanding of such effects, the air quality conditions that might elicit them, the potential for impacts in a natural ecosystem. Accordingly, the Administrator notes a lack of clarity in the characterization of these effects, and a lack of important quantitative information to consider such effects in this context such that it is not feasible to relate different patterns of O₃ concentrations with specific risks of such alterations. As a result, the Administrator concludes there is insufficient information to judge how particular ambient air concentrations of O₃ relate to the degree of impacts on public welfare related to these effects. Thus, he concludes there is insufficient information to judge the current standard inadequate or to identify an appropriate revision based on these effects.

Before focusing further on the key vegetation-related effects identified above, the Administrator first considers the strong evidence documenting tropospheric O₃ as a greenhouse gas causally related to radiative forcing, and likely causally related to subsequent effects on variables such as temperature and precipitation. In so doing, he takes note of the limitations and uncertainties in the evidence base that affect characterization of the extent of any relationships between O₃ concentrations in ambient air in the U.S. and climate-related effects, and preclude quantitative characterization of climate responses to changes in O₃ concentrations in ambient air at regional or national (vs global) scales, as summarized in sections III.A.2 above. As a result, he recognizes the lack of important quantitative tools with which to consider such effects in this context such that it is not feasible to relate different patterns of O₃ concentrations at the regional (or national) scale in the U.S. with specific risks of alterations in temperature, precipitation and other climate-related variables. The Administrator finds that these significant limitations and uncertainties together preclude his identification of an O₃ standard reasonably judged to provide requisite protection of the public welfare from adverse effects linked to O₃ influence on radiative forcing, and related climate effects. Thus, the Administrator concludes that the information available in this review is insufficient to judge the existing standard inadequate or to identify an appropriate revision based on O₃-related climate effects.

The Administrator turns now to vegetation-related effects, the evidence for which as a whole is extensive, spans several decades, and supports the Agency's conclusions of causal or likely to be causal relationship for O₃ in ambient air with an array of effect categories. These categories include reduced vegetation growth, reproduction, crop yield, productivity and carbon sequestration in terrestrial systems; increased tree mortality; alteration of terrestrial community composition, belowground biogeochemical cycles and ecosystem water cycling; and visible foliar injury (ISA, Appendix 8). As an initial matter, the Administrator notes the new ISA determination that the current evidence is sufficient to infer likely causal relationships of O₃ with increased tree mortality. With regard to the current evidence for this effect, the Administrator notes that the evidence does not indicate a potential for O₃ concentrations that occur in locations that meet the current standard to cause increased tree mortality, as summarized in section III.A.2.a above (PA, section 4.3.1). Accordingly, he finds it appropriate to focus on more sensitive effects, such as tree seedling growth, in his review of the standard. Thus, in considering the adequacy of protection provided by the current standard from adverse effects to the public welfare related to these effects, the Administrator begins by considering vegetation growth and conceptually related effects with a focus on RBL (described in section III.B.2 above), then turns to a specific consideration of crop yield loss and lastly, to consideration of visible foliar injury.

With regard to vegetation growth and related effects, the Administrator has considered discussions in the PA and in response to public comments in section III.B.2 above, and finds it appropriate for identification of the requisite protection to extend beyond consideration of a magnitude of growth effects, per se, that he may judge adverse to the public welfare. Rather, the Administrator extends his consideration beyond that, judging it appropriate to consider reduced growth (i.e., RBL) as a proxy for an array of other vegetation-related effects to the public welfare. As discussed in section III.B.2 above, these categories of effects include reduced vegetation growth, reproduction, productivity and carbon sequestration in terrestrial systems, and also alteration of terrestrial community composition, belowground biogeochemical cycles and ecosystem water cycling. In adopting RBL as a proxy for this array of effects, the Administrator notes that such a use is consistent with advice from CASAC, and that RBL was also adopted as a proxy for this array of effects by the prior Administrator, in consideration of advice from the prior CASAC.

In assessments of RBL estimated from O₃ exposure, the Administrator takes note of the PA consideration of the established E-R relationships for RBL in tree seedlings of 11 species with O₃ exposures in terms of W126 index (PA, Appendix 4A). In so doing, he agrees with the PA conclusion regarding 6% RBL, with which the CASAC concurred, as described in sections III.B.1.b and III.B.2 above), and judges that for his use of RBL as a proxy, maintaining O₃ concentrations such that associated estimates of RBL fall below 6%, as a median across the 11 species represented by the established E-R relationships would assure the appropriate protection. In making these judgments, he observes that they were also adopted by the prior Administrator, with consideration of advice from the prior CASAC.

Further, based on considerations discussed in the PA, advice from CASAC and discussion in section III.B.2 above, Administrator has considered the use of RBL in his judgment of the public welfare protection provided by the secondary standard. Based on those considerations, including uncertainties in the E-R relationships and their use in the way described here, the Administrator judges it appropriate for the standard to protect against W126 index values associated with a median RBL at or above 6% (while also controlling peak hourly concentrations, as discussed below). Based on this judgment, in addition to a recognition of uncertainty in these estimates (in light of the discussion in section III.B.2.b(ii) above regarding the appropriate duration or averaging for the W126 index metric) he concludes it appropriate for the standard to generally control exposures in terms of W126 index to a level of 17 ppm-hrs, recognizing that the RBL estimated for such a W126 index value is 5.3%, a value appreciably below 6%.

With regard to the appropriate O₃ exposure metric to employ in assessing adequacy of air quality control in protecting against RBL, the Administrator has considered the discussions in the PA, and in response to public comments in section III.B.2 above regarding the available evidence and air quality analyses. He has also considered this in the context of the court remand with regard to the EPA's use of a 3-year average W126 index to assess protection from RBL and the court's reference to advice from the prior CASAC on protection against “unusually damaging years” (described in section III.B.2 above). In so doing, the Administrator considers below the extent of conceptual similarities of the 3-year average W126 index with some aspects of the derivation approach for the established E-R functions, the context of RBL as a proxy (as recognized above), and limitations associated with a reliance solely on W126 index as a metric to control exposures that might be termed “unusually damaging.”

With regard to the established E-R functions used to describe the relationship of RBL with O₃ in terms of a seasonal W126 index, the Administrator recognizes that the E-R functions were derived mathematically from studies of different exposure durations (varying from shorter than one to multiple growing seasons) by applying adjustments so that they would yield estimates normalized to the same period of time (season), such that the estimates may represent average impact for a season, as summarized in section III.A.1.c(ii) above (PA, section 4.5.1.2, Appendix 4A, Attachment 1). He notes the compatibility of W126 index averaged over multiple growing seasons or years with these adjustments. He also notes the exposure levels represented in the data underlying the E-R functions are somewhat limited with regard to the relatively lower cumulative exposure levels most commonly associated with the current standard (e.g., at or below 17 ppm-hrs), indicating additional uncertainty for application to such levels. Further, he notes the PA observation that some of the underlying studies did not find statistically significant effects of O₃ at the lower exposure levels, indicating some uncertainty in predictions of an O₃-related RBL at those levels, as summarized in section III.A.1.c(ii) above (PA, section 4.5.1.2). He additionally notes the differing patterns of hourly concentrations of the elevated exposure levels in the datasets from which the E-R functions from the patterns in ambient air meeting the current standard across the U.S. today, as summarized in section III.B.2.b(ii). With these considerations regarding the E-R functions and their underlying datasets in mind, he also takes note of year-to-year variability of factors other than O₃ exposures that affect tree growth in the natural environment (e.g., related to variability in soil moisture, meteorological, plant-related and other factors), that have the potential to affect O₃ E-R relationships (ISA, Appendix 8, section 3.12; 2013 ISA section 9.4.8.3; PA, sections 4.3 and 4.5). Based on these considerations, the Administrator finds there to be a consistency of his use of the W126 index averaged over multiple years with the approach used in deriving the E-R function, and with other factors that may affect growth in the natural environment.

In light of such considerations, the Administrator agrees with the PA finding that several factors contribute uncertainty and some resulting imprecision or inexactitude to RBL estimated from single-year seasonal W126 index values, as discussed in section III.D.1.b(ii) of the proposal (85 FR 49900-01, August 14, 2020; PA sections 4.5.1.2 and 4.5.3). The Administrator additionally recognizes the qualitative and conceptual nature of our understanding, in many cases, of relationships of O₃ effects on plant growth and productivity with larger-scale impacts, such as those on populations, communities and ecosystems. From these considerations, he judges that use of a seasonal RBL averaged over multiple years, such as a 3-year average, is reasonable, and provides a more stable and well-founded RBL estimate for his purposes as a proxy to represent the array of vegetation-related effects identified above. The Administrator additionally takes note of the CASAC advice agreeing with the EPA's focus on a 3-year average W126 for this purpose, concluding such a focus to be reasonable and scientifically sound, as summarized in section III.B.1.b above. In light of these considerations, the Administrator finds there to be support for use of an average seasonal W126 index derived from multiple years (with their representation of variability in environmental factors), concluding the use of such averaging to provide an appropriate representation of the evidence and attention to considerations summarized above. In so doing, he finds that sole reliance on single year W126 estimates for reaching judgments with regard to magnitude of O₃ related RBL and associated judgments of public welfare protection would ascribe a greater specificity and certainty to such estimates than supported by the current evidence. Rather, he finds it appropriate, for purposes of considering public welfare protection from effects for which RBL is used as a proxy, to primarily consider W126 index in terms of a 3-year average metric.

In his consideration of the appropriateness of using a 3-year average W126 metric, the Administrator additionally takes note of the discussion in section III.B.2 above with regard to protection against “unusually damaging years,” a caution raised by the prior CASAC in considering a secondary standard in terms of a 3-year average W126 index (and an issue raised in the court remand). With regard to this caution, the Administrator finds informative the discussion in section III.B.2 above regarding the extent to which a standard in terms of a W126 metric might be expected to control exposure circumstances of concern (e.g., for growth effects, among others). This discussion and its focus on air quality analyses in the PA and additional analyses conducted in consideration of public comment investigate the annual occurrence of elevated hourly O₃ concentrations which may contribute to vegetation exposures of concern (PA, Appendix 2A, section 2A.2; Wells, 2020).

These air quality analyses illustrate limitations of the W126 index for purposes of controlling peak concentrations, and also the strengths of the current standard in this regard. As discussed more fully in section III.B.2.b(ii) above, the W126 index cannot, by virtue of its definition, always differentiate between air quality patterns with high peak concentrations and those without such concentrations. This is demonstrated in the air quality analyses which show that the form and averaging time of the existing standard is much more effective than the W126 index in limiting peak concentrations (e.g., hourly O₃ concentrations at or above 100 ppb) and in limiting number of days with any such hours (Wells, 2020, e.g., Figures 4, 5, 8, 9 compared to Figures 6, 7, 10 and 11). A similar finding is evidence in the historical data extending back to 2000. These data show the appreciable reductions in peak concentrations that have been achieved in the U.S. as air quality has improved under O₃ standards of the existing form and averaging time (Wells, 2020, Figures 12 and 13). From these analyses, the Administrator concludes that the form and averaging time of the current standard is effective in controlling peak hourly concentrations and that a W126 index based standard would be much less effective in providing the needed protection against years with such elevated and potentially damaging hourly concentrations. Thus, in light of the current evidence and quantitative air quality analyses, the Administrator notes that the W126 index, by its very definition, does not provide specificity with regard to year-to-year variability in elevated hourly O₃ concentrations with the potential to contribute to the “unusually damaging years” that the prior CASAC identified for increased concern. In so doing, he disagrees with the statement of the prior CASAC that a single-year W126 index would necessarily provide protection from such years. Further, he judges that a standard based on either a 3-year or a single-year W126 index would not be expected to provide effective control of the peak concentrations that may contribute to “unusually damaging years” for vegetation.

Thus, in considering the extent of protection provided by the current standard, in addition to considering seasonal W126 averaged over a 3-year period to estimate median RBL using the established E-R functions, the Administrator finds it appropriate to also consider other metrics, including peak hourly concentrations. While he recognizes that the evidence does not indicate a particular threshold number of hours at or above 100 ppb (or another reference point for elevated concentrations), he takes particular note of the evidence of greater impacts from higher concentrations (particularly with increased frequency) and of the air quality analyses that document variability in such concentrations for the same W126 index value. In light of these considerations, he judges such a multipronged approach to be needed to ensure appropriate consideration of exposures of concern and the associated protection from them afforded by the secondary standard. Thus, the Administrator concludes that use of a seasonal W126 averaged over a 3-year period, which is the design value period for the current standard, to estimate median RBL using the established E-R functions, in combination with a broader consideration of air quality patterns, such as peak hourly concentrations, is appropriate for considering the public welfare protection provided by the standard.

In the discussion above, the Administrator recognizes a number of public welfare policy judgments important to his review of the current standard that include the appropriateness of the W126 index, averaged across a 3-year period, for assessing the extent of protection afforded by the standard from cumulative seasonal O₃ exposures. In reflecting on these judgments, the current evidence presented in the ISA and the associated evaluations in the PA, the Administrator concludes that the currently available information supports such judgments, additionally noting the CASAC concurrence with regard to the scientific support for these judgments (Cox 2020a, Consensus Responses to Charge Questions p. 21). Accordingly, the Administrator concludes that the current evidence base and available information (qualitative and quantitative) continues to support consideration of the potential for O₃-related vegetation impacts in terms of the RBL estimates from established E-R functions as a quantitative tool within a larger framework of considerations pertaining to the public welfare significance of O₃ effects. Such consideration includes effects that are associated with effects on vegetation, and particularly those that conceptually relate to growth, and that are causally or likely causally related to O₃ in ambient air, yet for which there are greater uncertainties affecting estimates of impacts on public welfare. The Administrator additionally notes that this approach to weighing the available information in reaching judgments regarding the secondary standard additionally takes into account uncertainties regarding the magnitude of growth impact that might be expected in mature trees (e.g., compared to seedlings), and of related, broader, ecosystem-level effects for which the available tools for quantitative estimates are more uncertain and those for which the policy foundation for consideration of public welfare impacts is less well established.

In his consideration of the adequacy of protection provided by the current standard, the Administrator does not consider every possible instance of an effect on vegetation growth from O₃ to be adverse to public welfare, although he recognizes that, depending on factors including extent and severity, such vegetation-related effects have the potential to be adverse to public welfare. Comments from the current CASAC, in the context of its review of the draft PA, expressed the view that the strategy described by the prior Administrator for the secondary standard established in 2015 with its focus on limiting 3-year average W126 index values somewhat below those associated with a 6% RBL in the median species and associated W126 index target of 17 ppm-hrs (in terms of a 3-year average), at or below which the 2015 standard was expected to generally restrict cumulative seasonal exposure, is “still scientifically reasonable” and “still effective in particularly protecting the public welfare in light of vegetation impacts from ozone” (Cox, 2020a, Consensus Responses to Charge Questions p. 21). In light of this advice and based on the current evidence as evaluated in the PA, the Administrator judges that this approach or framework, with its focus on controlling cumulative seasonal exposures associated with an RBL of 6% or greater, by limiting air quality in terms of a 3-year average W126 index, to or below a target of 17 ppm-hrs, in combination with a broader consideration of air quality patterns, such as control of peak hourly concentrations, associated with meeting the current standard, is appropriate for his use in this review. In so doing, he additional notes the isolated, rare occurrences in locations meeting the current standard of such exposures at 19 ppm-hrs. Based on the current information to inform consideration of vegetation effects and their potential adversity to public welfare, he additionally judges that the RBL estimates associated with such marginally higher exposures in isolated, rare instances are not indicative of effects that would be adverse to the public welfare, particularly in light of variability in the array of environmental factors that can influence O₃ effects in different systems and uncertainties associated with estimates of effects associated with this magnitude of cumulative exposure in the natural environment.

With regard to O₃ effects on crop yield, the Administrator, as an initial matter, takes note of the long-standing evidence, qualitative and quantitative, of the reducing effect of O₃ on the yield of many crops, as summarized in the PA and current ISA and characterized in detail in past reviews (e.g., 2013 ISA, 2006 AQCD, 1997 AQCD, 2014 WREA). He additionally notes the established E-R functions for 10 crops and the estimates of RYL derived from them, as presented in the PA (PA, Appendix 4A, section 4A.1, Table 4A-4), and the potential public welfare significance of reductions in crop yield, as summarized in section III.A.2.b above. In so doing, however, he additionally recognizes that not every effect on crop yield will be adverse to public welfare. In the case of crops in particular there are a number of complexities related to the heavy management of many crops to obtain a particular output for commercial purposes, and related to other factors, that the Administrator takes into consideration in evaluating potential O₃-related public welfare impacts, as summarized in section III.B.2.b(iv) above (PA, sections 4.5.1.3 and 4.5.3).

Similarly, the Administrator concludes that the extensive management of agricultural crops that occurs to elicit optimum yields (e.g., through irrigation and usage of soil amendments, such as fertilizer) is relevant in evaluating the extent of RYL estimated from experimental O₃ exposures that should be judged adverse to the public welfare. He considers these opportunities in crop management for market objectives, as well as complications in judging relative adversity that relate to market responses and their effects on producers and consumers in evaluating the potential impact on public welfare of estimated crop yield losses. Further, the Administrator takes note of the conclusion of the CASAC that the available evidence does not call into question the adequacy of the current secondary standard and that it should be retained (Cox 2020a, p.1).

The Administrator also considered the public comments, discussed in section III.B.2.b(iv) above, suggesting that the proposed decision was not giving adequate consideration to crop yield effects and that his decision should consider a statement by the prior CASAC, raised in public comments, that a 5% RYL estimate, as the median based on the 10 E-R functions, “represents an adverse impact.” With regard to the prior CASAC statement, he notes the discussion in section III.B.2.b(iv) above regarding the unclear basis for the prior CASAC judgment, both with regard to a connection of an estimated 5% RYL to broader impacts and to judgments on significance of a 5% RYL to the public welfare. In considering the adequacy of protection of the public welfare from effects related to crop yield loss, the Administrator considers the air quality analyses and the W126 index levels commonly occurring in areas that meet the current standard. In so doing, he notes that W126 index values (3-year average) were at or below 17 ppm-hrs in virtually all monitoring sites with air quality meeting the current standard. Based on the established E-R functions, the median RYL estimate corresponding to 17 ppm-hrs is 5.1%. In considering single-year index values, as discussed in section III.B.2.b(vi), the vast majority are similarly low (with more than 99% less than or equal to 17 ppm-hrs), and the higher values predominantly occur in urban areas. The Administrator additionally takes note of the discussion in section III.B.2.b(ii) above regarding the role of elevated hourly concentrations in effects on vegetation growth and yield. In so doing, in addition to his consideration of W126 index occurring in areas that meet the current standard, he also takes note of the control of elevated hourly O₃ concentrations that is exerted by the current standard.

In light of all of the above, in reaching his judgment regarding public welfare implications of the W126 index values summarized here (and associated estimated RYL), including the isolated and rare occurrence of somewhat higher values, the Administrator notes that the secondary standard is not intended to protect against all known or anticipated O₃-related effects, but rather those that are judged to be adverse to the public welfare. He also takes into consideration the extensive management of agricultural crops, and the complexities associated with identifying adverse public welfare effects for market-traded goods (where producers and consumers may be impacted differently). Based on all of these factors, the Administrator disagrees with the prior CASAC statement that an estimated median RYL of 5% represents an adverse impact and further judges that an estimated median RYL of 5.1%, based on experimental exposures, would not constitute an adverse effect on public welfare. Accordingly, the Administrator notes that the current standard generally maintains air quality at a W126 index below 17 ppm-hrs, with few exceptions, and accordingly would limit the estimated RYL (based on experimental O₃ exposures) to this degree. Therefore, he concludes that the current standard provides adequate protection of public welfare related to crop yield loss and does not need to be revised to provide additional protection against this effect. In so doing, the Administrator notes the conclusions by the current CASAC that the evidence supports retaining the current standard, without revision.

Turning to consideration of visible foliar injury and protection afforded by the secondary standard from associated impacts to the public welfare, the Administrator takes note of the long-standing and well-established evidence base, updated in the ISA for this review, and of policy-relevant analyses presented in the PA to inform his judgments regarding a secondary standard that provides appropriate protection of the public welfare from this effect. In so doing, he has also taken into account issues raised by public comments, both with regard to our understanding of relationships between O₃ exposure circumstances and extent and severity of injury in natural areas across the U.S., and with regard to the extent of our understanding of the relationship of injury extent and severity to public welfare effects anticipated to be adverse, and the Murray Energy remand.

In considering public welfare implications of this effect, he notes the potential for this effect, when of a significant extent and severity, to reduce aesthetic and recreational values, such as the aesthetic value of scenic vistas in protected natural areas including national parks and wilderness areas, as well as other areas similarly protected by state and local governments for similar public uses. Based on these considerations, the Administrator recognizes that, depending on its severity and spatial extent, as well as the location(s) and the associated intended use, the impact of visible foliar injury on the physical appearance of plants has the potential to be significant to the public welfare. In this regard, he agrees with the PA statement that cases of widespread and relatively severe injury during the growing season (particularly when sustained across multiple years and accompanied by obvious impacts on the plant canopy) might reasonably be expected to have the potential to adversely impact the public welfare in scenic and/or recreational areas, particularly in areas with special protection, such as Class I areas (PA, sections 4.3.2 and 4.5.1). In so doing, the Administrator notes that the secondary standard is not meant to protect against all known or anticipated O₃-related welfare effects, but rather those that are judged to be adverse to the public welfare, and further notes that there are not established measures for when such welfare effects should be judged adverse to the public welfare. Rather, the level of protection from known or anticipated adverse effects to public welfare that is requisite for the secondary standard is a public welfare policy judgment to be made by the Administrator.

While recognizing there to be a paucity of established approaches for interpreting specific levels of severity and extent of foliar injury in natural areas with regard to impacts on the public welfare (e.g., related to recreational services), the Administrator recognizes that injury to whole stands of trees of a severity apparent to the casual observer (e.g., when viewed as a whole from a distance) would reasonably be expected to affect recreational values and thus pose a risk of adverse effects to the public welfare. He further notes that the available information does not provide for specific characterization of the incidence and severity that would not be expected to be apparent to the casual observer, nor for clear identification of the pattern of O₃ concentrations that would provide for such a situation.

In this context, the Administrator takes note of the system developed by the USFS for its monitoring program to categorize BI scores of visible foliar injury at biosites (sites with O₃-sensitive vegetation assessed for visible foliar injury) in natural vegetated areas by severity levels (described in section III.A.2.c(ii) above). While recognizing that quantitative analyses and evidence are lacking that might support a more precise conclusion with regard to a magnitude of BI score coupled with an extent of occurrence that might be specifically identified as adverse to the public welfare, the Administrator also takes note of the D.C. Circuit's holding that substantial uncertainty about the level at which visible foliar injury may become adverse to public welfare does not necessarily provide a basis for declining to evaluate whether the existing standard provides requisite protection against such effects. See Murray Energy Corp. v. EPA, 936 F.3d 597, 619-20 (D.C. Cir. 2019). In this context, he considers the discussion in the PA and in sections III.A.2.b, III.A.2.c and III.B.2 above regarding the USFS biosite monitoring program. He finds the scale of this program's objectives, which focus on natural settings in the U.S. and forests as opposed to individual plants, to be suited for his consideration with regard to the public welfare protection afforded by the current standard, and consequently, he finds the data and analyses generated by the program informative in such considerations.

In this context, he takes note of the USFS system, including its descriptors for BI scores of differing magnitude intended for that Agency's consideration in identifying areas of potential impact to forest resources. As described in section III.A.2.b(iii) above, very low BI scores (at or below 5) are described by the USFS scheme as “little or no foliar injury” (Smith et al., 2007; Smith et al., 2012). The Administrator notes that BI scores above 15 are categorized as moderate to severe (and scores above 25 as severe). In so doing, in light of considerations raised in the PA and consideration of public comment, he recognizes the lower categories of BI scores as indicative of injury of generally lesser risk to the natural area or to public enjoyment, which he judges unlikely to be indicative of injury of such a magnitude or extent as to pose risk of adverse effects to the public welfare. Thus, the Administrator reaches the conclusion that occurrence of the lower categories of BI scores does not pose concern for the public welfare, but that findings of BI scores categorized as “moderate to severe” injury by the USFS scheme would be an indication of visible foliar injury occurrence that, depending on extent and severity, may raise public welfare concerns. In this framework, the Administrator considers the PA evaluations of the currently available information and what it indicates with regard to patterns of air quality of concern for such an occurrence, and the extent to which they are expected to occur in areas that meet the current standard.

In so doing, the Administrator takes particular note of the USFS biosite monitoring program studies of the occurrence, extent and severity of visible foliar injury in indicator species in defined plots or biosites in natural areas across the U.S. These studies of data for USFS biosites (sites with O₃-sensitive vegetation assessed for visible foliar injury) have often summarized O₃ concentrations in terms of cumulative exposure metrics (e.g., SUM06 or W126 index). Some of these studies, particularly those examining such data across multiple years and multiple regions of the U.S., have reported that variation in cumulative O₃ exposure, in terms of such metrics, does not completely explain the patterns of occurrence and severity of injury observed. Although the availability of detailed analyses that have explored multiple exposure metrics and other influential variables is limited, multiple studies have indicated a potential role for an additional metric, one related to the occurrence of days with relatively high concentrations (e.g., number of days with a 1-hour concentration at or above 100 ppb), as summarized in section III.A.2.c above (PA, section 4.5.1.2). Thus, the Administrator takes note of this evidence indicating an influence of peak concentrations on BI scores (beyond an influence of cumulative exposure). He also finds noteworthy the extensive evidence of trends across the past nearly 20 years that indicate reductions in severity of visible foliar injury that parallel reductions in peak concentrations that have been suggested to be influential in the severity of visible foliar injury.

Further, the Administrator considers the PA analysis of a dataset developed from USFS biosite index scores, combined with W126 estimates and soil moisture categories, summarized in section III.A.2.c above. In so doing, he takes note of the PA observation that important uncertainties remain in the understanding of the O₃ exposure conditions that will elicit visible foliar injury of varying severity and extent in natural areas, and particularly in light of the other environmental variables that influence its occurrence, and of the recognition by the CASAC that “uncertainties continue to hamper efforts to quantitatively characterize the relationship of [visible foliar injury] occurrence and relative severity with ozone exposures” (Cox 2020a, Consensus Responses to Charge Questions, p. 20). Notwithstanding, and while being mindful of, such uncertainties with regard to predictive O₃ metric or metrics and a quantitative function relating them to incidence and severity of visible foliar injury in natural areas (as also noted in the USFS studies referenced above), the Administrator takes note of the PA finding that the incidence of nonzero BI scores, and, particularly of relatively higher scores (such as scores above 15 which are indicative of “moderate to severe” injury in the USFS scheme) appears to markedly increase only with W126 index values above 25 ppm-hrs, as summarized in section III.B.2.b above (PA, section 4.3.3 and Appendix 4C).

In light of these observations, the Administrator finds the current evidence to be incomplete with regard to information to support a quantitative characterization of air quality that would be anticipated to result in visible foliar injury of an extent and severity to cause adverse effects to the public welfare. The Administrator also considers discussion in the court's remand of the 2015 standard with regard to visible foliar injury (Murray Energy Corp. v EPA, 936 F.3d at 619-20). The court concluded that the EPA had failed to offer a reasoned explanation for deciding not to specify a level of air quality to protect against adverse effects related to visible foliar injury. In particular, the court stated that the EPA had not explained why it was unable to choose such a level although the prior CASAC had provided advice with regard to a specific level. The EPA's disagreement with the prior CASAC on its identified level is explained in section III.B.2 above, as is the reason why the EPA did not find the analysis on which the prior CASAC based its advice to be appropriate for such a conclusion. This and other associated issues raised by the court have been raised in public comments on the proposal for this action and are addressed in section III.B.2 above.

Based on the evidence and quantitative analyses available in the present review, and advice from the current CASAC, the Administrator considers the question of a level of air quality that would provide protection against visible foliar injury related effects known or anticipated to cause adverse effects to the public welfare. Based on the evidence and associated quantitative analyses in this review, the Administrator's judgment reflects his recognition of less confidence and greater uncertainty in the existence of adverse public welfare effects with lower O₃ exposures. In this context, the Administrator judges that W126 index values at or below 25 ppm-hrs, when in combination with infrequent occurrences of hourly concentrations at or above 100 ppb, would not be anticipated to pose risk of visible foliar injury of an extent and severity so as to be adverse to the public welfare.

With these conclusions in mind, the Administrator considers the air quality analyses presented in the PA and the additional analyses developed in response to public comment. In so doing, he notes that a W126 index above 25 ppm-hrs (either as a 3-year average or in a single year) is not seen to occur at monitoring locations (including in or near Class I areas) where the current standard is met, and that, in fact, values above 17 or 19 ppm-hrs are rare, as summarized in section III.A.3 above (PA, Appendix 4C, section 4C.3; Appendix 4D, section 4D.3.2.3). Further, the Administrator takes note of the PA consideration of the USFS publications that identify an influence of peak concentrations on BI scores (beyond an influence of cumulative exposure) and the PA observation of the appreciable control of peak concentrations exerted by the form and averaging time of the current standard, as evidenced by the air quality analyses which document reductions in 1-hour daily maximum concentrations with declining design values. He also notes, as discussed above, the uncommonness of days with any hours at or above 100 ppb at monitoring sites that meet the current standard, as well as the minimal number of hours on any such days (Wells, 2020). Based on these considerations, the Administrator concludes that the current standard provides control of air quality conditions that contribute to increased BI scores and to scores of a magnitude indicative of “moderate to severe” foliar injury.

The Administrator further takes note of the PA finding that the current information, particularly in locations meeting the current standard or with W126 index estimates likely to occur under the current standard, does not indicate a significant extent and degree of injury (e.g., based on analyses of BI scores in the PA, Appendix 4C) or specific impacts on recreational or related services for areas, such as wilderness areas or national parks. Thus, he gives credence to the associated PA conclusion that the evidence indicates that areas that meet the current standard are unlikely to have BI scores reasonably considered to be impacts of public welfare significance. Based on all of the considerations raised here, the Administrator concludes that the current standard provides sufficient protection of natural areas, including particularly protected areas such as Class I areas, from O₃ concentrations in the ambient air that might be expected to elicit visible foliar injury of such an incidence and severity as would reasonably be judged adverse to the public welfare.

With a primary focus on RBL in its role as proxy, the Administrator further considers the analyses available in this review of recent air quality at sites across the U.S., particularly including those sites in or near Class I areas, and also the analyses of historical air quality. In so doing, the Administrator recognizes that these analyses are distributed across all nine NOAA climate regions and 50 states, although some geographic areas within specific regions and states may be more densely covered and represented by monitors than others, as summarized in section III.C of the proposal (PA, Appendix 4D). The Administrator notes that the findings from both the analysis of the air quality data from the most recent period and from the larger analysis of historical air quality data extending back to 2000, as presented in the PA and summarized in section III.A.3 above, are consistent with the air quality analyses available in the last review. That is, in virtually all design value periods and all locations at which the current standard was met across the 19 years and 17 design value periods (in more than 99.9% of such observations), the 3-year average W126 metric was at or below 17 ppm-hrs. Further, in all such design value periods and locations the 3-year average W126 index was at or below 19 ppm-hrs. The Administrator additionally considers the protection provided by the current standard from the occurrence of O₃ exposures within a single year with potentially damaging consequences, such as a significantly increased incidence of areas with visible foliar injury that might be judged moderate to severe, as discussed in section III.B.2 above. In so doing, he takes notes of the PA analyses, summarized in section III.A.2.c above, of USFS BI scores, giving particular focus to scores above 15, termed “moderate to severe injury” by the USFS categorization scheme, as described in section III.A.2.b above (PA, sections 4.3.3.2, 4.5.1.2 and Appendix 4C). He notes the PA finding that incidence of sites with BI scores above 15 markedly increases with W126 index estimates above 25 ppm-hrs. In this context, he additionally takes note of the air quality analysis finding of a scarcity of single-year W126 index values above 25 ppm-hrs at sites that meet the current standard, with just a single occurrence across all U.S. sites with design values meeting the current standard in the 19-year historical dataset dating back to 2000 (PA, section 4.4 and Appendix 4D). Further, in light of the evidence indicating that peak short-term concentrations (e.g., of durations as short as one hour) may also play a role in the occurrence of visible foliar injury, the Administrator additionally takes note of the air quality analyses in the PA and in the additional analysis documented in Wells (2020). These analyses of data from the past 20 years show a declining trend in 1-hour daily maximum concentrations mirroring the declining trend in design values, supporting the PA conclusion that the form and averaging time of the current standard provides appreciable control of peak 1-hour concentrations. Furthermore, these analyses indicate there to be only a few days among sites meeting the current standard, with hourly concentrations at or above 100 ppb (just seven in the period from 2000 through 2018) (Wells, 2020). In light of these findings from the air quality analyses and considerations in the PA, both with regard to 3-year average W126 index values at sites meeting the current standard and the rarity of such values at or above 19 ppm-hrs, and with regard to single-year W126 index values at sites meeting the current standard, and the rarity of such values above 25 ppm-hrs, as well as with regard to the appreciable control of 1-hour daily maximum concentrations, the Administrator judges that the current standard provides adequate protection from air quality conditions with the potential to be adverse to the public welfare.

In reaching his conclusion on the current secondary O₃ standard, the Administrator recognizes, as is the case in NAAQS reviews in general, his decision depends on a variety of factors, including science policy judgments and public welfare policy judgments, as well as the currently available information. With regard to the current review, the Administrator gives primary attention to the principal effects of O₃ as recognized in the current ISA, the 2013 ISA and past AQCDs, and for which the evidence is strongest (e.g., growth, reproduction, and related larger-scale effects, as well as, visible foliar injury). With regard to growth and the categories of effects identified above for which RBL has been identified for use as a proxy, based on all of the considerations above, including the discussion of air quality immediately above, the Administrator judges the current standard to provide adequate protection for air quality conditions with the potential to be adverse to the public welfare. Further, with regard to visible foliar injury, the Administrator concludes that the currently available information on visible foliar injury and with regard to air quality analyses that may be informative with regard to air quality conditions associated with appreciably increased incidence and severity of BI scores at USFS biomonitoring sites, and with particular attention to Class I and other areas afforded special protection, indicates the current standard to provide adequate protection from visible foliar injury of an extent or severity that might be anticipated to be adverse to the public welfare.

In summary, the Administrator has based his decision on the public welfare protection afforded by the secondary O₃ standard from identified O₃-related welfare effects, and from their potential to present adverse effects to the public welfare, on judgments regarding what the available evidence, quantitative information, and associated uncertainties and limitations (such as those identified above) indicate with regard to the protection provided from the array of O₃ welfare effects. He finds that, as a whole, this information, as summarized above, and presented in detail in the ISA and PA, does not indicate the current standard to allow air quality conditions with implications of concern for the public welfare. He has additionally considered the advice from the CASAC in this review, including its finding “that the available evidence does not reasonably call into question the adequacy of the current secondary ozone standard and concurs that it should be retained” (Cox, 2020a, p. 1), and well as public comment on the proposed decision. Based on all of the above considerations, including his consideration of the currently available evidence and quantitative exposure/risk information, the Administrator concludes that the current secondary standard is requisite to protect the public welfare from known or anticipated adverse effects of O₃ and related photochemical oxidants in ambient air, and thus that the current standard should be retained, without revision.

C. Decision on the Secondary Standard

For the reasons discussed above and taking into account information and assessments presented in the ISA and PA, the advice from the CASAC, and consideration of public comments, the Administrator concludes that the current secondary O₃ standard is requisite to protect the public welfare from known or anticipated adverse effects, and is retaining the current standard without revision.

IV. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

The Office of Management and Budget (OMB) has determined that this action is a significant regulatory action and it was submitted to OMB for review. Any changes made in response to OMB recommendations have been documented in the docket. Because this action does not change the existing O₃ NAAQS, it does not impose costs or benefits relative to the baseline of continuing with the current NAAQS in effect. EPA has thus not prepared a Regulatory Impact Analysis for this action.

B. Executive Order 13771: Reducing Regulations and Controlling Regulatory Costs

This action is not an Executive Order 13771 regulatory action. There are no quantified cost estimates for this action because EPA is retaining the current standards.

C. Paperwork Reduction Act (PRA)

This action does not impose an information collection burden under the PRA. There are no information collection requirements directly associated with a decision to retain a NAAQS without any revision under section 109 of the CAA, and this action retains the existing O₃ NAAQS without any revisions.

D. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA. This action will not impose any requirements on small entities. Rather, this action retains, without revision, existing national standards for allowable concentrations of O₃ in ambient air as required by section 109 of the CAA. See also American Trucking Associations v. EPA, 175 F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant impacts upon small entities because NAAQS themselves impose no regulations upon small entities), rev'd in part on other grounds, Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

E. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect small governments. This action imposes no enforceable duty on any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government.

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

This action does not have tribal implications, as specified in Executive Order 13175. It does not have a substantial direct effect on one or more Indian Tribes. This action does not change existing regulations; it retains the existing O₃ NAAQS, without revision. Executive Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

This action is not subject to Executive Order 13045 because it is not economically significant as defined in Executive Order 12866. The health effects evidence and risk assessment information for this action, which focuses on children and people (of all ages) with asthma as key at-risk populations, is summarized in section II.A.2 and II.A.3 above and described in the ISA and PA, copies of which are in the public docket for this action.

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution or Use

This action is not a “significant energy action” for purposes of Executive Order 13211. The action is not likely to have a significant adverse effect on the supply, distribution, or use of energy. This action retains the current O₃ NAAQS. This decision does not change existing requirements. The Administrator of the Office of Information and Regulatory Affairs has not otherwise designated this action as a significant energy action. Thus, this decision does not constitute a significant energy action as defined in Executive Order 13211.

J. National Technology Transfer and Advancement Act

This action does not involve technical standards.

K. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

The EPA believes that this action does not have disproportionately high and adverse human health or environmental effects on minority, low-income populations and/or indigenous peoples, as specified in Executive Order 12898 (59 FR 7629, February 16, 1994). The action described in this document is to retain without revision the existing O₃ NAAQS based on the Administrator's conclusions that the existing primary standard protects public health, including the health of sensitive groups, with an adequate margin of safety, and that the existing secondary standard protects public welfare from known or anticipated adverse effects. As discussed in section II above, the EPA expressly considered the available information regarding health effects among at-risk populations in reaching the decision that the existing standard is requisite.

L. Determination Under Section 307(d)

Section 307(d)(1)(V) of the CAA provides that the provisions of section 307(d) apply to “such other actions as the Administrator may determine.” Pursuant to section 307(d)(1)(V), the Administrator determines that this action is subject to the provisions of section 307(d).

M. Congressional Review Act

The EPA will submit a rule report to each House of the Congress and to the Comptroller General of the United States. This action is not a “major rule” as defined by 5 U.S.C. 804(2).

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List of Subjects in 40 CFR Part 50

• Environmental protection
• Air pollution control
• Carbon monoxide
• Lead
• Nitrogen dioxide
• Ozone
• Particulate matter
• Sulfur oxides