Occupational Exposure to Respirable Crystalline Silica

AGENCY:

Occupational Safety and Health Administration (OSHA), Department of Labor.

ACTION:

Proposed rule; request for comments.

SUMMARY:

The Occupational Safety and Health Administration (OSHA) proposes to amend its existing standards for occupational exposure to respirable crystalline silica. The basis for issuance of this proposal is a preliminary determination by the Assistant Secretary of Labor for Occupational Safety and Health that employees exposed to respirable crystalline silica face a significant risk to their health at the current permissible exposure limits and that promulgating these proposed standards will substantially reduce that risk.

This document proposes a new permissible exposure limit, calculated as an 8-hour time-weighted average, of 50 micrograms of respirable crystalline silica per cubic meter of air (50 μg/m³). OSHA also proposes other ancillary provisions for employee protection such as preferred methods for controlling exposure, respiratory protection, medical surveillance, hazard communication, and recordkeeping. OSHA is proposing two separate regulatory texts—one for general industry and maritime, and the other for construction—in order to tailor requirements to the circumstances found in these sectors.

DATES:

Written comments. Written comments, including comments on the information collection determination described in Section IX of the preamble (OMB Review under the Paperwork Reduction Act of 1995), must be submitted (postmarked, sent, or received) by December 11, 2013.

Informal public hearings. The Agency plans to hold informal public hearings beginning on March 4, 2014, in Washington, DC. OSHA expects the hearings to last from 9:30 a.m. to 5:30 p.m., local time; a schedule will be released prior to the start of the hearings. The exact daily schedule may be amended at the discretion of the presiding administrative law judge (ALJ). If necessary, the hearings will continue at the same time on subsequent days. Peer reviewers of OSHA's Health Effects Literature Review and Preliminary Quantitative Risk Assessment will be present in Washington, DC to hear testimony on the second day of the hearing, March 5, 2014; see Section XV for more information on the peer review process.

Notice of intention to appear at the hearings. Interested persons who intend to present testimony or question witnesses at the hearings must submit (transmit, send, postmark, deliver) a notice of their intention to do so by November 12, 2013. The notice of intent must indicate if the submitter requests to present testimony in the presence of the peer reviewers.

Hearing testimony and documentary evidence. Interested persons who request more than 10 minutes to present testimony, or who intend to submit documentary evidence, at the hearings must submit (transmit, send, postmark, deliver) the full text of their testimony and all documentary evidence by December 11, 2013. See Section XV below for details on the format and how to file a notice of intention to appear, submit documentary evidence at the hearing, and request an appropriate amount of time to present testimony.

ADDRESSES:

Written comments. You may submit comments, identified by Docket No. OSHA-2010-0034, by any of the following methods:

Electronically: You may submit comments and attachments electronically at http://www.regulations.gov,, which is the Federal e-Rulemaking Portal. Follow the instructions on-line for making electronic submissions.

Fax: If your submissions, including attachments, are not longer than 10 pages, you may fax them to the OSHA Docket Office at (202) 693-1648.

Mail, hand delivery, express mail, messenger, or courier service: You must submit your comments to the OSHA Docket Office, Docket No. OSHA-2010-0034, U.S. Department of Labor, Room N-2625, 200 Constitution Avenue NW., Washington, DC 20210, telephone (202) 693-2350 (OSHA's TTY number is (877) 889-5627). Deliveries (hand, express mail, messenger, or courier service) are accepted during the Department of Labor's and Docket Office's normal business hours, 8:15 a.m.-4:45 p.m., E.T.

Instructions: All submissions must include the Agency name and the docket number for this rulemaking (Docket No. OSHA-2010-0034). All comments, including any personal information you provide, are placed in the public docket without change and may be made available online at http://www.regulations.gov. Therefore, OSHA cautions you about submitting personal information such as social security numbers and birthdates.

If you submit scientific or technical studies or other results of scientific research, OSHA requests (but is not requiring) that you also provide the following information where it is available: (1) Identification of the funding source(s) and sponsoring organization(s) of the research; (2) the extent to which the research findings were reviewed by a potentially affected party prior to publication or submission to the docket, and identification of any such parties; and (3) the nature of any financial relationships (e.g., consulting agreements, expert witness support, or research funding) between investigators who conducted the research and any organization(s) or entities having an interest in the rulemaking. If you are submitting comments or testimony on the Agency's scientific and technical analyses, OSHA requests that you disclose: (1) The nature of any financial relationships you may have with any organization(s) or entities having an interest in the rulemaking; and (2) the extent to which your comments or testimony were reviewed by an interested party prior to its submission. Disclosure of such information is intended to promote transparency and scientific integrity of data and technical information submitted to the record. This request is consistent with Executive Order 13563, issued on January 18, 2011, which instructs agencies to ensure the objectivity of any scientific and technological information used to support their regulatory actions. OSHA emphasizes that all material submitted to the rulemaking record will be considered by the Agency to develop the final rule and supporting analyses.

Informal public hearings. The Washington, DC hearing will be held in the auditorium of the U.S. Department of Labor, 200 Constitution Avenue NW., Washington, DC 20210.

Notice of intention to appear, hearing testimony and documentary evidence. You may submit (transmit, send, postmark, deliver) your notice of intention to appear, hearing testimony, and documentary evidence, identified by docket number (OSHA-2010-0034), by any of the following methods:

Electronically: http://www.regulations.gov. Follow the instructions online for electronic submission of materials, including attachments.

Fax: If your written submission does not exceed 10 pages, including attachments, you may fax it to the OSHA Docket Office at (202) 693-1648.

Regular mail, express delivery, hand delivery, and messenger and courier service: Submit your materials to the OSHA Docket Office, Docket No. OSHA-2010-0034, U.S. Department of Labor, Room N-2625, 200 Constitution Avenue NW., Washington, DC 20210; telephone (202) 693-2350 (TTY number (877) 889-5627). Deliveries (express mail, hand delivery, and messenger and courier service) are accepted during the Department of Labor's and OSHA Docket Office's normal hours of operation, 8:15 a.m. to 4:45 p.m., ET.

Instructions: All submissions must include the Agency name and docket number for this rulemaking (Docket No. OSHA-2010-0034). All submissions, including any personal information, are placed in the public docket without change and may be available online at http://www.regulations.gov. Therefore, OSHA cautions you about submitting certain personal information, such as social security numbers and birthdates. Because of security-related procedures, the use of regular mail may cause a significant delay in the receipt of your submissions. For information about security-related procedures for submitting materials by express delivery, hand delivery, messenger, or courier service, please contact the OSHA Docket Office. For additional information on submitting notices of intention to appear, hearing testimony or documentary evidence, see Section XV of this preamble, Public Participation.

Docket: To read or download comments, notices of intention to appear, and materials submitted in response to this Federal Register notice, go to Docket No. OSHA-2010-0034 at http://www.regulations.gov or to the OSHA Docket Office at the address above. All comments and submissions are listed in the http://www.regulations.gov index; however, some information (e.g., copyrighted material) is not publicly available to read or download through that Web site. All comments and submissions are available for inspection and, where permissible, copying at the OSHA Docket Office.

Electronic copies of this Federal Register document are available at http://regulations.gov. Copies also are available from the OSHA Office of Publications, Room N-3101, U.S. Department of Labor, 200 Constitution Avenue NW., Washington, DC 20210; telephone (202) 693-1888. This document, as well as news releases and other relevant information, is also available at OSHA's Web site at http://www.osha.gov.

FOR FURTHER INFORMATION CONTACT:

For general information and press inquiries, contact Frank Meilinger, Director, Office of Communications, Room N-3647, OSHA, U.S. Department of Labor, 200 Constitution Avenue NW., Washington, DC 20210; telephone (202) 693-1999. For technical inquiries, contact William Perry or David O'Connor, Directorate of Standards and Guidance, Room N-3718, OSHA, U.S. Department of Labor, 200 Constitution Avenue NW., Washington, DC 20210; telephone (202) 693-1950 or fax (202) 693-1678. For hearing inquiries, contact Frank Meilinger, Director, Office of Communications, Room N-3647, OSHA, U.S. Department of Labor, 200 Constitution Avenue NW., Washington, DC 20210; telephone (202) 693-1999; email meilinger.francis2@dol.gov.

SUPPLEMENTARY INFORMATION:

The preamble to the proposed standard on occupational exposure to respirable crystalline silica follows this outline:

I. Issues

II. Pertinent Legal Authority

III. Events Leading to the Proposed Standards

IV. Chemical Properties and Industrial Uses

V. Health Effects Summary

VI. Summary of the Preliminary Quantitative Risk Assessment

VII. Significance of Risk

VIII. Summary of the Preliminary Economic Analysis and Initial Regulatory Flexibility Analysis

IX. OMB Review Under the Paperwork Reduction Act of 1995

X. Federalism

XI. State Plans

XII. Unfunded Mandates

XIII. Protecting Children From Environmental Health and Safety Risks

XIV. Environmental Impacts

XV. Public Participation

XVI. Summary and Explanation of the Standards

(a) Scope and Application

(b) Definitions

(c) Permissible Exposure Limit (PEL)

(d) Exposure Assessment

(e) Regulated Areas and Access Control

(f) Methods of Compliance

(g) Respiratory Protection

(h) Medical Surveillance

(i) Communication of Respirable Crystalline Silica Hazards to Employees

(j) Recordkeeping

(k) Dates

XVII. References

XVIII. Authority and Signature

OSHA currently enforces permissible exposure limits (PELs) for respirable crystalline silica in general industry, construction, and shipyards. These PELs were adopted in 1971, shortly after the Agency was created, and have not been updated since then. The PEL for quartz (the most common form of crystalline silica) in general industry is a formula that is approximately equivalent to 100 micrograms per cubic meter of air (μg/m³) as an 8-hour time-weighted average. The PEL for quartz in construction and shipyards is a formula based on a now-obsolete particle count sampling method that is approximately equivalent to 250 μg/m³. The current PELs for two other forms of crystalline silica (cristobalite and tridymite) are one-half of the values for quartz in general industry. OSHA is proposing a new PEL for respirable crystalline silica (quartz, cristobalite, and tridymite) of 50 μg/m³ in all industry sectors covered by the rule. OSHA is also proposing other elements of a comprehensive health standard, including requirements for exposure assessment, preferred methods for controlling exposure, respiratory protection, medical surveillance, hazard communication, and recordkeeping.

OSHA's proposal is based on the requirements of the Occupational Safety and Health Act (OSH Act) and court interpretations of the Act. For health standards issued under section 6(b)(5) of the OSH Act, OSHA is required to promulgate a standard that reduces significant risk to the extent that it is technologically and economically feasible to do so. See Section II of this preamble, Pertinent Legal Authority, for a full discussion of OSHA legal requirements.

OSHA has conducted an extensive review of the literature on adverse health effects associated with exposure to respirable crystalline silica. The Agency has also developed estimates of the risk of silica-related diseases assuming exposure over a working lifetime at the proposed PEL and action level, as well as at OSHA's current PELs. These analyses are presented in a background document entitled “Respirable Crystalline Silica—Health Effects Literature Review and Preliminary Quantitative Risk Assessment” and are summarized in this preamble in Section V, Health Effects Summary, and Section VI, Summary of OSHA's Preliminary Quantitative Risk Assessment, respectively. The available evidence indicates that employees exposed to respirable crystalline silica well below the current PELs are at increased risk of lung cancer mortality and silicosis mortality and morbidity. Occupational exposures to respirable crystalline silica also may result in the development of kidney and autoimmune diseases and in death from other nonmalignant respiratory diseases, including chronic obstructive pulmonary disease (COPD). As discussed in Section VII, Significance of Risk, in this preamble, OSHA preliminarily finds that worker exposure to respirable crystalline silica constitutes a significant risk and that the proposed standard will substantially reduce this risk.

Section 6(b) of the OSH Act requires OSHA to determine that its standards are technologically and economically feasible. OSHA's examination of the technological and economic feasibility of the proposed rule is presented in the Preliminary Economic Analysis and Initial Regulatory Flexibility Analysis (PEA), and is summarized in Section VIII of this preamble. For general industry and maritime, OSHA has preliminarily concluded that the proposed PEL of 50 μg/m³ is technologically feasible for all affected industries. For construction, OSHA has preliminarily determined that the proposed PEL of 50 μg/m³ is feasible in 10 out of 12 of the affected activities. Thus, OSHA preliminarily concludes that engineering and work practices will be sufficient to reduce and maintain silica exposures to the proposed PEL of 50 μg/m³ or below in most operations most of the time in the affected industries. For those few operations within an industry or activity where the proposed PEL is not technologically feasible even when workers use recommended engineering and work practice controls, employers can supplement controls with respirators to achieve exposure levels at or below the proposed PEL.

OSHA developed quantitative estimates of the compliance costs of the proposed rule for each of the affected industry sectors. The estimated compliance costs were compared with industry revenues and profits to provide a screening analysis of the economic feasibility of complying with the revised standard and an evaluation of the potential economic impacts. Industries with unusually high costs as a percentage of revenues or profits were further analyzed for possible economic feasibility issues. After performing these analyses, OSHA has preliminarily concluded that compliance with the requirements of the proposed rule would be economically feasible in every affected industry sector.

OSHA directed Inforum—a not-for-profit corporation (based at the University of Maryland) well recognized for its macroeconomic modeling—to run its LIFT (Long-term Interindustry Forecasting Tool) model of the U.S. economy to estimate the industry and aggregate employment effects of the proposed silica rule. Inforum developed estimates of the employment impacts over the ten-year period from 2014-2023 by feeding OSHA's year-by-year and industry-by-industry estimates of the compliance costs of the proposed rule into its LIFT model. Based on the resulting Inforum estimates of employment impacts, OSHA has preliminarily concluded that the proposed rule would have a negligible—albeit slightly positive—net impact on aggregate U.S. employment.

OSHA believes that a new PEL, expressed as a gravimetric measurement of respirable crystalline silica, will improve compliance because the PEL is simple and relatively easy to understand. In comparison, the existing PELs require application of a formula to account for the crystalline silica content of the dust sampled and, in the case of the construction and shipyard PELs, a conversion from particle count to mg/m³ as well. OSHA also expects that the approach to methods of compliance for construction operations included in this proposal will improve compliance with the standard. This approach, which specifies exposure control methods for selected construction operations, gives employers a simple option to identify the control measures that are appropriate for these operations. Alternately, employers could conduct exposure assessments to determine if worker exposures are in compliance with the PEL. In either case, the proposed rule would provide a basis for ensuring that appropriate measures are in place to limit worker exposures.

The Regulatory Flexibility Act, as amended by the Small Business Regulatory Enforcement Fairness Act (SBREFA), requires that OSHA either certify that a rule would not have a significant economic impact on a substantial number of small firms or prepare a regulatory flexibility analysis and hold a Small Business Advocacy Review (SBAR) Panel prior to proposing the rule. OSHA has determined that a regulatory flexibility analysis is needed and has provided this analysis in Section VIII.G of this preamble. OSHA also previously held a SBAR Panel for this rule. The recommendations of the Panel and OSHA's response to them are summarized in Section VIII.G of this preamble.

Executive Orders 13563 and 12866 direct agencies to assess all costs and benefits of available regulatory alternatives. Executive Order 13563 emphasizes the importance of quantifying both costs and benefits, of reducing costs, of harmonizing rules, and of promoting flexibility. This rule has been designated an economically significant regulatory action under section 3(f)(1) of Executive Order 12866. Accordingly, the rule has been reviewed by the Office of Management and Budget, and the remainder of this section summarizes the key findings of the analysis with respect to costs and benefits of the rule and then presents several possible alternatives to the rule.

Table SI-1—which, like all the tables in this section, is derived from material presented in Section VIII of this preamble—provides a summary of OSHA's best estimate of the costs and benefits of the proposed rule using a discount rate of 3 percent. As shown, the proposed rule is estimated to prevent 688 fatalities and 1,585 silica-related illnesses annually once it is fully effective, and the estimated cost of the rule is $637 million annually. Also as shown in Table SI-1, the discounted monetized benefits of the proposed rule are estimated to be $5.3 billion annually, and the proposed rule is estimated to generate net benefits of $4.6 billion annually. These estimates are for informational purposes only and have not been used by OSHA as the basis for its decision concerning the choice of a PEL or of other ancillary requirements for this proposed silica rule. The courts have ruled that OSHA may not use benefit-cost analysis or a criterion of maximizing net benefits as a basis for setting OSHA health standards.

Both the costs and benefits of Table SI-1 reflect the incremental costs and benefits associated with achieving full compliance with the proposed rule. They do not include (a) costs and benefits associated with current compliance that have already been achieved with regard to the new requirements, or (b) costs and benefits associated with achieving compliance with existing requirements, to the extent that some employers may currently not be fully complying with applicable regulatory requirements. They also do not include costs or benefits associated with relatively rare, extremely high exposures that can lead to acute silicosis.

Subsequent to completion of the PEA, OSHA identified an industry, hydraulic fracturing, that would be impacted by the proposed standard. Hydraulic fracturing, sometimes called “fracking,” is a process used to extract natural gas and oil deposits from shale and other tight geologic formations. A recent cooperative study by the National Institute for Occupational Safety and Health (NIOSH) and industry partners identified overexposures to silica among workers conducting hydraulic fracturing operations. An industry focus group has been working with OSHA and NIOSH to disseminate information about this hazard, share best practices, and develop engineering controls to limit worker exposures to silica. OSHA finds that there are now sufficient data to provide the main elements of the economic analysis for this rapidly growing industry and has done so in Appendix A to the PEA.

Based on recent data from the U.S. Census Bureau and industry sources, OSHA estimates that roughly 25,000 workers in 444 establishments (operated by 200 business entities) in hydraulic fracturing would be affected by the proposed standard. Annual benefits of the proposed 50 μg/m³ PEL include approximately 12 avoided fatalities—2.9 avoided lung cancers (mid-point estimate), 6.3 prevented non-cancer respiratory illnesses, and 2.3 prevented cases of renal failure—and 40.8 avoided cases of silicosis morbidity. Monetized benefits are expected to range from $75.1 million at a seven percent discount rate to $105.4 million at a three percent discount rate to undiscounted benefits of $140.3 million. OSHA estimates that under the proposed standard, annualized compliance costs for the hydraulic fracturing industry will total $28.6 million at a discount rate of 7 percent or $26.4 million at a discount rate of 3 percent.

In addition to the proposed rule itself, this preamble discusses several regulatory alternatives to the proposed OSHA silica standard. These are presented below as well as in Section VIII of this preamble. OSHA believes that this presentation of regulatory alternatives serves two important functions. The first is to explore the possibility of less costly ways (than the proposed rule) to provide an adequate level of worker protection from exposure to respirable crystalline silica. The second is tied to the Agency's statutory requirement, which underlies the proposed rule, to reduce significant risk to the extent feasible. If, based on evidence presented during notice and comment, OSHA is unable to justify its preliminary findings of significant risk and feasibility as presented in this preamble to the proposed rule, the Agency must then consider regulatory alternatives that do satisfy its statutory obligations.

Each regulatory alternative presented here is described and analyzed relative to the proposed rule. Where appropriate, the Agency notes whether the regulatory alternative, to be a legitimate candidate for OSHA consideration, requires evidence contrary to the Agency's findings of significant risk and feasibility. To facilitate comment, the regulatory alternatives have been organized into four categories: (1) Alternative PELs to the proposed PEL of 50 μg/m³; (2) regulatory alternatives that affect proposed ancillary provisions; (3) a regulatory alternative that would modify the proposed methods of compliance; and (4) regulatory alternatives concerning when different provisions of the proposed rule would take effect.

In addition, OSHA would like to draw attention to one possible modification to the proposed rule, involving methods of compliance, that the Agency would not consider to be a legitimate regulatory alternative: To permit the use of respiratory protection as an alternative to engineering and work practice controls as a primary means to achieve the PEL.

As described in Section XVI of the preamble, Summary and Explanation of the Proposed Standards, OSHA is proposing to require primary reliance on engineering controls and work practices because reliance on these methods is consistent with long-established good industrial hygiene practice, with the Agency's experience in ensuring that workers have a healthy workplace, and with the Agency's traditional adherence to a hierarchy of preferred controls. The Agency's adherence to the hierarchy of controls has been successfully upheld by the courts (see AFL-CIO v. Marshall, 617 F.2d 636 (D.C. Cir. 1979) (cotton dust standard); United Steelworkers v. Marshall, 647 F.2d 1189 (D.C. Cir. 1980), cert. denied, 453 U.S. 913 (1981) (lead standard); ASARCO v. OSHA, 746 F.2d 483 (9th Cir. 1984) (arsenic standard); Am. Iron & Steel v. OSHA, 182 F.3d 1261 (11th Cir. 1999) (respiratory protection standard); Pub. Citizen v. U.S. Dep't of Labor, 557 F.3d 165 (3rd Cir. 2009) (hexavalent chromium standard)).

Engineering controls are reliable, provide consistent levels of protection to a large number of workers, can be monitored, allow for predictable performance levels, and can efficiently remove a toxic substance from the workplace. Once removed, the toxic substance no longer poses a threat to employees. The effectiveness of engineering controls does not generally depend on human behavior to the same extent as personal protective equipment does, and the operation of equipment is not as vulnerable to human error as is personal protective equipment.

Respirators are another important means of protecting workers. However, to be effective, respirators must be individually selected; fitted and periodically refitted; conscientiously and properly worn; regularly maintained; and replaced as necessary. In many workplaces, these conditions for effective respirator use are difficult to achieve. The absence of any of these conditions can reduce or eliminate the protection that respirators provide to some or all of the employees who wear them.

In addition, use of respirators in the workplace presents other safety and health concerns. Respirators impose substantial physiological burdens on some employees. Certain medical conditions can compromise an employee's ability to tolerate the physiological burdens imposed by respirator use, thereby placing the employee wearing the respirator at an increased risk of illness, injury, and even death. Psychological conditions, such as claustrophobia, can also impair the effective use of respirators by employees. These concerns about the burdens placed on workers by the use of respirators are the basis for the requirement that employers provide a medical evaluation to determine the employee's ability to wear a respirator before the employee is fit tested or required to use a respirator in the workplace. Although experience in industry shows that most healthy workers do not have physiological problems wearing properly chosen and fitted respirators, common health problems can sometime preclude an employee from wearing a respirator. Safety problems created by respirators that limit vision and communication must also be considered. In some difficult or dangerous jobs, effective vision or communication is vital. Voice transmission through a respirator can be difficult and fatiguing.

Because respirators are less reliable than engineering and work practice controls and may create additional problems, OSHA believes that primary reliance on respirators to protect workers is generally inappropriate when feasible engineering and work practice controls are available. All OSHA substance-specific health standards have recognized and required employers to observe the hierarchy of controls, favoring engineering and work practice controls over respirators. OSHA's PELs, including the current PELs for respirable crystalline silica, also incorporate this hierarchy of controls. In addition, the industry consensus standards for crystalline silica (ASTM E 1132-06, Standard Practice for Health Requirements Relating to Occupational Exposure to Respirable Crystalline Silica, and ASTM E 2626-09, Standard Practice for Controlling Occupational Exposure to Respirable Crystalline Silica for Construction and Demolition Activities) incorporate the hierarchy of controls.

It is important to note that the very concept of technological feasibility for OSHA standards is grounded in the hierarchy of controls. As indicated in Section II of this preamble, Pertinent Legal Authority, the courts have clarified that a standard is technologically feasible if OSHA proves a reasonable possibility,

. . . within the limits of the best available evidence . . . that the typical firm will be able to develop and install engineering and work practice controls that can meet the PEL in most of its operations. [See United Steelworkers v. Marshall, 647 F.2d 1189, 1272 (D.C. Cir. 1980)]

Allowing use of respirators instead of engineering and work practice controls would be at odds with this framework for evaluating the technological feasibility of a PEL.

Alternative PELs

OSHA has examined two regulatory alternatives (named Regulatory Alternatives #1 and #2) that would modify the PEL for the proposed rule. Under Regulatory Alternative #1, the proposed PEL would be changed from 50 μg/m³ to 100 μg/m³ for all industry sectors covered by the rule, and the action level would be changed from 25 μg/m³ to 50 μg/m³ (thereby keeping the action level at one-half of the PEL). Under Regulatory Alternative #2, the proposed PEL would be lowered from 50 μg/m³ to 25 μg/m³ for all industry sectors covered by the rule, while the action level would remain at 25 μg/m³ (because of difficulties in accurately measuring exposure levels below 25 μg/m³).

Tables SI-2 and SI-3 present, for informational purposes, the estimated costs, benefits, and net benefits of the proposed rule under the proposed PEL of 50 μg/m³ and for the regulatory alternatives of a PEL of 100 μg/m³ and a PEL of 25 μg/m³ (Regulatory Alternatives #1 and #2), using alternative discount rates of 3 and 7 percent. These two tables also present the incremental costs, the incremental benefits, and the incremental net benefits of going from a PEL of 100 μg/m³ to the proposed PEL of 50 μg/m³ and then of going from the proposed PEL of 50 μg/m³ to a PEL of 25 μg/m³. Table SI-2 breaks out costs by provision and benefits by type of disease and by morbidity/mortality, while Table SI-3 breaks out costs and benefits by major industry sector.

As Tables SI-2 and SI-3 show, going from a PEL of 100 μg/m³ to a PEL of 50 μg/m³ would prevent, annually, an additional 357 silica-related fatalities and an additional 632 cases of silicosis. Based on its preliminary findings that the proposed PEL of 50 μg/m³ significantly reduces worker risk from silica exposure (as demonstrated by the number of silica-related fatalities and silicosis cases avoided) and is both technologically and economically feasible, OSHA cannot propose a PEL of 100 μg/m³ (Regulatory Alternative #1) without violating its statutory obligations under the OSH Act. However, the Agency will consider evidence that challenges its preliminary findings.

As previously noted, Tables SI-2 and SI-3 also show the costs and benefits of a PEL of 25 μg/m³ (Regulatory Alternative #2), as well as the incremental costs and benefits of going from the proposed PEL of 50 μg/m³ to a PEL of 25 μg/m³. Because OSHA preliminarily determined that a PEL of 25 μg/m³ would not be feasible (that is, engineering and work practices would not be sufficient to reduce and maintain silica exposures to a PEL of 25 μg/m³ or below in most operations most of the time in the affected industries), the Agency did not attempt to identify engineering controls or their costs for affected industries to meet this PEL. Instead, for purposes of estimating the costs of going from a PEL of 50 μg/m³ to a PEL of 25 μg/m³, OSHA assumed that all workers exposed between 50 μg/m³ and 25 μg/m³ would have to wear respirators to achieve compliance with the 25 μg/m³ PEL. OSHA then estimated the associated additional costs for respirators, exposure assessments, medical surveillance, and regulated areas (the latter three for ancillary requirements specified in the proposed rule).

As shown in Tables SI-2 and SI-3, going from a PEL of 50 μg/m³ to a PEL of 25 μg/m³ would prevent, annually, an additional 335 silica-related fatalities and an additional 186 cases of silicosis. These estimates support OSHA's preliminarily finding that there is significant risk remaining at the proposed PEL of 50 μg/m³. However, the Agency has preliminarily determined that a PEL of 25 μg/m³ (Regulatory Alternative #2) is not technologically feasible, and for that reason, cannot propose it without violating its statutory obligations under the OSH Act.

Regulatory Alternatives That Affect Ancillary Provisions

The proposed rule contains several ancillary provisions (provisions other than the PEL), including requirements for exposure assessment, medical surveillance, training, and regulated areas or access control. As shown in Table SI-2, these ancillary provisions represent approximately $223 million (or about 34 percent) of the total annualized costs of the rule of $658 million (using a 7 percent discount rate). The two most expensive of the ancillary provisions are the requirements for medical surveillance, with annualized costs of $79 million, and the requirements for exposure monitoring, with annualized costs of $74 million.

As proposed, the requirements for exposure assessment are triggered by the action level. As described in this preamble, OSHA has defined the action level for the proposed standard as an airborne concentration of respirable crystalline silica of 25 μg/m³ calculated as an eight-hour time-weighted average. In this proposal, as in other standards, the action level has been set at one-half of the PEL.

Because of the variable nature of employee exposures to airborne concentrations of respirable crystalline silica, maintaining exposures below the action level provides reasonable assurance that employees will not be exposed to respirable crystalline silica at levels above the PEL on days when no exposure measurements are made. Even when all measurements on a given day may fall below the PEL (but are above the action level), there is some chance that on another day, when exposures are not measured, the employee's actual exposure may exceed the PEL. When exposure measurements are above the action level, the employer cannot be reasonably confident that employees have not been exposed to respirable crystalline silica concentrations in excess of the PEL during at least some part of the work week. Therefore, requiring periodic exposure measurements when the action level is exceeded provides the employer with a reasonable degree of confidence in the results of the exposure monitoring.

The action level is also intended to encourage employers to lower exposure levels in order to avoid the costs associated with the exposure assessment provisions. Some employers would be able to reduce exposures below the action level in all work areas, and other employers in some work areas. As exposures are lowered, the risk of adverse health effects among workers decreases.

OSHA's preliminary risk assessment indicates that significant risk remains at the proposed PEL of 50 μg/m³. Where there is continuing significant risk, the decision in the Asbestos case (Bldg. and Constr. Trades Dep't, AFL-CIO v. Brock, 838 F.2d 1258, 1274 (D.C. Cir. 1988)) indicated that OSHA should use its legal authority to impose additional requirements on employers to further reduce risk when those requirements will result in a greater than de minimis incremental benefit to workers' health. OSHA's preliminary conclusion is that the requirements triggered by the action level will result in a very real and necessary, but non-quantifiable, further reduction in risk beyond that provided by the PEL alone. OSHA's choice of proposing an action level for exposure monitoring of one-half of the PEL is based on the Agency's successful experience with other standards, including those for inorganic arsenic (29 CFR 1910.1018), ethylene oxide (29 CFR 1910.1047), benzene (29 CFR 1910.1028), and methylene chloride (29 CFR 1910.1052).

As specified in the proposed rule, all workers exposed to respirable crystalline silica above the PEL of 50 μg/m³ are subject to the medical surveillance requirements. This means that the medical surveillance requirements would apply to 15,172 workers in general industry and 336,244 workers in construction. OSHA estimates that 457 possible silicosis cases will be referred to pulmonary specialists annually as a result of this medical surveillance.

OSHA has preliminarily determined that these ancillary provisions will: (1) Help ensure that the PEL is not exceeded, and (2) minimize risk to workers given the very high level of risk remaining at the PEL. OSHA did not estimate, and the benefits analysis does not include, monetary benefits resulting from early discovery of illness.

Because medical surveillance and exposure assessment are the two most costly ancillary provisions in the proposed rule, the Agency has examined four regulatory alternatives (named Regulatory Alternatives #3, #4, #5, and #6) involving changes to one or the other of these ancillary provisions. These four regulatory alternatives are defined below and the incremental cost impact of each is summarized in Table SI-4. In addition, OSHA is including a regulatory alternative (named Regulatory Alternative #7) that would remove all ancillary provisions.

Under Regulatory Alternative #3, the action level would be raised from 25 μg/m³ to 50 μg/m³ while keeping the PEL at 50 μg/m³. As a result, exposure monitoring requirements would be triggered only if workers were exposed above the proposed PEL of 50 μg/m³. As shown in Table SI-4, Regulatory Option #3 would reduce the annualized cost of the proposed rule by about $62 million, using a discount rate of either 3 percent or 7 percent.

Under Regulatory Alternative #4, the action level would remain at 25 μg/m³ but medical surveillance would now be triggered by the action level, not the PEL. As a result, medical surveillance requirements would be triggered only if workers were exposed at or above the proposed action level of 25 μg/m³. As shown in Table SI-4, Regulatory Option #4 would increase the annualized cost of the proposed rule by about $143 million, using a discount rate of 3 percent (and by about $169 million, using a discount rate of 7 percent).

Under Regulatory Alternative #5, the only change to the proposed rule would be to the medical surveillance requirements. Instead of requiring workers exposed above the PEL to have a medical check-up every three years, those workers would be required to have a medical check-up annually. As shown in Table SI-4, Regulatory Option #5 would increase the annualized cost of the proposed rule by about $69 million, using a discount rate of 3 percent (and by about $66 million, using a discount rate of 7 percent).

Regulatory Alternative #6 would essentially combine the modified requirements in Regulatory Alternatives #4 and #5. Under Regulatory Alternative #6, medical surveillance would be triggered by the action level, not the PEL, and workers exposed at or above the action level would be required to have a medical check-up annually rather than triennially. The exposure monitoring requirements in the proposed rule would not be affected. As shown in Table SI-4, Regulatory Option #6 would increase the annualized cost of the proposed rule by about $342 million, using a discount rate of either 3 percent or 7 percent.

OSHA is not able to quantify the effects of these preceding four regulatory alternatives on protecting workers exposed to respirable crystalline silica at levels at or below the proposed PEL of 50 μg/m³—where significant risk remains. The Agency solicits comment on the extent to which these regulatory options may improve or reduce the effectiveness of the proposed rule.

The final regulatory alternative affecting ancillary provisions, Regulatory Alternative #7, would eliminate all of the ancillary provisions of the proposed rule, including exposure assessment, medical surveillance, training, and regulated areas or access control. However, it should be carefully noted that elimination of the ancillary provisions does not mean that all costs for ancillary provisions would disappear. In order to meet the PEL, employers would still commonly need to do monitoring, train workers on the use of controls, and set up some kind of regulated areas to indicate where respirator use would be required. It is also likely that employers would increasingly follow the many recommendations to provide medical surveillance for employees. OSHA has not attempted to estimate the extent to which the costs of these activities would be reduced if they were not formally required, but OSHA welcomes comment on the issue.

As indicated previously, OSHA preliminarily finds that there is significant risk remaining at the proposed PEL of 50 μg/m³. However, the Agency has also preliminarily determined that 50 μg/m³ is the lowest feasible PEL. Therefore, the Agency believes that it is necessary to include ancillary provisions in the proposed rule to further reduce the remaining risk. OSHA anticipates that these ancillary provisions will reduce the risk beyond the reduction that will be achieved by a new PEL alone.

OSHA's reasons for including each of the proposed ancillary provisions are detailed in Section XVI of this preamble, Summary and Explanation of the Standards. In particular, OSHA believes that requirements for exposure assessment (or alternately, using specified exposure control methods for selected construction operations) would provide a basis for ensuring that appropriate measures are in place to limit worker exposures. Medical surveillance is particularly important because individuals exposed above the PEL (which triggers medical surveillance in the proposed rule) are at significant risk of death and illness. Medical surveillance would allow for identification of respirable crystalline silica-related adverse health effects at an early stage so that appropriate intervention measures can be taken. OSHA believes that regulated areas and access control are important because they serve to limit exposure to respirable crystalline silica to as few employees as possible. Finally, OSHA believes that worker training is necessary to inform employees of the hazards to which they are exposed, along with associated protective measures, so that employees understand how they can minimize potential health hazards. Worker training on silica-related work practices is particularly important in controlling silica exposures because engineering controls frequently require action on the part of workers to function effectively.

OSHA expects that the benefits estimated under the proposed rule will not be fully achieved if employers do not implement the ancillary provisions of the proposed rule. For example, OSHA believes that the effectiveness of the proposed rule depends on regulated areas or access control to further limit exposures and on medical surveillance to identify disease cases when they do occur.

Both industry and worker groups have recognized that a comprehensive standard is needed to protect workers exposed to respirable crystalline silica. For example, the industry consensus standards for crystalline silica, ASTM E 1132-06, Standard Practice for Health Requirements Relating to Occupational Exposure to Respirable Crystalline Silica, and ASTM E 2626-09, Standard Practice for Controlling Occupational Exposure to Respirable Crystalline Silica for Construction and Demolition Activities, as well as the draft proposed silica standard for construction developed by the Building and Construction Trades Department, AFL-CIO, have each included comprehensive programs. These recommended standards include provisions for methods of compliance, exposure monitoring, training, and medical surveillance (ASTM, 2006; 2009; BCTD 2001). Moreover, as mentioned previously, where there is continuing significant risk, the decision in the Asbestos case (Bldg. and Constr. Trades Dep't, AFL-CIO v. Brock, 838 F.2d 1258, 1274 (D.C. Cir. 1988)) indicated that OSHA should use its legal authority to impose additional requirements on employers to further reduce risk when those requirements will result in a greater than de minimis incremental benefit to workers' health. OSHA preliminarily concludes that the additional requirements in the ancillary provisions of the proposed standard clearly exceed this threshold.

A Regulatory Alternative That Modifies the Methods of Compliance

The proposed standard in general industry and maritime would require employers to implement engineering and work practice controls to reduce employees' exposures to or below the PEL. Where engineering and/or work practice controls are insufficient, employers would still be required to implement them to reduce exposure as much as possible, and to supplement them with a respiratory protection program. Under the proposed construction standard, employers would be given two options for compliance. The first option largely follows requirements for the general industry and maritime proposed standard, while the second option outlines, in Table 1 (Exposure Control Methods for Selected Construction Operations) of the proposed rule, specific construction exposure control methods. Employers choosing to follow OSHA's proposed control methods would be considered to be in compliance with the engineering and work practice control requirements of the proposed standard, and would not be required to conduct certain exposure monitoring activities.

One regulatory alternative (Regulatory Alternative #8) involving methods of compliance would be to eliminate Table 1 as a compliance option in the construction sector. Under that regulatory alternative, OSHA estimates that there would be no effect on estimated benefits but that the annualized costs of complying with the proposed rule (without the benefit of the Table 1 option in construction) would increase by $175 million, totally in exposure monitoring costs, using a 3 percent discount rate (and by $178 million using a 7 percent discount rate), so that the total annualized compliance costs for all affected establishments in construction would increase from $495 to $670 million using a 3 percent discount rate (and from $511 to $689 million using a 7 percent discount rate).

Regulatory Alternatives That Affect the Timing of the Standard

The proposed rule would become effective 60 days following publication of the final rule in the Federal Register. Provisions outlined in the proposed standard would become enforceable 180 days following the effective date, with the exceptions of engineering controls and laboratory requirements. The proposed rule would require engineering controls to be implemented no later than one year after the effective date, and laboratory requirements would be required to begin two years after the effective date.

OSHA will strongly consider alternatives that would reduce the economic impact of the rule and provide additional flexibility for firms coming into compliance with the requirements of the rule. The Agency solicits comment and suggestions from stakeholders, particularly small business representatives, on options for phasing in requirements for engineering controls, medical surveillance, and other provisions of the rule (e.g., over 1, 2, 3, or more years). These options will be considered for specific industries (e.g., industries where first-year or annualized cost impacts are highest), specific size-classes of employers (e.g., employers with fewer than 20 employees), combinations of these factors, or all firms covered by the rule.

Although OSHA did not explicitly develop or quantitatively analyze the multitude of potential regulatory alternatives involving longer-term or more complex phase-ins of the standard, the Agency is soliciting comments on this issue. Such a particularized, multi-year phase-in could have several advantages, especially from the viewpoint of impacts on small businesses. First, it would reduce the one-time initial costs of the standard by spreading them out over time, a particularly useful mechanism for small businesses that have trouble borrowing large amounts of capital in a single year. Second, a differential phase-in for smaller firms would aid very small firms by allowing them to gain from the control experience of larger firms. Finally, a phase-in would be useful in certain industries—such as foundries, for example—by allowing employers to coordinate their environmental and occupational safety and health control strategies to minimize potential costs. However a phase-in would also postpone the benefits of the standard.

OSHA analyzed one regulatory alternative (Regulatory Alternative #9) involving the timing of the standard which would arise if, contrary to OSHA's preliminary findings, a PEL of 50 µ g/m³ with an action level of 25 µ g/m³ were found to be technologically and economically feasible some time in the future (say, in five years), but not feasible immediately. In that case, OSHA might issue a final rule with a PEL of 50 µ g/m³ and an action level of 25 µ g/m³ to take effect in five years, but at the same time issue an interim PEL of 100 µ g/m³ and an action level of 50 µ g/m³ to be in effect until the final rule becomes feasible. Under this regulatory alternative, and consistent with the public participation and “look back” provisions of Executive Order 13563, the Agency could monitor compliance with the interim standard, review progress toward meeting the feasibility requirements of the final rule, and evaluate whether any adjustments to the timing of the final rule would be needed. Under Regulatory Alternative #9, the estimated costs and benefits would be somewhere between those estimated for a PEL of 100 µ g/m³ with an action level of 50 µ g/m³ and those estimated for a PEL of 50 µ g/m³ with an action level of 25 µ g/m³, the exact estimates depending on the length of time until the final rule is phased in. OSHA emphasizes that this regulatory alternative is contrary to the Agency's preliminary findings of economic feasibility and, for the Agency to consider it, would require specific evidence introduced on the record to show that the proposed rule is not now feasible but would be feasible in the future.

OSHA requests comments on these regulatory alternatives, including the Agency's choice of regulatory alternatives (and whether there are other regulatory alternatives the Agency should consider) and the Agency's analysis of them.

I. Issues

OSHA requests comment on all relevant issues, including health effects, risk assessment, significance of risk, technological and economic feasibility, and the provisions of the proposed regulatory text. In addition, OSHA requests comments on all of the issues raised by the Small Business Regulatory Fairness Enforcement Act (SBREFA) Panel, as summarized in Table VIII-H-4 in Section VIII.H of this preamble.

OSHA is including Section I on issues at the beginning of the document to assist readers as they review the proposal and consider any comments they may want to submit. However, to fully understand the questions in this section and provide substantive input in response to them, the parts of the preamble that address these issues in detail should be read and reviewed. These include: Section V, Health Effects Summary; Section VI, Summary of the Preliminary Quantitative Risk Assessment; Section VII, Significance of Risk; Section VIII, Summary of the Preliminary Economic Analysis and Initial Regulatory Flexibility Analysis; and Section XVI, Summary and Explanation of the Standards. In addition, OSHA invites comment on additional technical questions and discussions of economic issues presented in the Preliminary Economic Analysis (PEA) of the proposed standards. Section XIX is the text of the standards and is the final authority on what is required in them.

OSHA requests that comments be organized, to the extent possible, around the following issues and numbered questions. Comment on particular provisions should contain a heading setting forth the section and the paragraph in the standard that the comment is addressing. Comments addressing more than one section or paragraph will have correspondingly more headings.

Submitting comments in an organized manner and with clear reference to the issue raised will enable all participants to easily see what issues the commenter addressed and how they were addressed. This is particularly important in a rulemaking such as silica, which has multiple adverse health effects and affects many diverse processes and industries. Many commenters, especially small businesses, are likely to confine their interest (and comments) to the issues that affect them, and they will benefit from being able to quickly identify comments on these issues in others' submissions. Of course, the Agency welcomes comments concerning this proposal that fall outside the issues raised in this section. However, OSHA is especially interested in responses, supported by evidence and reasons, to the following questions:

Health Effects

1. OSHA has described a variety of studies addressing the major adverse health effects that have been associated with exposure to respirable crystalline silica. Has OSHA adequately identified and documented all critical health impairments associated with occupational exposure to respirable crystalline silica? If not, what adverse health effects should be added? Are there any additional studies, other data, or information that would affect the information discussed or significantly change the determination of material health impairment? Submit any relevant information, data, or additional studies (or the citations), and explain your reasoning for recommending the inclusion of any studies you suggest.

2. Using currently available epidemiologic and experimental studies, OSHA has made a preliminary determination that respirable crystalline silica presents risks of lung cancer, silicosis, and non-malignant respiratory disease (NMRD) as well as autoimmune and renal disease risks to exposed workers. Is this determination correct? Are there additional studies or other data OSHA should consider in evaluating any of these adverse health risks? If so, submit the studies (or citations) and other data and include your reasons for finding them germane to determining adverse health effects of exposure to crystalline silica.

Risk Assessment

3. OSHA has relied upon risk models using cumulative respirable crystalline silica exposure to estimate the lifetime risk of death from occupational lung cancer, silicosis, and NMRD among exposed workers. Additionally, OSHA has estimated the lifetime risk of silicosis morbidity among exposed workers. Is cumulative exposure the correct metric for exposure for each of these models? If not, what exposure measure should be used?

4. Some of the literature OSHA reviewed indicated that the risk of contracting accelerated silicosis and lung cancer may be non-linear at very high exposures and may be described by an exposure dose rate health effect model. OSHA used the more conservative model of cumulative exposure that is more protective to the worker. Are there additional data to support or rebut any of these models used by OSHA? Are there other models that OSHA should consider for estimating lung cancer, silicosis, or NMRD risk? If so, describe the models and the rationale for their use.

5. Are there additional studies or sources of data that OSHA should have included in its qualitative and quantitative risk assessments? What are these studies and have they been peer-reviewed, or are they soon to be peer-reviewed? What is the rationale for recommending the studies or data?

6. Steenland et al. (2001a) pooled data from 10 cohort studies to conduct an analysis of lung cancer mortality among silica-exposed workers. Can you provide quantitative lung cancer risk estimates from other data sources? Have or will the data you submit be peer-reviewed? OSHA is particularly interested in quantitative risk analyses that can be conducted using the industrial sand worker studies by McDonald, Hughes, and Rando (2001) and the pooled center-based case-control study conducted by Cassidy et al. (2007).

7. OSHA has made a preliminary determination that the available data are not sufficient or suitable for quantitative analysis of the risk of autoimmune disease, stomach cancer, and other cancer and non-cancer health effects. Do you have, or are you aware of, studies, data, and rationale that would be suitable for a quantitative risk assessment for these adverse health effects? Submit the studies (or citations), data, and rationale.

Profile of Affected Industries

8. In its PEA of the proposed rule, summarized in Section VIII of this preamble, OSHA presents a profile of the affected worker population. The profile includes estimates of the number of affected workers by industry sector or operation and job category, and the distribution of exposures by job category. If your company has potential worker exposures to respirable crystalline silica, is your industry among those listed by North American Industry Classification System (NAICS) code as affected industries? Are there additional data that will enable the Agency to refine its profile of the worker population exposed to respirable crystalline silica? If so, provide or reference such data and explain how OSHA should use these data to revise the profile.

Technological and Economic Feasibility of the Proposed PEL

9. What are the job categories in which employees are potentially exposed to respirable crystalline silica in your company or industry? For each job category, provide a brief description of the operation and describe the job activities that may lead to respirable crystalline silica exposure. How many employees are exposed, or have the potential for exposure, to respirable crystalline silica in each job category in your company or industry? What are the frequency, duration, and levels of exposures to respirable crystalline silica in each job category in your company or industry? Where responders are able to provide exposure data, OSHA requests that, where available, exposure data be personal samples with clear descriptions of the length of the sample, analytical method, and controls in place. Exposure data that provide information concerning the controls in place are more valuable than exposure data without such information.

10. Please describe work environments or processes that may expose workers to cristobalite. Please provide supporting evidence, or explain the basis of your knowledge.

11. Have there been technological changes within your industry that have influenced the magnitude, frequency, or duration of exposure to respirable crystalline silica or the means by which employers attempt to control such exposures? Describe in detail these technological changes and their effects on respirable crystalline silica exposures and methods of control.

12. Has there been a trend within your industry or an effort in your firm to reduce or eliminate respirable crystalline silica from production processes, products, and services? If so, please describe the methods used and provide an estimate of the percentage reduction in respirable crystalline silica, and the extent to which respirable crystalline silica is still necessary in specific processes within product lines or production activities. If you have substituted another substance(s) for crystalline silica, identify the substance(s) and any adverse health effects associated with exposure to the substitute substances, and the cost impact of substitution (cost of materials, productivity impact). OSHA also requests that responders describe any health hazards or technical, economic, or other deterrents to substitution.

13. Has your industry or firm used outsourcing or subcontracting, or concentrated high exposure tasks in-house, in order to expose fewer workers to respirable crystalline silica? An example would be subcontracting for the removal of hardened concrete from concrete mixing trucks, a task done typically 2-4 times a year, to a specialty subcontractor. What methods have you used to reduce the number of workers exposed to respirable crystalline silica and how were they implemented? Describe any trends related to concentration of high exposure tasks and provide any supporting information.

14. Does any job category or employee in your workplace have exposures to respirable crystalline silica that air monitoring data do not adequately portray due to the short duration, intermittent or non-routine nature, or other unique characteristics of the exposure? Explain your response and indicate peak levels, duration, and frequency of exposures for employees in these job categories.

15. OSHA requests the following information regarding engineering and work practice controls to control exposure to crystalline silica in your workplace or industry:

a. Describe the operations and tasks in which the proposed PEL is being achieved most of the time by means of engineering and work practice controls.

b. What engineering and work practice controls have been implemented in these operations and tasks?

c. For all operations and tasks in facilities where respirable crystalline silica is used, what engineering and work practice controls have been implemented to control respirable crystalline silica? If you have installed engineering controls or adopted work practices to reduce exposure to respirable crystalline silica, describe the exposure reduction achieved and the cost of these controls.

d. Where current work practices include the use of regulated areas and hygiene facilities, provide data on the implementation of these controls, including data on the costs of installation, operation, and maintenance associated with these controls.

e. Describe additional engineering and work practice controls that could be implemented in each operation where exposure levels are currently above the proposed PEL to further reduce exposure levels.

f. When these additional controls are implemented, to what levels can exposure be expected to be reduced, or what percent reduction is expected to be achieved?

g. What amount of time is needed to develop, install, and implement these additional controls? Will the added controls affect productivity? If so, how?

h. Are there any processes or operations for which it is not reasonably possible to implement engineering and work practice controls within one year to achieve the proposed PEL? If so, how much additional time would be necessary?

16. OSHA requests information on whether there are any specific conditions or job tasks involving exposure to respirable crystalline silica where engineering and work practice controls are not available or are not capable of reducing exposure levels to or below the proposed PEL most of the time. Provide data and evidence to support your response.

17. OSHA has made a preliminary determination that compliance with the proposed PEL can be achieved in most operations most of the time through the use of engineering and work practice controls. OSHA has further made a preliminary determination that the proposed rule is technologically feasible. OSHA solicits comments on the reasonableness of these preliminary determinations.

Compliance Costs

18. In its PEA (summarized in Section VIII.3 of this preamble), OSHA developed its estimate of the costs of the proposed rule. The Agency requests comment on the methodological and analytical assumptions applied in the cost analysis. Of particular importance are the unit cost estimates provided in tables and text in Chapter V of the PEA for all major provisions of the proposed rule. OSHA requests the following information regarding unit and total compliance costs:

a. If you have installed engineering controls or adopted work practices to reduce exposure to respirable crystalline silica, describe these controls and their costs. If you have substituted another substance(s) for crystalline silica, what has been the cost impact of substitution (cost of materials, productivity impact)?

b. OSHA has proposed to limit the prohibition on dry sweeping to situations where this activity could contribute to exposure that exceeds the PEL and estimated the costs for the use of wet methods to control dust. OSHA requests comment on the use of wet methods as a substitute for dry sweeping and whether the prohibition on dry sweeping is feasible and cost-effective.

c. In its PEA, OSHA presents estimated baseline levels of use of personal protective equipment (PPE) and the incremental PPE costs associated with the proposed rule. Are OSHA's estimated PPE compliance rates reasonable? Are OSHA's estimates of PPE costs, and the assumptions underlying these estimates, consistent with current industry practice? If not, provide data and evidence describing current industry PPE practices.

d. Do you currently conduct exposure monitoring for respirable crystalline silica? Are OSHA's estimates of exposure assessment costs reasonable? Would your company require outside consultants to perform exposure monitoring?

e. Are OSHA's estimates for medical surveillance costs—including direct medical costs, the opportunity cost of worker time for offsite travel and for the health screening, and recordkeeping costs—reasonable?

f. In its PEA, OSHA presents estimated baseline levels of training and information concerning respirable crystalline silica-related hazards and the incremental costs associated with the additional requirements for training and information in the proposed rule. OSHA requests information on information and training programs addressing respirable crystalline silica that are currently being implemented by employers and any necessary additions to those programs that are anticipated in response to the proposed rule. Are OSHA's baseline estimates and unit costs for training reasonable and consistent with current industry practice?

g. Are OSHA's estimated costs for regulated areas and written access control plans reasonable?

h. The cost estimates in the PEA take the much higher labor turnover rates in construction into account when calculating costs. For the proposed rule, OSHA used the most recent BLS turnover rate of 64 percent for construction (versus a turnover rate of 27.2 percent for general industry). OSHA believes that the estimates in the PEA capture the effect of high turnover rates in construction and solicits comments on this issue.

i. Has OSHA omitted any costs that would be incurred to comply with the proposed rule?

Effects on Small Entities

19. OSHA has considered the effects on small entities raised during its SBREFA process and addressed these concerns in Chapter VIII of the PEA. Are there additional difficulties small entities may encounter when attempting to comply with requirements of the proposed rule? Can any of the proposal's requirements be deleted or simplified for small entities, while still providing equivalent protection of the health of employees? Would allowing additional time for small entities to comply make a difference in their ability to comply? How much additional time would be necessary?

Economic Impacts

20. OSHA, in its PEA, has estimated compliance costs per affected entity and the likely impacts on revenues and profits. OSHA requests that affected employers provide comment on OSHA's estimate of revenue, profit, and the impacts of costs for their industry or application group. The Agency also requests that employers provide data on their revenues, profits, and the impacts of cost, if available. Are there special circumstances—such as unique cost factors, foreign competition, or pricing constraints—that OSHA needs to consider when evaluating economic impacts for particular applications and industry groups?

21. OSHA seeks comment as to whether establishments will be able to finance first-year compliance costs from cash flow, and under what circumstances a phase-in approach will assist firms in complying with the proposed rule.

22. The Agency invites comment on potential employment impacts of the proposed silica rule, and on Inforum's estimates of the employment impacts of the proposed silica rule on the U.S. economy.

Outreach and Compliance Assistance

23. If the proposed rule is promulgated, OSHA will provide outreach materials on the provisions of the standards in order to encourage and assist employers in complying. Are there particular materials that would make compliance easier for your company or industry? What materials would be especially useful for small entities? Submit recommendations or samples.

Benefits and Net Benefits

24. OSHA requests comments on any aspect of its estimation of benefits and net benefits from the proposed rule, including the following:

a. The use of willingness-to-pay measures and estimates based on compensating wage differentials.

b. The data and methods used in the benefits calculations.

c. The choice of discount rate for annualizing the monetized benefits of the proposed rule.

d. Increasing the monetary value of a statistical life over time resulting from an increase in real per capita income and the estimated income elasticity of the value of life.

e. Extending the benefits analysis beyond the 60-year period used in the PEA.

f. The magnitude of non-quantified health benefits arising from the proposed rule and methods for better measuring these effects. An example would be diagnosing latent tuberculosis (TB) in the silica-exposed population and thereby reducing the risk of TB being spread to the population at large.

Overlapping and Duplicative Regulations

25. Do any federal regulations duplicate, overlap, or conflict with the proposed respirable crystalline silica rule? If so, provide or cite to these regulations.

Alternatives/Ways to Simplify a New Standard

26. Comment on the alternative to new comprehensive standards (which have ancillary provisions in addition to a permissible exposure limit) that would be simply improved outreach and enforcement of the existing standards (which is only a permissible exposure limit with no ancillary provisions). Do you believe that improved outreach and enforcement of the existing permissible exposure limits would be sufficient to reduce significant risks of material health impairment in workers exposed to respirable crystalline silica? Provide information to support your position.

27. OSHA solicits comments on ways to simplify the proposed rule without compromising worker protection from exposure to respirable crystalline silica. In particular, provide detailed recommendations on ways to simplify the proposed standard for construction. Provide evidence that your recommended simplifications would result in a standard that was effective, to the extent feasible, in reducing significant risks of material health impairment in workers exposed to respirable crystalline silica.

Environmental Impacts

28. Submit data, information, or comments pertaining to possible environmental impacts of adopting this proposal, including any positive or negative environmental effects and any irreversible commitments of natural resources that would be involved. In particular, consideration should be given to the potential direct or indirect impacts of the proposal on water and air pollution, energy use, solid waste disposal, or land use. Would compliance with the silica rule require additional actions to comply with federal, state, or local environmental requirements?

29. Some small entity representatives advised OSHA that the use of water as a control measure is limited at their work sites due to potential water and soil contamination. OSHA believes these limits may only apply in situations where crystalline silica is found with other toxic substances such as during abrasive blasting of metal or painted metal structures, or in locations where state and local requirements are more restrictive than EPA requirements. OSHA seeks comments on this issue, including cites to applicable requirements.

a. Are there limits on the use of water controls in your operations due to environmental regulations? If so, are the limits due to the non-silica components of the waste stream? What are these non-silica components?

b. What metals or other toxic chemicals are in your silica waste streams and what are the procedures and costs to filter out these metals or other toxic chemicals from your waste streams? Provide documentation to support your cost estimates.

Provisions of the Standards

Scope

30. OSHA's Advisory Committee on Construction Safety and Health (ACCSH) has historically advised the Agency to take into consideration the unique nature of construction work environments by either setting separate standards or making accommodations for the differences in work environments in construction as compared to general industry. ASTM, for example, has separate silica standards of practice for general industry and construction, E 1132-06 and E 2625-09, respectively. To account for differences in the workplace environments for these different sectors, OSHA has proposed separate standards for general industry/maritime and construction. Is this approach necessary and appropriate? What other approaches, if any, should the Agency consider? Provide a rationale for your response.

31. OSHA has proposed that the scope of the construction standard include all occupational exposures to respirable crystalline silica in construction work as defined in 29 CFR 1910.12(b) and covered under 29 CFR part 1926, rather than restricting the application of the rule to specific construction operations. Should OSHA modify the scope to limit what is covered? What should be included and what should be excluded? Provide a rationale for your position. Submit your proposed language for the scope and application provision.

32. OSHA has not proposed to cover agriculture because the Agency does not have data sufficient to determine the feasibility of the proposed PEL in agricultural operations. Should OSHA cover respirable crystalline silica exposure in agriculture? Provide evidence to support your position. OSHA seeks information on agricultural operations that involve respirable crystalline silica exposures, including information that identifies particular activities or crops (e.g., hand picking fruit and vegetables, shaking branches and trees, harvesting with combines, loading storage silos, planting) associated with exposure, information indicating levels of exposure, and information relating to available control measures and their effectiveness. OSHA also seeks information related to the development of respirable crystalline silica-related adverse health effects and diseases among workers in the agricultural sector.

33. Should OSHA limit coverage of the rule to materials that contain a threshold concentration (e.g., 1%) of crystalline silica? For example, OSHA's Asbestos standard defines “asbestos-containing material” as any material containing more than 1% asbestos, for consistency with EPA regulations. OSHA has not proposed a comparable limitation to the definition of respirable crystalline silica. Is this approach appropriate? Provide the rationale for your position.

34. OSHA has proposed to cover shipyards under the general industry standard. Are there any unique circumstances in shipyard employment that would justify development of different provisions or a separate standard for the shipyard industry? What are the circumstances and how would they not be adequately covered by the general industry standard?

Definitions

35. Competent person. OSHA has proposed limited duties for a competent person relating to establishment of an access control plan. The Agency did not propose specific requirements for training of a competent person. Is this approach appropriate? Should OSHA include a competent person provision? If so, should the Agency add to, modify, or delete any of the duties of a competent person as described in the proposed standard? Provide the basis for your recommendations.

36. Has OSHA defined “respirable crystalline silica” appropriately? If not, provide the definition that you believe is appropriate. Explain the basis for your response, and provide any data that you believe are relevant.

37. The proposed rule defines “respirable crystalline silica” in part as “airborne particles that contain quartz, cristobalite, and/or tridymite.” OSHA believes that tridymite is rarely found in nature or in the workplace. Please describe any instances of occupational exposure to tridymite of which you are aware. Please provide supporting evidence, or explain the basis of your knowledge. Should tridymite be included in the scope of this proposed rule? Please provide any evidence to support your position.

PEL and Action Level

38. OSHA has proposed a TWA PEL for respirable crystalline silica of 50 µg/m³ for general industry, maritime, and construction. The Agency has made a preliminary determination that this is the lowest level that is technologically feasible. The Agency has also determined that a PEL of 50 µg/m³ will substantially reduce, but not eliminate, significant risk of material health impairment. Is this PEL appropriate, given the Agency's obligation to reduce significant risk of material health impairment to the extent feasible? If not, what PEL would be more appropriate? The Agency also solicits comment on maintaining the existing PELs for respirable crystalline silica. Provide evidence to support your response.

39. OSHA has proposed a single PEL for respirable crystalline silica (quartz, cristobalite, and tridymite). Is a single PEL appropriate, or should the Agency maintain separate PELs for the different forms of respirable crystalline silica? Provide the rationale for your position.

40. OSHA has proposed an action level for respirable crystalline silica exposure of 25 µg/m³ in general industry, maritime, and construction. Is this an appropriate approach and level, and if not, what approach or level would be more appropriate and why? Should an action level be included in the final rule? Provide the rationale for your position.

41. If an action level is included in the final rule, which provisions, if any, should be triggered by exposure above or below the action level? Provide the basis for your position and include supporting information.

42. If no action level is included in the final rule, which provisions should apply to all workers exposed to respirable crystalline silica? Which provisions should be triggered by the PEL? Are there any other appropriate triggers for the requirements of the rule?

Exposure Assessment

43. OSHA is proposing to allow employers to initially assess employee exposures using air monitoring or objective data. Has OSHA defined “objective data” sufficiently for an employer to know what data may be used? If not, submit an alternative definition. Is it appropriate to allow employers to use objective data to perform exposure assessments? Explain why or why not.

44. The proposed rule provides two options for periodic exposure assessment: (1) A fixed schedule option, and (2) a performance option. The performance option provides employers flexibility in the methods used to determine employee exposures, but requires employers to accurately characterize employee exposures. The proposed approach is explained in the Summary and Explanation for paragraph (d) Exposure Assessment. OSHA solicits comments on this proposed exposure assessment provision. Is the wording of the performance option in the regulatory text understandable and does it clearly indicate what would constitute compliance with the provision? If not, suggest alternative language that would clarify the provision, enabling employers to more easily understand what would constitute compliance.

45. Do you conduct initial air monitoring or do you rely on objective data to determine respirable crystalline silica exposures? If objective data, what data do you use? Have you conducted historical exposure monitoring of your workforce that is representative of current process technology and equipment use? Describe any other approaches you have implemented for assessing an employee's initial exposure to respirable crystalline silica.

46. OSHA is proposing specific requirements for laboratories that perform analyses of respirable crystalline silica samples. The rationale is to improve the precision in individual laboratories and reduce the variability of results between laboratories, so that sampling results will be more reliable. Are these proposed requirements appropriate? Will the laboratory requirements add necessary reliability and reduce inter-lab variability, or might they be overly proscriptive? Provide the basis for your response.

47. Has OSHA correctly described the accuracy and precision of existing methods of sampling and analysis for respirable crystalline silica at the proposed action level and PEL? Can worker exposures be accurately measured at the proposed action level and PEL? Explain the basis for your response, and provide any data that you believe are relevant.

48. OSHA has not addressed the performance of the analytical method with respect to tridymite since we have found little available data. Please comment on the performance of the analytical method with respect to tridymite and provide any data to support your position.

Regulated Areas and Access Control

49. Where exposures exceed the PEL, OSHA has proposed to provide employers with the option of either establishing a regulated area or establishing a written access control plan. For which types of work operations would employers be likely to establish a written access control plan? Will employees be protected by these options? Provide the basis for your position and include supporting information.

50. The Summary and Explanation for paragraph (e) Regulated Areas and Access Control clarifies how the regulated area requirements would apply to multi-employer worksites in the proposed standard. OSHA solicits comments on this issue.

51. OSHA is proposing limited requirements for protective clothing in the silica rule. Is this appropriate? Are you aware of any situations where more or different protective clothing would be needed for silica exposures? If so, what type of protective clothing and equipment should be required? Are there additional provisions related to protective clothing that should be incorporated into this rule that will enhance worker protection? Provide the rationale and data that support your conclusions.

Methods of Compliance

52. In OSHA's cadmium standard (29 CFR 1910.1027(f)(1)(ii),(iii), and (iv)), the Agency established separate engineering control air limits (SECALs) for certain processes in selected industries. SECALs were established where compliance with the PEL by means of engineering and work practice controls was infeasible. For these industries, a SECAL was established at the lowest feasible level that could be achieved by engineering and work practice controls. The PEL was set at a lower level, and could be achieved by any allowable combination of controls, including respiratory protection. In OSHA's chromium (VI) standard (29 CFR 1910.1026), an exception similar to SECALs was made for painting airplanes and airplane parts. Should OSHA follow this approach for respirable crystalline silica in any industries or processes? If so, in what industries or processes, and at what exposure levels, should the SECALs be established? Provide the basis for your position and include supporting information.

53. The proposed standards do not contain a requirement for a written exposure control program. The two ASTM standards for general industry and construction (E 1132-06, section 4.2.6, and E 2626-09, section 4.2.5) state that, where overexposures are persistent (such as in regulated areas or abrasive blasting operations), a written exposure control plan shall establish engineering and administrative controls to bring the area into compliance, if feasible. In addition, the proposed regulatory language developed by the Building and Construction Trades Department, AFL-CIO contains provisions for a written program. The ASTM standards recommend that, where there are regulated areas with persistent exposures or tasks, tools, or operations that tend to cause respirable crystalline silica exposure, the employer will conduct a formal analysis and implement a written control plan (an abatement plan) on how to bring the process into compliance. If that is not feasible, the employer is to indicate the respiratory protection and other protective procedures that will be used to protect employee(s) permanently or until compliance will be achieved. Should OSHA require employers to develop and implement a written exposure control plan and, if so, what should be required to be in the plans?

54. Table 1 in the proposed construction standard specifies engineering and work practice controls and respiratory protection for selected construction operations, and exempts employers who implement these controls from exposure assessment requirements. Is this approach appropriate? Are there other operations that should be included, or listed operations that should not be included? Are the specified control measures effective? Should any other changes be made in Table 1? How should OSHA update Table 1 in the future to account for development of new technologies? Provide data and information to support your position.

55. OSHA requests comments on the degree of specificity used for the engineering and work practice controls for tasks identified in Table 1, including maintenance requirements. Should OSHA require an evaluation or inspection checklist for controls? If so, how frequently should evaluations or inspections be conducted? Provide any examples of such checklists, along with information regarding their frequency of use and effectiveness.

56. In the proposed construction standard, when employees perform an operation listed in Table 1 and the employer fully implements the engineering controls, work practices, and respiratory protection described in Table 1 for that operation, the employer is not required to assess the exposure of the employees performing such operations. However, the employer must still ensure compliance with the proposed PEL for that operation. OSHA seeks comment on whether employers fully complying with Table 1 for an operation should still need to comply with the proposed PEL for that operation. Instead, should OSHA treat compliance with Table 1 as automatically meeting the requirements of the proposed PEL?

57. Are the descriptions of the operations (specific task or tool descriptions) and control technologies in Table 1 clear and precise enough so that employers and workers will know what controls they should be using for the listed operations? Identify the specific operation you are addressing and whether your assessment is based on your anecdotal experience or research. For each operation, are the data and other supporting information sufficient to predict the range of expected exposures under the controlled conditions? Identify operations, if any, where you believe the data are not sufficient. Provide the reasoning and data that support your position.

58. In one specific example from Table 1, OSHA has proposed the option of using a wet method for hand-operated grinders, with respirators required only for operations lasting four hours or more. Please comment and provide OSHA with additional information regarding wet grinding and the adequacy of this control strategy. OSHA is also seeking additional information on the second option (commercially available shrouds and dust collection systems) to confirm that this control strategy (including the use of half-mask respirators) will reduce workers' exposure to or below the PEL.

59. For impact drilling operations lasting four hours or less, OSHA is proposing in Table 1 to allow workers to use water delivery systems without the use of respiratory protection, as the Agency believes that this dust suppression method alone will provide consistent, sufficient protection. Is this control strategy appropriate? Please provide the basis for your position and any supporting evidence or additional information that addresses the appropriateness of this control strategy.

60. In the case of rock drilling, in order to ensure that workers are adequately protected from the higher exposures that they would experience working under shrouds, OSHA is proposing in Table 1 that employers ensure that workers use half-mask respirators when working under shrouds at the point of operation. Is this specification appropriate? Please provide the basis for your position and any supporting evidence or additional information that addresses the appropriateness of this specification.

61. OSHA has specified a control strategy for concrete drilling in Table 1 that includes use of a dust collection system as well as a low-flow water spray. Please provide to OSHA any data that you have that describes the efficacy of these controls. Is the control strategy in Table 1 adequate? Please provide the basis for your position and any supporting evidence or additional information regarding the adequacy of this control strategy.

62. One of the control options in Table 1 in the proposed construction standard for rock-crushing operations is local exhaust ventilation. However, OSHA is aware of difficulties in applying this control to this operation. Is this control strategy appropriate and practical for rock-crushing operations? Please provide any information that you have addressing this issue.

63. OSHA has not proposed to prohibit the use of crystalline silica as an abrasive blasting agent. Abrasive blasting, similar to other operations that involve respirable crystalline silica exposures, must follow the hierarchy of controls, which means, if feasible, that substitution, engineering, or administrative controls or a combination of these controls must be used to minimize or eliminate the exposure hazard. Is this approach appropriate? Provide the basis for your position and any supporting evidence.

64. The technological feasibility study (PEA, Chapter 4) indicates that employers use substitutes for crystalline silica in a variety of operations. If you are aware of substitutes for crystalline silica that are currently being used in any operation not considered in the feasibility study, please provide to OSHA relevant information that contains data supporting the effectiveness, in reducing exposure to crystalline silica, of those substitutes. Provide any information you may have on the health hazards associated with exposure to these substitutes.

65. Information regarding the effectiveness of dust control kits that incorporate local exhaust ventilation in the railroad transportation industry in reducing worker exposure to crystalline silica is not available from the manufacturer. If you have any relevant information on the effectiveness of such kits, please provide it to OSHA.

66. The proposed rule prohibits the use of compressed air and dry brushing and sweeping for cleaning of surfaces and clothing in general industry, maritime, and construction and promotes the use of wet methods and HEPA-filter vacuuming as alternatives. Are there any circumstances in general industry, maritime, or construction work where dry sweeping is the only kind of sweeping that can be done? Have you done dry sweeping and, if so, what has been your experience with it? What methods have you used to minimize dust when dry sweeping? Can exposure levels be kept below the proposed PEL when dry sweeping is conducted? How? Provide exposure data for periods when you conducted dry sweeping. If silica respirable dust samples are not available, provide real time respirable dust or gravimetric respirable dust data. Is water available at most sites to wet down dust prior to sweeping? How effective is the use of water? Does the use of water cause other problems for the worksite? Are there other substitutes that are effective?

67. A 30-day exemption from the requirement to implement engineering and work practice controls was not included in the proposed standard for construction, and has been removed from the proposed standard for general industry and maritime. OSHA requests comment on this issue.

68. The proposed prohibition on employee rotation is explained in the Summary and Explanation for paragraph (f) Methods of Compliance. OSHA solicits comment on the prohibition of employee rotation to achieve compliance when exposure levels exceed the PEL.

Medical Surveillance

69. Is medical surveillance being provided for respirable crystalline silica-exposed employees at your worksite? If so:

a. How do you determine which employees receive medical surveillance (e.g., by exposure level or other factors)?

b. Who administers and implements the medical surveillance (e.g., company doctor or nurse, outside doctor or nurse)?

c. What examinations, tests, or evaluations are included in the medical surveillance program? Does your medical surveillance program include testing for latent TB? Do you include pulmonary function testing in your medical surveillance program?

d. What benefits (e.g., health, reduction in absenteeism, or financial) have been achieved from the medical surveillance program?

e. What are the costs of your medical surveillance program? How do your costs compare with OSHA's estimated unit costs for the physical examination and employee time involved in the medical surveillance program? Are OSHA's baseline assumptions and cost estimates for medical surveillance consistent with your experiences providing medical surveillance to your employees?

f. How many employees are included in your medical surveillance program?

g. What NAICS code describes your workplace?

70. Is the content and frequency of proposed examinations appropriate? If not, how should content and frequency be modified?

71. Is the specified content of the physician or other licensed health care professional's (PLHCP) written medical opinion sufficiently detailed to enable the employer to address the employee's needs and potential workplace improvements, and yet appropriately limited so as to protect the employee's medical privacy? If not, how could the medical opinion be improved?

72. Is the requirement for latent TB testing appropriate? Does the proposed rule implement this requirement in a cost-effective manner? Provide the data or cite references that support your position.

73. Is the requirement for pulmonary function testing initially and at three-year intervals appropriate? Is there an alternate strategy or schedule for conducting follow-up testing that is better? Provide data or cite references to support your position.

74. Is the requirement for chest X-rays initially and at three-year intervals appropriate? Is there an alternate strategy or schedule for conducting follow-up chest X-rays that you believe would be better? Provide data or cite references to support your position.

75. Are there other tests that should be included in medical surveillance?

76. Do you provide medical surveillance to employees under another OSHA standard or as a matter of company policy? If so, describe your program in terms of what standards the program addresses and such factors as content and frequency of examinations and referrals, and reports to the employer.

77. Is exposure for 30 days at or above the PEL the appropriate number of days to trigger medical surveillance? Should the appropriate reference for medical monitoring be the PEL or the action level? Is 30 days from initial assignment a reasonable amount of time to provide a medical exam? Indicate the basis for your position.

78. Are PLHCPs available in your geographic area to provide medical surveillance to workers who are covered by the proposed rule? For example, do you have access to qualified X-ray technicians, NIOSH-certified B-readers, and pulmonary specialists? Describe any difficulties you may have with regard to access to PLHCPs to provide surveillance for the rule. Note what you consider your “geographic area” in responding to this question.

79. OSHA is proposing to allow an “equivalent diagnostic study” in place of requirements to use a chest X-ray (posterior/anterior view; no less than 14 x 17 inches and no more than 16 x 17 inches at full inspiration; interpreted and classified according to the International Labour Organization (ILO) International Classification of Radiographs of Pneumoconioses by a NIOSH-certified “B” reader). Two other radiological test methods, computed tomography (CT) and high resolution computed tomography (HRCT), could be considered “equivalent diagnostic studies” under paragraph (h)(2)(iii) of the proposal. However, the benefits of CT or HRCT should be balanced with risks, including higher radiation doses. Also, standardized methods for interpreting and reporting results of CT or HRCT are not currently available. The Agency requests comment on whether CT and HRCT should be considered “equivalent diagnostic studies” under the rule. Provide a rationale and evidence to support your position.

80. OSHA has not included requirements for medical removal protection (MRP) in the proposed rule, because OSHA has made a preliminary determination that there are few instances where temporary worker removal and MRP will be useful. The Agency requests comment as to whether the respirable crystalline silica rule should include provisions for the temporary removal and extension of MRP benefits to employees with certain respirable crystalline silica-related health conditions. In particular, what medical conditions or findings should trigger temporary removal and for what maximum amount of time should MRP benefits be extended? OSHA also seeks information on whether or not MRP is currently being used by employers with respirable crystalline silica-exposed workers, and the costs of such programs.

Hazard Communication and Training

81. OSHA has proposed that employers provide hazard information to employees in accordance with the Agency's Hazard Communication standard (29 CFR 1910.1200). Compliance with the Hazard Communication standard would mean that there would be a requirement for a warning label for substances that contain more than 0.1 percent crystalline silica. Should this requirement be changed so that warning labels would only be required of substances more than 1 percent by weight of silica? Provide the rationale for your position. The Agency also has proposed additional training specific to work with respirable crystalline silica. Should OSHA include these additional requirements in the final rule, or are the requirements of the Hazard Communication standard sufficient?

82. OSHA is providing an abbreviated training section in this proposal as compared to ASTM consensus standards (see ASTM E 1132-06, sections 4.8.1-5). The Hazard Communication standard is comprehensive and covers most of the training requirements traditionally included in an OSHA health standard. Do you concur with OSHA that performance-based training specified in the Hazard Communication standard, supplemented by the few training requirements of this section, is sufficient in its scope and depth? Are there any other training provisions you would add?

83. The proposed rule does not alter the requirements for substances to have warning labels, specify wording for labels, or otherwise modify the provisions of the OSHA's Hazard Communication standard. OSHA invites comment on these issues.

Recordkeeping

84. OSHA is proposing to require recordkeeping for air monitoring data, objective data, and medical surveillance records. The proposed rule's recordkeeping requirements are discussed in the Summary and Explanation for paragraph (j) Recordkeeping. The Agency seeks comment on the utility of these recordkeeping requirements as well as the costs of making and maintaining these records. Provide evidence to support your position.

Dates

85. OSHA requests comment on the time allowed for compliance with the provisions of the proposed rule. Is the time proposed appropriate, or should there be a longer or shorter phase-in of requirements? In particular, should requirements for engineering controls and/or medical surveillance be phased in over a longer period of time (e.g., over 1, 2, 3, or more years)? Should an extended phase-in period be provided for specific industries (e.g., industries where first-year or annualized cost impacts are highest), specific size-classes of employers (e.g., employers with fewer than 20 employees), combinations of these factors, or all firms covered by the rule? Identify any industries, processes, or operations that have special needs for additional time, the additional time required, and the reasons for the request.

86. OSHA is proposing a two-year start-up period to allow laboratories time to achieve compliance with the proposed requirements, particularly with regard to requirements for accreditation and round robin testing. OSHA also recognizes that requirements for monitoring in the proposed rule will increase the required capacity for analysis of respirable crystalline silica samples. Do you think that this start-up period is enough time for laboratories to achieve compliance with the proposed requirements and to develop sufficient analytic capacity? If you think that additional time is needed, please tell OSHA how much additional time is required and give your reasons for this request.

Appendices

87. Some OSHA health standards include appendices that address topics such as the hazards associated with the regulated substance, health screening considerations, occupational disease questionnaires, and PLHCP obligations. In this proposed rule, OSHA has included a non-mandatory appendix to clarify the medical surveillance provisions of the rule. What would be the advantages and disadvantages of including such an appendix in the final rule? If you believe it should be included, comment on the appropriateness of the information included. What additional information, if any, should be included in the appendix?

II. Pertinent Legal Authority

The purpose of the Occupational Safety and Health Act, 29 U.S.C. 651 et seq. (“the Act”), is to “. . . assure so far as possible every working man and woman in the nation safe and healthful working conditions and to preserve our human resources.” 29 U.S.C. 651(b).

To achieve this goal Congress authorized the Secretary of Labor (the Secretary) to promulgate and enforce occupational safety and health standards. 29 U.S.C. 654(b) (requiring employers to comply with OSHA standards), 655(a) (authorizing summary adoption of existing consensus and federal standards within two years of the Act's enactment), and 655(b) (authorizing promulgation, modification or revocation of standards pursuant to notice and comment).

The Act provides that in promulgating health standards dealing with toxic materials or harmful physical agents, such as this proposed standard regulating occupational exposure to respirable crystalline silica, the Secretary, shall set the standard which most adequately assures, to the extent feasible, on the basis of the best available evidence that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard dealt with by such standard for the period of his working life. 29 U.S.C. 655(b)(5).

The Supreme Court has held that before the Secretary can promulgate any permanent health or safety standard, she must make a threshold finding that significant risk is present and that such risk can be eliminated or lessened by a change in practices. Industrial Union Dept., AFL-CIO v. American Petroleum Institute, 448 U.S. 607, 641-42 (1980) (plurality opinion) (“The Benzene case”). Thus, section 6(b)(5) of the Act requires health standards to reduce significant risk to the extent feasible. Id.

The Court further observed that what constitutes “significant risk” is “not a mathematical straitjacket” and must be “based largely on policy considerations.” The Benzene case, 448 U.S. at 655. The Court gave the example that if,

. . . the odds are one in a billion that a person will die from cancer . . . the risk clearly could not be considered significant. On the other hand, if the odds are one in one thousand that regular inhalation of gasoline vapors that are 2% benzene will be fatal, a reasonable person might well consider the risk significant. [Id.]

OSHA standards must be both technologically and economically feasible. United Steelworkers v. Marshall, 647 F.2d 1189, 1264 (D.C. Cir. 1980) (“The Lead I case”). The Supreme Court has defined feasibility as “capable of being done.” Am. Textile Mfrs. Inst. v. Donovan, 452 U.S. 490, 509-510 (1981) (“The Cotton Dust case”). The courts have further clarified that a standard is technologically feasible if OSHA proves a reasonable possibility,

. . . within the limits of the best available evidence . . . that the typical firm will be able to develop and install engineering and work practice controls that can meet the PEL in most of its operations. [See The Lead I case, 647 F.2d at 1272]

With respect to economic feasibility, the courts have held that a standard is feasible if it does not threaten massive dislocation to or imperil the existence of the industry. Id. at 1265. A court must examine the cost of compliance with an OSHA standard,

. . . in relation to the financial health and profitability of the industry and the likely effect of such costs on unit consumer prices . . . [T]he practical question is whether the standard threatens the competitive stability of an industry, . . . or whether any intra-industry or inter-industry discrimination in the standard might wreck such stability or lead to undue concentration. [Id. (citing Indus. Union Dep't, AFL-CIO v. Hodgson, 499 F.2d 467 (D.C. Cir. 1974))]

The courts have further observed that granting companies reasonable time to comply with new PELs may enhance economic feasibility. The Lead I case at 1265. While a standard must be economically feasible, the Supreme Court has held that a cost-benefit analysis of health standards is not required by the Act because a feasibility analysis is required. The Cotton Dust case, 453 U.S. at 509.

Finally, sections 6(b)(7) and 8(c) of the Act authorize OSHA to include among a standard's requirements labeling, monitoring, medical testing, and other information-gathering and -transmittal provisions. 29 U.S.C. 655(b)(7), 657(c).

III. Events Leading to the Proposed Standards

OSHA's current standards for workplace exposure to respirable crystalline silica were adopted in 1971, pursuant to section 6(a) of the OSH Act (36 FR 10466, May 29, 1971). Section 6(a) provided that in the first two years after the effective date of the Act, OSHA had to promulgate “start-up” standards, on an expedited basis and without public hearing or comment, based on national consensus or established Federal standards that improved employee safety or health. Pursuant to that authority, OSHA in 1971 promulgated approximately 425 permissible exposure limits (PELs) for air contaminants, including silica, derived principally from Federal standards applicable to government contractors under the Walsh-Healey Public Contracts Act, 41 U.S.C. 35, and the Contract Work Hours and Safety Standards Act (commonly known as the Construction Safety Act), 40 U.S.C. 333. The Walsh-Healey Act and Construction Safety Act standards, in turn, had been adopted primarily from recommendations of the American Conference of Governmental Industrial Hygienists (ACGIH).

For general industry (see 29 CFR 1910.1000, Table Z-3), the PEL for crystalline silica in the form of respirable quartz is based on two alternative formulas: (1) A particle-count formula, PEL_mppcf = 250/(% quartz + 5); and (2) a mass formula proposed by ACGIH in 1968, PEL = (10 mg/m)/(% quartz + 2). The general industry PELs for cristobalite and tridymite are one-half of the value calculated from either of the above two formulas. For construction (29 CFR 1926.55, Appendix A) and shipyards (29 CFR 1915.1000, Table Z), the formula for the PEL for crystalline silica in the form of quartz (PEL_mppcf = 250/(% quartz + 5)), which requires particle counting, is derived from the 1970 ACGIH threshold limit value (TLV). The formula based on particle-counting technology used in the general industry, construction, and shipyard PELs is now considered obsolete.

In 1974, the National Institute for Occupational Safety and Health (NIOSH) evaluated crystalline silica as a workplace hazard and issued criteria for a recommended standard on occupational exposure to crystalline silica (NIOSH, 1974). NIOSH recommended that occupational exposure to crystalline silica be controlled so that no worker is exposed to a time-weighted average (TWA) of free (respirable crystalline) silica greater than 50 μg/m³ as determined by a full-shift sample for up to a 10-hour workday, 40-hour workweek. The document also recommended a number of ancillary provisions for a standard, such as exposure monitoring and medical surveillance.

In December 1974, OSHA published an Advanced Notice of Proposed Rulemaking (ANPRM) based on the recommendations in the NIOSH criteria document (39 FR 44771, Dec. 27, 1974). In the ANPRM, OSHA solicited “public participation on the issues of whether a new standard for crystalline silica should be issued on the basis of the [NIOSH] criteria or any other information, and, if so, what should be the contents of a proposed standard for crystalline silica.” OSHA also set forth the particular issues of concern on which comments were requested. The Agency did not pursue a final rule for crystalline silica at that time.

As information developed during the 1980s and 1990s, national and international classification organizations came to recognize crystalline silica as a human carcinogen. In June 1986, the International Agency for Research on Cancer (IARC) evaluated the available evidence regarding crystalline silica carcinogenicity and concluded that it was “probably carcinogenic to humans” (IARC, 1987). An IARC working group met again in October 1996 to evaluate the complete body of research, including research that had been conducted since the initial 1986 evaluation. IARC concluded that “crystalline silica inhaled in the form of quartz or cristobalite from occupational sources is carcinogenic to humans” (IARC, 1997).

In 1991, in the Sixth Annual Report on Carcinogens, the U.S. National Toxicology Program (NTP) concluded that respirable crystalline silica was “reasonably anticipated to be a human carcinogen” (NTP, 1991). NTP reevaluated the available evidence and concluded, in the Ninth Report on Carcinogens (NTP, 2000), that “respirable crystalline silica (RCS), primarily quartz dust occurring in industrial and occupational settings, is known to be a human carcinogen, based on sufficient evidence of carcinogenicity from studies in humans indicating a causal relationship between exposure to RCS and increased lung cancer rates in workers exposed to crystalline silica dust” (NTP, 2000). ACGIH listed respirable crystalline silica (in the form of quartz) as a suspected human carcinogen in 2000, while lowering the TLV to 0.05 mg/m³ (ACGIH, 2001). ACGIH subsequently lowered the TLV for crystalline silica to 0.025 mg/m³ in 2006, which is the current value (ACGIH, 2010).

In 1989, OSHA established 8-hour TWA PELs of 0.1 for quartz and 0.05 mg/m³ for cristobalite and tridymite, as part of the Air Contaminants final rule for general industry (54 FR 2332, Jan. 19, 1989). OSHA stated that these limits presented no substantial change from the Agency's former formula limits, but would simplify sampling procedures. In providing comments on the proposed rule, NIOSH recommended that crystalline silica be considered a potential carcinogen.

In 1992, OSHA, as part of the Air Contaminants proposed rule for maritime, construction, and agriculture, proposed the same PELs as for general industry, to make the PELs consistent across all the OSHA-regulated sectors (57 FR 26002, June 12, 1992). However, on July 7 of the same year, the U.S. Court of Appeals for the Eleventh Circuit vacated the 1989 Air Contaminants final rule for general industry (Am. Fed'n of Labor and Cong. of Indus. Orgs. v. OSHA, 965 F.2d 962 (1992)), which also mooted the proposed rule for maritime, construction, and agriculture. The Court's decision to vacate the rule forced the Agency to return to the PELs adopted in the 1970s.

In 1994, OSHA launched a process to determine which safety and health hazards in the U.S. needed most attention. A priority planning committee included safety and health experts from OSHA, NIOSH, and the Mine Safety and Health Administration (MSHA). The committee reviewed available information on occupational deaths, injuries, and illnesses and held an extensive dialogue with representatives of labor, industry, professional and academic organizations, the States, voluntary standards organizations, and the public. The National Advisory Committee on Occupational Safety and Health and the Advisory Committee on Construction Safety and Health also made recommendations. Rulemaking for crystalline silica exposure was one of the priorities designated by this process. OSHA indicated that crystalline silica would be added to the Agency's regulatory agenda as other standards were completed and resources became available.

In August 1996, the Agency initiated enforcement efforts under a Special Emphasis Program (SEP) on crystalline silica. The SEP was intended to reduce worker silica dust exposures that can cause silicosis. It included extensive outreach as well as inspections. Among the outreach materials available were slides presenting information on hazard recognition and crystalline silica control technology, a video on crystalline silica and silicosis, and informational cards for workers explaining crystalline silica, health effects related to exposure, and methods of control. The SEP provided guidance for targeting inspections of worksites with employees at risk of developing silicosis.

As a follow-up to the SEP, OSHA undertook numerous non-regulatory actions to address silica exposures. For example, in October of 1996, OSHA launched a joint silicosis prevention effort with MSHA, NIOSH, and the American Lung Association (DOL, 1996). This public education campaign involved distribution of materials on how to prevent silicosis, including a guide for working safely with silica and stickers for hard hats to remind workers of crystalline silica hazards. Spanish language versions of these materials were also made available. OSHA and MSHA inspectors distributed materials at mines, construction sites, and other affected workplaces. The joint silicosis prevention effort included a National Conference to Eliminate Silicosis in Washington, DC, in March of 1997, which brought together approximately 650 participants from labor, business, government, and the health and safety professions to exchange ideas and share solutions to reach the goal of eliminating silicosis. The conference highlighted the best methods of eliminating silicosis and included problem-solving workshops on how to prevent the disease in specific industries and job operations; plenary sessions with senior government, labor, and corporate officials; and opportunities to meet with safety and health professionals who had implemented successful silicosis prevention programs.

In 2003, OSHA examined enforcement data for the years between 1997 and 2002 and identified high rates of noncompliance with the OSHA respirable crystalline silica PEL, particularly in construction. This period covers the first five years of the SEP. These enforcement data, presented in Table 1, indicate that 24 percent of silica samples from the construction industry and 13 percent from general industry were at least three times the OSHA PEL. The data indicate that 66 percent of the silica samples obtained during inspections in general industry were in compliance with the PEL, while only 58 percent of the samples collected in construction were in compliance.

In an effort to expand the 1996 SEP, on January 24, 2008, OSHA implemented a National Emphasis Program (NEP) to identify and reduce or eliminate the health hazards associated with occupational exposure to crystalline silica (OSHA, 2008). The NEP targeted worksites with elevated exposures to crystalline silica and included new program evaluation procedures designed to ensure that the goals of the NEP were measured as accurately as possible, detailed procedures for conducting inspections, updated information for selecting sites for inspection, development of outreach programs by each Regional and Area Office emphasizing the formation of voluntary partnerships to share information, and guidance on calculating PELs in construction and shipyards. In each OSHA Region, at least two percent of inspections every year are silica-related inspections. Additionally, the silica-related inspections are conducted at a range of facilities reasonably representing the distribution of general industry and construction work sites in that region.

A recent analysis of OSHA enforcement data from January 2003 to December 2009 (covering the period of continued implementation of the SEP and the first two years of the NEP) shows that considerable noncompliance with the PEL continues to occur. These enforcement data, presented in Table 2, indicate that 14 percent of silica samples from the construction industry and 19 percent for general industry were at least three times the OSHA PEL during this period. The data indicate that 70 percent of the silica samples obtained during inspections in general industry were in compliance with the PEL, and 75 percent of the samples collected in construction were in compliance.

Both industry and worker groups have recognized that a comprehensive standard is needed to protect workers exposed to respirable crystalline silica. For example, ASTM (originally known as the American Society for Testing and Materials) has published recommended standards for addressing the hazards of crystalline silica, and the Building and Construction Trades Department, AFL-CIO also has recommended a comprehensive program standard. These recommended standards include provisions for methods of compliance, exposure monitoring, training, and medical surveillance. The National Industrial Sand Association has also developed exposure assessment, medical surveillance, and training guidance products.

In 1997, OSHA announced in its Unified Agenda under Long-Term Actions that it planned to publish a proposed rule on crystalline silica “because the agency has concluded that there will be no significant progress in the prevention of silica-related diseases without the adoption of a full and comprehensive silica standard, including provisions for product substitution, engineering controls, training and education, respiratory protection and medical screening and surveillance. A full standard will improve worker protection, ensure adequate prevention programs, and further reduce silica-related diseases.” (62 FR 57755, 57758, Oct. 29, 1997). In November 1998, OSHA moved “Occupational Exposure to Crystalline Silica” to the pre-rule stage in the Regulatory Plan (63 FR 61284, 61303-304, Nov. 9, 1998). OSHA held a series of stakeholder meetings in 1999 and 2000 to get input on the rulemaking. Stakeholder meetings for all industry sectors were held in Washington, Chicago, and San Francisco. A separate stakeholder meeting for the construction sector was held in Atlanta.

OSHA initiated Small Business Regulatory Enforcement Fairness Act (SBREFA) proceedings in 2003, seeking the advice of small business representatives on the proposed rule (68 FR 30583, 30584, May 27, 2003). The SBREFA panel, including representatives from OSHA, the Small Business Administration (SBA), and the Office of Management and Budget (OMB), was convened on October 20, 2003. The panel conferred with small entity representatives (SERs) from general industry, maritime, and construction on November 10 and 12, 2003, and delivered its final report, which included comments from the SERs and recommendations to OSHA for the proposed rule, to OSHA's Assistant Secretary on December 19, 2003 (OSHA, 2003).

Throughout the crystalline silica rulemaking process, OSHA has presented information to, and has consulted with, the Advisory Committee on Construction Safety and Health (ACCSH) and the Maritime Advisory Committee on Occupational Safety and Health (MACOSH). In December of 2009, OSHA representatives met with ACCSH to discuss the rulemaking and receive their comments and recommendations. On December 11, ACCSH passed motions supporting the concept of Table 1 in the draft proposed construction rule and recognizing that the controls listed in Table 1 are effective. (As discussed with regard to paragraph (f) of the proposed rule, Table 1 presents specified control measures for selected construction operations.) ACCSH also recommended that OSHA maintain the protective clothing provision found in the SBREFA panel draft regulatory text and restore the “competent person” requirement and responsibilities to the proposed rule. Additionally, the group recommended that OSHA move forward expeditiously with the rulemaking process.

In January 2010, OSHA completed a peer review of the draft Health Effects analysis and Preliminary Quantitative Risk Assessment following procedures set forth by OMB in the Final Information Quality Bulletin for Peer Review, published on the OMB Web site on December 16, 2004 (see 70 FR 2664, Jan. 14, 2005). Each peer reviewer submitted a written report to OSHA. The Agency revised its draft documents as appropriate and made the revised documents available to the public as part of this Notice of Proposed Rulemaking. OSHA also made the written charge to the peer reviewers, the peer reviewers' names, the peer reviewers' reports, and the Agency's response to the peer reviewers' reports publicly available with publication of this proposed rule. OSHA will schedule time during the informal rulemaking hearing for participants to testify on the Health Effects analysis and Preliminary Quantitative Risk Assessment in the presence of peer reviewers and will request the peer reviewers to submit any amended final comments they may wish to add to the record. The Agency will consider amended final comments received from the peer reviewers during development of a final rule and will make them publicly available as part of the silica rulemaking record.

IV. Chemical Properties and Industrial Uses

Silica is a compound composed of the elements silicon and oxygen (chemical formula SiO₂). Silica has a molecular weight of 60.08, and exists in crystalline and amorphous states, both in the natural environment and as produced during manufacturing or other processes. These substances are odorless solids, have no vapor pressure, and create non-explosive dusts when particles are suspended in air (IARC, 1997).

Silica is classified as part of the “silicate” class of minerals, which includes compounds that are composed of silicon and oxygen and which may also be bonded to metal ions or their oxides (Hurlbut, 1966). The basic structural units of silicates are silicon tetrahedrons (SiO₄), pyramidal structures with four triangular sides where a silicon atom is located in the center of the structure and an oxygen atom is located at each of the four corners. When silica tetrahedrons bond exclusively with other silica tetrahedrons, each oxygen atom is bonded to the silicon atom of its original ion, as well as to the silicon atom from another silica ion. This results in a ratio of one atom of silicon to two atoms of oxygen, expressed as SiO₂. The silicon-oxygen bonds within the tetrahedrons use only one-half of each oxygen's total bonding energy. This leaves negatively charged oxygen ions available to bond with available positively charged ions. When they bond with metal and metal oxides, commonly of iron, magnesium, aluminum, sodium, potassium, and calcium, they form the silicate minerals commonly found in nature (Bureau of Mines, 1992).

In crystalline silica, the silicon and oxygen atoms are arranged in a three-dimensional repeating pattern. Silica is said to be polymorphic, as different forms are created when the silica tetrahedrons combine in different crystalline structures. The primary forms of crystalline silica are quartz, cristobalite, and tridymite. In an amorphous state, silicon and oxygen atoms are present in the same proportions but are not organized in a repeating pattern. Amorphous silica includes natural and manufactured glasses (vitreous and fused silica, quartz glass), biogenic silica, and opals which are amorphous silica hydrates (IARC, 1997).

Quartz is the most common form of crystalline silica and accounts for almost 12% by volume of the earth's crust. Alpha quartz, the quartz form that is stable below 573 °C, is the most prevalent form of crystalline silica found in the workplace. It accounts for the overwhelming majority of naturally found silica and is present in varying amounts in almost every type of mineral. Alpha quartz is found in igneous, sedimentary, and metamorphic rock, and all soils contain at least a trace amount of quartz (Bureau of Mines, 1992). Alpha quartz is used in many products throughout various industries and is a common component of building materials (Madsen et al., 1995). Common trade names for commercially available quartz include: CSQZ, DQ 12, Min-U-Sil, Sil-Co-Sil, Snowit, Sykron F300, and Sykron F600 (IARC, 1997).

Cristobalite is a form of crystalline silica that is formed at high temperatures (>1470 °C). Although naturally occurring cristobalite is relatively rare, volcanic eruptions, such as Mount St. Helens, can release cristobalite dust into the air. Cristobalite can also be created during some processes conducted in the workplace. For example, flux-calcined diatomaceous earth is a material used as a filtering aid and as a filler in other products (IARC, 1997). It is produced when diatomaceous earth (diatomite), a geological product of decayed unicellular organisms called diatoms, is heated with flux. The finished product can contain between 40 and 60 percent cristobalite. Also, high temperature furnaces are often lined with bricks that contain quartz. When subjected to prolonged high temperatures, this quartz can convert to cristobalite.

Tridymite is another material formed at high temperatures (>870 °C) that is associated with volcanic activity. The creation of tridymite requires the presence of a flux such as sodium oxide. Tridymite is rarely found in nature and rarely reported in the workplace (Smith, 1998).

When heated or cooled sufficiently, crystalline silica can transition between the polymorphic forms, with specific transitions occurring at different temperatures. At higher temperatures the linkages between the silica tetrahedrons break and reform, resulting in new crystalline structures. Quartz converts to cristobalite at 1470 °C, and at 1723 °C cristobalite loses its crystalline structure and becomes amorphous fused silica. These high temperature transitions reverse themselves at extremely slow rates, with different forms co-existing for a long time after the crystal cools.

Other types of transitions occur at lower temperatures when the silica-oxygen bonds in the silica tetrahedron rotate or stretch, resulting in a new crystalline structure. These low-temperature, or alpha to beta, transitions are readily and rapidly reversed as the crystal cools. At temperatures encountered by workers, only the alpha form of crystalline silica exists (IARC, 1997).

Crystalline silica minerals produce distinct X-ray diffraction patterns, specific to their crystalline structure. The patterns can be used to distinguish the crystalline polymorphs from each other and from amorphous silica (IARC, 1997).

The specific gravity and melting point of silica vary between polymorphs. Silica is insoluble in water at 20 °C and in most acids, but its solubility increases with higher temperatures and pH, and it dissolves readily in hydrofluoric acid. Solubility is also affected by the presence of trace metals and by particle size. Under humid conditions water vapor in the air reacts with the surface of silica particles to form an external layer of silinols (SiOH). When these silinols are present the crystalline silica becomes more hydrophilic. Heating or acid washing reduces the amount of silinols on the surface area of crystalline silica particles. There is an external amorphous layer found in aged quartz, called the Beilby layer, which is not found on freshly cut quartz. This amorphous layer is more water soluble than the underlying crystalline core. Etching with hydrofluoric acid removes the Beilby layer as well as the principal metal impurities on quartz.

Crystalline silica has limited chemical reactivity. It reacts with alkaline aqueous solutions, but does not readily react with most acids, with the exception of hydrofluoric acid. In contrast, amorphous silica and most silicates react with most mineral acids and alkaline solutions. Analytical chemists relied on this difference in acid reactivity to develop the silica point count analytical method that was widely used prior to the current X-ray diffraction and infrared methods (Madsen et al., 1995).

Crystalline silica is used in industry in a wide variety of applications. Sand and gravel are used in road building and concrete construction. Sand with greater than 98% silica is used in the manufacture of glass and ceramics. Silica sand is used to form molds for metal castings in foundries, and in abrasive blasting operations. Silica is also used as a filler in plastics, rubber, and paint, and as an abrasive in soaps and scouring cleansers. Silica sand is used to filter impurities from municipal water and sewage treatment plants, and in hydraulic fracturing for oil and gas recovery. Silica is also used to manufacture artificial stone products used as bathroom and kitchen countertops, and the silica content in those products can exceed 93 percent (Kramer et al., 2012).

There are over thirty major industries and operations where exposures to crystalline silica can occur. They include such diverse workplaces as foundries, dental laboratories, concrete products and paint and coating manufacture, as well as construction activities including masonry cutting, grinding and tuckpointing, operating heavy equipment, and road work. A more detailed discussion of the industries affected by the proposed standard is presented in Section VIII of this preamble. Crystalline silica exposures can also occur in mining, and in agriculture during plowing and harvesting.

V. Health Effects Summary

This section presents a summary of OSHA's review of the health effects literature for respirable crystalline silica. OSHA's full analysis is contained in Section I of the background document entitled “Respirable Crystalline Silica—Health Effects Literature Review and Preliminary Quantitative Risk Assessment,” which has been placed in rulemaking docket OSHA-2010-0034. OSHA's review of the literature on the adverse effects associated with exposure to crystalline silica covers the following topics:

(1) Silicosis (including relevant data from U.S. disease surveillance efforts);

(2) Lung cancer and cancer at other sites;

(3) Non-malignant respiratory disease (other than silicosis);

(4) Renal and autoimmune effects; and

(5) Physical factors affecting the toxicity of crystalline silica.

The purpose of the Agency's scientific review is to present OSHA's preliminary findings on the nature of the hazards presented by exposure to respirable crystalline silica, and to present an adequate basis for the quantitative risk assessment section to follow. OSHA's review reflects the relevant literature identified by the Agency through previously published reviews, literature searches, and contact with outside experts. Most of the evidence that describes the health risks associated with exposure to silica consists of epidemiological studies of worker populations; in addition, animal and in vitro studies on mode of action and molecular toxicology are also described. OSHA's review of the silicosis literature focused on a few particular issues, such as the factors that affect progression of the disease and the relationship between the appearance of radiological abnormalities indicative of silicosis and pulmonary function decline. Exposure to respirable crystalline silica is the only known cause of silicosis and there are literally thousands of research papers and case studies describing silicosis among working populations. OSHA did not review every one of these studies, because many of them do not relate to the issues that are of interest to OSHA.

OSHA's health effects literature review addresses exposure only to airborne respirable crystalline silica since there is no evidence that dermal or oral exposure presents a hazard to workers. This review is also confined to issues related to inhalation of respirable dust, which is generally defined as particles that are capable of reaching the gas-exchange region of the lung (i.e., particles less than 10 μm in aerodynamic diameter). The available studies include populations exposed to quartz or cristobalite, the two forms of crystalline silica most often encountered in the workplace. OSHA was unable to identify any relevant epidemiological literature concerning a third polymorph, tridymite, which is also currently regulated by OSHA and included in the scope of OSHA's proposed crystalline silica standard.

OSHA's approach in this review is based on a weight-of-evidence approach, in which studies (both positive and negative) are evaluated for their overall quality, and causal inferences are drawn based on a determination of whether there is substantial evidence that exposure increases the risk of a particular effect. Factors considered in assessing the quality of studies include size of the cohort studied and power of the study to detect a sufficiently low level of disease risk; duration of follow-up of the study population; potential for study bias (such as selection bias in case-control studies or survivor effects in cross-sectional studies); and adequacy of underlying exposure information for examining exposure-response relationships. Studies were deemed suitable for inclusion in OSHA's Preliminary Quantitative Risk Assessment where there was adequate quantitative information on exposure and disease risks and the study was judged to be sufficiently high quality according to the criteria described above. The Preliminary Quantitative Risk Assessment is included in Section II of the background document and is summarized in Section VI of this preamble.

A draft health effects review document was submitted for external scientific peer review in accordance with the Office of Management and Budget's “Final Information Quality Bulletin for Peer Review” (OMB, 2004). A summary of OSHA's responses to the peer reviewers' comments appears in Section III of the background document. Since the draft health effects review document was submitted for external scientific peer review, new studies or reviews examining possible associations between occupational exposure to respirable crystalline silica and lung cancer have been published. OSHA's analysis of that new information is presented in a supplemental literature review and is available in the docket (OSHA, 2013).

A. Silicosis and Disease Progression

1. Pathology and Diagnosis

Silicosis is a progressive disease in which accumulation of respirable crystalline silica particles causes an inflammatory reaction in the lung, leading to lung damage and scarring, and, in some cases, progresses to complications resulting in disability and death. Three types of silicosis have been described: an acute form following intense exposure to respirable dust of high crystalline silica content for a relatively short period (i.e., a few months or years); an accelerated form, resulting from about 5 to 15 years of heavy exposure to respirable dusts of high crystalline silica content; and, most commonly, a chronic form that typically follows less intense exposure of usually more than 20 years (Becklake, 1994; Balaan and Banks, 1992). In both the accelerated and chronic form of the disease, lung inflammation leads to the formation of excess connective tissue, or fibrosis, in the lung. The hallmark of the chronic form of silicosis is the silicotic islet or nodule, one of the few agent-specific lesions in pathology (Balaan and Banks, 1992). As the disease progresses, these nodules, or fibrotic lesions, increase in density and can develop into large fibrotic masses, resulting in progressive massive fibrosis (PMF). Once established, the fibrotic process of chronic silicosis is thought to be irreversible (Becklake, 1994), and there is no specific treatment for silicosis (Davis, 1996; Banks, 2005). Unlike chronic silicosis, the acute form of the disease almost certainly arises from exposures well in excess of current OSHA standards and presents a different pathological picture, one of pulmonary alveolar proteinosis.

Chronic silicosis is the most frequently observed type of silicosis in the U.S. today. Affected workers may have a dry chronic cough, sputum production, shortness of breath, and reduced pulmonary function. These symptoms result from airway restriction and/or obstruction caused by the development of fibrotic scarring in the alveolar sacs and lower region of the lung. The scarring can be detected by chest x-ray or computerized tomography (CT) when the lesions become large enough to appear as visible opacities. The result is restriction of lung volumes and decreased pulmonary compliance with concomitant reduced gas transfer (Balaan and Banks, 1992). Early stages of chronic silicosis can be referred to as either simple or nodular silicosis; later stages are referred to as either pulmonary massive fibrosis (PMF), complicated, or advanced silicosis.

The clinical diagnosis of silicosis has three requisites (Balaan and Banks, 1992; Banks, 2005). The first is the recognition by the physician that exposure to crystalline silica adequate to cause this disease has occurred. The second is the presence of chest radiographic abnormalities consistent with silicosis. The third is the absence of other illnesses that could resemble silicosis on chest radiograph, e.g., pulmonary fungal infection or miliary tuberculosis. To describe the presence and severity of silicosis from chest x-ray films or digital radiographic images, a standardized system exists to classify the opacities seen on chest radiographs (the International Labor Organization (ILO) International Classification of Radiographs of the Pneumoconioses (ILO, 1980, 2002, 2011; Merchant and Schwartz, 1998; NIOSH, 2011). This system standardizes the description of chest x-ray films or digital radiographic images with respect to the size, shape, and density of opacities, which together indicate the severity and extent of lung involvement. The density of opacities seen on chest x-ray films or digital radiographic images is classified on a 4-point major category scale (0, 1, 2, or 3), with each major category divided into three subcategories, giving a 12-point scale between 0/0 and 3/+. (For each subcategory, the top number indicates the major category that the profusion most closely resembles, and the bottom number indicates the major category that was given secondary consideration.) Major category 0 indicates the absence of visible opacities and categories 1 to 3 reflect increasing profusion of opacities and a concomitant increase in severity of disease. Biopsy is not necessary to make a diagnosis and a diagnosis does not require that chest x-ray films or digital radiographic images be rated using the ILO system (NIOSH, 2002). In addition, an assessment of pulmonary function, though not itself necessary to confirm a diagnosis of silicosis, is important to evaluate whether the individual has impaired lung function.

Although chest x-ray is typically used to examine workers exposed to respirable crystalline silica for the presence of silicosis, it is a fairly insensitive tool for detecting lung fibrosis (Hnizdo et al., 1993; Craighead and Vallyathan, 1980; Rosenman et al., 1997). To address the low sensitivity of chest x-rays for detecting silicosis, Hnizdo et al. (1993) recommended that radiographs consistent with an ILO category of 0/1 or greater be considered indicative of silicosis among workers exposed to a high concentration of silica-containing dust. In like manner, to maintain high specificity, chest x-rays classified as category 1/0 or 1/1 should be considered as a positive diagnosis of silicosis.

Newer imaging technologies with both research and clinical applications include computed tomography, and high resolution tomography. High- resolution computed tomography (HRCT) uses thinner image slices and a different reconstruction algorithm to improve spatial resolution over CT. Recent studies of high-resolution computerized tomography (HRCT) have found HRCT to be superior to chest x-ray imaging for detecting small opacities and for identifying PMF (Sun et al., 2008; Lopes et al., 2008; Blum et al., 2008).

The causal relationship between exposure to crystalline silica and silicosis has long been accepted in the scientific and medical communities. Of greater interest to OSHA is the quantitative relationship between exposure to crystalline silica and development of silicosis. A large number of cross-sectional and retrospective studies have been conducted to evaluate this relationship (Kreiss and Zhen, 1996; Love et al., 1999; Ng and Chan, 1994; Rosenman et al., 1996; Hughes et al., 1998; Muir et al., 1989a, 1989b; Park et al., 2002; Chen et al., 2001; Hnizdo and Sluis-Cremer, 1993; Miller et al., 1998; Buchanan et al., 2003; Steenland and Brown, 1995b). In general, these studies, particularly those that included retirees, have found a risk of radiological silicosis (usually defined as x-ray films classified ILO major category 1 or greater) among workers exposed near the range of cumulative exposure permitted by current exposure limits. These studies are presented in detail in OSHA's Preliminary Quantitative Risk Assessment (Section II of the background document and summarized in Section VI of this preamble).

2. Silicosis in the United States

Unlike most occupational diseases, surveillance statistics are available that provide information on the prevalence of silicosis mortality and morbidity in the U.S. The most comprehensive and current source of surveillance data in the U.S. related to occupational lung diseases, including silicosis, is the National Institute for Occupational Safety and Health (NIOSH) Work-Related Lung Disease (WoRLD) Surveillance System; the WoRLD Surveillance Report is compiled from the most recent data from the WoRLD System (NIOSH, 2008c). National statistics on mortality associated with occupational lung diseases are also compiled in the National Occupational Respiratory Mortality System (NORMS, available on the Internet at http://webappa.cdc.gov/ords/norms.html ), a searchable database administered by NIOSH. In addition, NIOSH published a recent review of mortality statistics in its MMWR Report Silicosis Mortality, Prevention, and Control—United States, 1968-2002 (CDC, 2005). For each of these sources, data are compiled from death certificates reported to state vital statistics offices, which are collected by the National Center for Health Statistics (NCHS). Data on silicosis morbidity are available from only a few states that administer occupational disease surveillance systems, and from data on hospital discharges. OSHA believes that the mortality and morbidity statistics compiled in these sources and summarized below indicate that silicosis remains a significant occupational health problem in the U.S. today.

From 1968 to 2002, silicosis was recorded as an underlying or contributing cause of death on 16,305 death certificates; of these, a total of 15,944 (98 percent) deaths occurred in males (CDC, 2005). From 1968 to 2002, the number of silicosis deaths decreased from 1,157 (8.91 per million persons aged ≥ 15 years) to 148 (0.66 per million), corresponding to a 93-percent decline in the overall mortality rate. In its most recent WoRLD Report (NIOSH, 2008c), NIOSH reported that the number of silicosis deaths in 2003, 2004, and 2005 were 179, 166, and 161, respectively, slightly higher than that reported in 2002. The number of silicosis deaths identified each year has remained fairly constant since the late 1990's.

NIOSH cited two main factors that were likely responsible for the declining trend in silicosis mortality since 1968. First, many of the deaths in the early part of the study period occurred among persons whose main exposure to crystalline silica dust probably occurred before introduction of national standards for silica dust exposure established by OSHA and the Mine Safety and Health Administration (MSHA) (i.e., permissible exposure limits (PELs)) that likely led to reduced silica dust exposure. Second, there has been declining employment in heavy industries (e.g., foundries) where silica exposure was prevalent (CDC, 2005). Although the factors described by NIOSH are reasonable explanations for the steep reduction in silicosis-related mortality, it should be emphasized that the surveillance data are insufficient for the analysis of residual risk associated with current occupational exposure limits for crystalline silica. Analyses designed to explore this question must make use of appropriate exposure-response data, as is presented in OSHA's Preliminary Quantitative Risk Assessment (summarized in Section VI of this preamble).

Although the number of deaths from silicosis overall has declined since 1968, the number of silicosis-associated deaths reported among persons aged 15 to 44 had not declined substantially prior to 1995 (CDC 1998). Unfortunately, it is not known to what extent these deaths among younger workers were caused by acute or accelerated forms of silicosis.

Silicosis deaths among workers of all ages result in significant premature mortality; between 1996 and 2005, a total of 1,746 deaths resulted in a total of 20,234 years of life lost from life expectancy, with an average of 11.6 years of life lost. For the same period, among 307 decedents who died before age 65, or the end of a working life, there were 3,045 years of life lost to age 65, with an average of 9.9 years of life lost from a working life (NIOSH, 2008c).

Data on the prevalence of silicosis morbidity are available from only three states (Michigan, Ohio, and New Jersey) that have administered disease surveillance programs over the past several years. These programs rely primarily on hospital discharge records, reporting of cases from the medical community, workers' compensation programs, and death certificate data. For the reporting period 1993-2002, the last year for which data are available, three states (Michigan, New Jersey and Ohio) recorded 879 cases of silicosis (NIOSH 2008c). Hospital discharge records represent the primary ascertainment source for all three states. It should be noted that hospital discharge records most likely include cases of acute silicosis or very advance chronic silicosis since it is unlikely that there would be a need for hospitalization in cases with early radiographic signs of silicosis, such as for an ILO category 1/0 x-ray. Nationwide hospital discharge data compiled by NIOSH (2008c) and the Council of State and Territorial Epidemiologists (CSTE, 2005) indicates that there are at least 1,000 hospitalizations each year due to silicosis.

Data on silicosis mortality and morbidity are likely to understate the true impact of exposure of U.S. workers to crystalline silica. This is in part due to underreporting that is characteristic of passive case-based disease surveillance systems that rely on the health care community to generate records (Froines et al., 1989). Health care professionals play the main role in such surveillance by virtue of their unique role in recognizing and diagnosing diseases, but most health care professionals do not take occupational histories (Goldman and Peters, 1981; Rutstein et al., 1983). In addition to the lack of information about exposure histories, difficulty in recognizing occupational illnesses that have long latency periods, like silicosis, contributes to under-recognition and underreporting by health care providers. Based on an analysis of data from Michigan's silicosis surveillance activities, Rosenman et al. (2003) estimated that the true incidence of silicosis mortality and morbidity were understated by a factor of between 2.5 and 5, and that there were estimated to be from 3,600 to 7,300 new cases of silicosis occurring in the U.S. annually between 1987 and 1996. Taken with the surveillance data presented above, OSHA believes that exposure to crystalline silica remains a cause of significant mortality and morbidity in the U.S.

3. Progression of Silicosis and Its Associated Impairment

As described above, silicosis is a progressive lung disease that is usually first detected by the appearance of a diffuse nodular fibrosis on chest x-ray films. To evaluate the clinical significance of radiographic signs of silicosis, OSHA reviewed several studies that have examined how exposure affects progression of the disease (as seen by chest radiography) as well as the relationship between radiologic findings and pulmonary function. The following summarizes OSHA's preliminary findings from this review.

Of the several studies reviewed by OSHA that documented silicosis progression in populations of workers, four studies (Hughes et al., 1982; Hessel et al., 1988; Miller et al., 1998; Ng et al., 1987a) included quantitative exposure data that were based on either current or historical measurements of respirable quartz. The exposure variable most strongly associated in these studies with progression of silicosis was cumulative respirable quartz (or silica) exposure (Hessel et al., 1988; Hughes et al., 1982; Miller et al., 1998; Ng et al., 1987a), though both average concentration of respirable silica (Hughes et al., 1982; Ng et al., 1987a) and duration of employment in dusty jobs have also been found to be associated with the progression of silicosis (Hughes et al., 1982; Ogawa et al., 2003).

The study reflecting average exposures most similar to current exposure conditions is that of Miller et al. (1998), which followed a group of 547 British coal miners in 1990-1991 to evaluate chest x-ray changes that had occurred after the mines closed in 1981. This study had data available from chest x-rays taken during health surveys conducted between 1954 and 1978, as well as data from extensive exposure monitoring conducted between 1964 and 1978. The mean and maximum cumulative exposure reported in the study correspond to average concentrations of 0.12 and 0.55 mg/m³, respectively, over the 15-year sampling period. However, between 1971 and 1976, workers experienced unusually high concentrations of respirable quartz in one of the two coal seams in which the miners worked. For some occupations, quarterly mean quartz concentrations ranged from 1 to 3 mg/m³, and for a brief period, concentrations exceeded 10 mg/m³ for one job. Some of these high exposures likely contributed to the extent of disease progression seen in these workers; in its Preliminary Quantitative Risk Assessment, OSHA reviewed a study by Buchanan et al. (2003), who found that short-term exposures to high (>2 mg/m³) concentrations of silica can increase the silicosis risk by 3-fold over what would be predicted by cumulative exposure alone (see Section VI).

Among the 504 workers whose last chest x-ray was classified as ILO 0/0 or 0/1, 20 percent had experienced onset of silicosis (i.e., chest x-ray was classified as ILO 1/0 by the time of follow up in 1990-1991), and 4.8 percent progressed to at least category 2. However, there are no data available to continue following the progression of this group because there have been no follow-up surveys of this cohort since 1991.

In three other studies examining the progression of silicosis, (Hessel et al., 1988; Hughes et al., 1982; Ng et al., 1987a) cohorts were comprised of silicotics (individuals already diagnosed with silicosis) that were followed further to evaluate disease progression. These studies reflect exposures of workers to generally higher average concentrations of respirable quartz than are permitted by OSHA's current exposure limit. Some general findings from this body of literature follow. First, size of opacities on initial radiograph is a determinant for further progression. Individuals with large opacities on initial chest radiograph have a higher probability of further disease progression than those with small opacities (Hughes et al., 1982; Lee, et al., 2001; Ogawa et al., 2003). Second, although silicotics who continue to be exposed are more likely to progress than silicotics who are not exposed (Hessel et al., 1988), once silicosis has been detected there remains a likelihood of progression in the absence of additional exposure to silica (Hessel et al., 1988; Miller et al., 1998; Ogawa, et al., 2003; Yang et al., 2006). There is some evidence in the literature that the probability of progression is likely to decline over time following the end of the exposure, although this observation may also reflect a survivor effect (Hughes et al., 1982; Lee et al., 2001). In addition, of borderline statistical significance was the association of tuberculosis with increased likelihood of silicosis progression (Lee et al., 2001).

Of the four studies reviewed by OSHA that provided quantitative exposure information, two studies (Miller et al., 1998; Ng et al., 1987a) provide the information most relevant to current exposure conditions. The range of average concentration of respirable crystalline silica to which workers were exposed in these studies (0.12 to 0.48 mg/m³, respectively) is relatively narrow and is of particular interest to OSHA because current enforcement data indicate that exposures in this range or not much lower are common today, especially in construction and foundries, and sandblasting operations. These studies reported the percentage of workers whose chest x-rays show signs of progression at the time of follow-up; the annual rate at which workers showed disease progression were similar, 2 percent and 6 percent, respectively.

Several cross-sectional and longitudinal studies have examined the relationship between progressive changes observed on radiographs and corresponding declines in lung-function parameters. In general, the results are mixed: some studies have found that pulmonary function losses correlate with the extent of fibrosis seen on chest x-ray films, and others have not found such correlations. The lack of a correlation in some studies between degree of fibrotic profusion seen on chest x-rays and pulmonary function have led some to suggest that pulmonary function loss is an independent effect of exposure to respirable crystalline silica, or may be a consequence of emphysematous changes that have been seen in conjunction with radiographic silicosis.

Among studies that have reported finding a relationship between pulmonary function and x-ray abnormalities, Ng and Chan (1992) found that forced expiratory volume (FEV₁) and forced vital capacity (FVC) were statistically significantly lower for workers whose x-ray films were classified as ILO profusion categories 2 and 3, but not among workers with ILO category 1 profusion compared to those with a profusion score of 0/0. As expected, highly significant reductions in FEV₁, FVC, and FEV₁/FVC were noted in subjects with large opacities. The authors concluded that chronic simple silicosis, except that classified as profusion category 1, is associated with significant lung function impairment attributable to fibrotic disease.

Similarly, Moore et al. (1988) also found chronic silicosis to be associated with significant lung function loss, especially among workers with chest x-rays classified as ILO profusion categories 2 and 3. For those classified as category 1, lung function was not diminished. Bégin et al. (1988) also found a correlation between decreased lung function (FVC and the ratio of FEV₁/FVC) and increased profusion and coalescence of opacities as determined by CT scan. This study demonstrated increased impairment among workers with higher imaging categories (3 and 4), as expected, but also impairment (significantly reduced expiratory flow rates) among persons with more moderate pulmonary fibrosis (group 2).

In a population of gold miners, Cowie (1998) found that lung function declined more rapidly in men with silicosis than those without. In addition to the 24 ml./yr. decrements expected due to aging, this study found an additional loss of 8 ml. of FEV₁ per year would be expected from continued exposure to dust in the mines. An earlier cross-sectional study by these authors (Cowie and Mabena, 1991), which examined 1,197 black underground gold miners who had silicosis, found that silicosis (analyzed as a continuous variable based on chest x-ray film classification) was associated with reductions in FVC, FEV_1, FEV₁/FVC, and carbon monoxide diffusing capacity (DL_co), and these relationships persisted after controlling for duration and intensity of exposure and smoking.

In contrast to these studies, other investigators have reported finding pulmonary function decrements in exposed workers independent of radiological evidence of silicosis. Hughes et al. (1982) studied a representative sample of 83 silicotic sandblasters, 61 of whom were followed for one to seven years. A multiple regression analysis showed that the annual reductions in FVC, FEV₁ and DL_co were related to average silica concentrations but not duration of exposure, smoking, stage of silicosis, or time from initial exposure. Ng et al. (1987b) found that, among male gemstone workers in Hong Kong with x-rays classified as either Category 0 or 1, declines in FEV₁ and FVC were not associated with radiographic category of silicosis after adjustment for years of employment. The authors concluded that there was an independent effect of respirable dust exposure on pulmonary function. In a population of 61 gold miners, Wiles et al. (1992) also found that radiographic silicosis was not associated with lung function decrements. In a re-analysis and follow-up of an earlier study, Hnizdo (1992) found that silicosis was not a significant predictor of lung function, except for FEV₁ for non-smokers.

Wang et al. (1997) observed that silica-exposed workers (both nonsmokers and smokers), even those without radiographic evidence of silicosis, had decreased spirometric parameters and diffusing capacity (DL_co). Pulmonary function was further decreased in the presence of silicosis, even those with mild to moderate disease (ILO categories 1 and 2). The authors concluded that functional abnormalities precede radiographic changes of silicosis.

A number of studies were conducted to examine the role of emphysematous changes in the presence of silicosis in reducing lung function; these have been reviewed by Gamble et al. (2004), who concluded that there is little evidence that silicosis is related to development of emphysema in the absence of PMF. In addition, Gamble et al. (2004) found that, in general, studies found that the lung function of those with radiographic silicosis in ILO category 1 was indistinguishable from those in category 0, and that those in category 2 had small reductions in lung function relative to those with category 0 and little difference in the prevalence of emphysema. There were slightly greater decrements in lung function with category 3 and more significant reductions with progressive massive fibrosis. In studies for which information was available on both silicosis and emphysema, reduced lung function was more strongly related to emphysema than to silicosis.

In conclusion, many studies reported finding an association between pulmonary function decrements and ILO category 2 or 3 background profusion of small opacities; this appears to be consistent with the histopathological view, in which individual fibrotic nodules conglomerate to form a massive fibrosis (Ng and Chan, 1992). Emphysema may also play a role in reducing lung function in workers with higher grades of silicosis. Pulmonary function decrements have not been reported in some studies among workers with silicosis scored as ILO category 1. However, a number of other studies have documented declines in pulmonary function in persons exposed to silica and whose radiograph readings are in the major ILO category 1 (i.e. 1/0, 1/1, 1/2), or even before changes were seen on chest x-ray (Bégin et al., 1988; Cowie, 1998; Cowie and Mabena, 1991; Ng et al., 1987a; Wang et al., 1997). It may also be that studies designed to relate x-ray findings with pulmonary function declines are further confounded by pulmonary function declines caused by chronic obstructive pulmonary disease (COPD) seen among silica-exposed workers absent radiological silicosis, as has been seen in many investigations of COPD. OSHA's review of the literature on crystalline silica exposure and development of COPD appears in section II.D of the background document and is summarized in section V.D below.

OSHA believes that the literature reviewed above demonstrates decreased lung function among workers with radiological evidence of silicosis consistent with an ILO classification of major category 2 or higher. Also, given the evidence of functional impairment in some workers prior to radiological evidence of silicosis, and given the low sensitivity of radiography, particularly in detecting early silicosis, OSHA believes that exposure to silica impairs lung function in at least some individuals before silicosis can be detected on chest radiograph.

4. Pulmonary Tuberculosis

As silicosis progresses, it may be complicated by severe mycobacterial infections, the most common of which is pulmonary tuberculosis (TB). Active tuberculosis infection is a well-recognized complication of chronic silicosis, and such infections are known as silicotuberculosis (IARC, 1997; NIOSH, 2002). The risk of developing TB infection is higher in silicotics than non-silicotics (Balmes, 1990; Cowie, 1994; Hnizdo and Murray, 1998; Kleinschmidt and Churchyard, 1997; and Murray et al., 1996). There also is evidence that exposure to silica increases the risk for pulmonary tuberculosis independent of the presence of silicosis (Cowie, 1994; Hnizdo and Murray, 1998; teWaterNaude et al., 2006). In a summary of the literature on silica-related disease mechanisms, Ding et al. (2002) noted that it is well documented that exposure to silica can lead to impaired cell-mediated immunity, increasing susceptibility to mycobacterial infection. Reduced numbers of T-cells, increased numbers of B-cells, and alterations of serum immunoglobulin levels have been observed in workers with silicosis. In addition, according to Ng and Chan (1991), silicosis and TB act synergistically to increase fibrotic scar tissue (leading to massive fibrosis) or to enhance susceptibility to active mycobacterial infection. Lung fibrosis is common to both diseases and both diseases decrease the ability of alveolar macrophages to aid in the clearance of dust or infectious particles.

B. Carcinogenic Effects of Silica (Cancer of the Lung and Other Sites)

OSHA conducted an independent review of the epidemiological literature on exposure to respirable crystalline silica and lung cancer, covering more than 30 occupational groups in over a dozen industrial sectors. In addition, OSHA reviewed a pooled case-control study, a large national death certificate study, two national cancer registry studies, and six meta-analyses. In all, OSHA's review included approximately 60 primary epidemiological studies.

Based on its review, OSHA preliminarily concludes that the human data summarized in this section provides ample evidence that exposure to respirable crystalline silica increases the risk of lung cancer among workers. The strongest evidence comes from the worldwide cohort and case-control studies reporting excess lung cancer mortality among workers exposed to respirable crystalline silica dust as quartz in various industrial sectors, including the granite/stone quarrying and processing, industrial sand, mining, and pottery and ceramic industries, as well as to cristobalite in diatomaceous earth and refractory brick industries. The 10-cohort pooled case-control analysis by Steenland et al. (2001a) confirms these findings. A more recent clinic-based pooled case-control analysis of seven European countries by Cassidy et al. (2007) as well as two national death certificate registry studies (Pukkala et al., 2005 in Finland; Calvert et al., 2003 in the United States) support the findings from the cohort and case-control analysis.

1. Overall and Industry Sector-Specific Findings

Associations between exposure to respirable crystalline silica and lung cancer have been reported in worker populations from many different industrial sectors. IARC (1997) concluded that crystalline silica is a confirmed human carcinogen based largely on nine studies of cohorts in four industry sectors that IARC considered to be the least influenced by confounding factors (sectors included quarries and granite works, gold mining, ceramic/pottery/refractory brick industries, and the diatomaceous earth industry). IARC (2012) recently reaffirmed that crystalline silica is a confirmed human carcinogen. NIOSH (2002) also determined that crystalline silica is a human carcinogen after evaluating updated literature.

OSHA believes that the strongest evidence for carcinogenicity comes from studies in five industry sectors. These are:

• Diatomaceous Earth Workers (Checkoway et al., 1993, 1996, 1997, and 1999; Seixas et al., 1997);
• British Pottery Workers (Cherry et al., 1998; McDonald et al., 1995);
• Vermont Granite Workers (Attfield and Costello, 2004; Graham et al., 2004; Costello and Graham, 1988; Davis et al., 1983);
• North American Industrial Sand Workers (Hughes et al., 2001; McDonald et al., 2001, 2005; Rando et al., 2001; Sanderson et al., 2000; Steenland and Sanderson, 2001); and
• British Coal Mining (Miller et al., 2007; Miller and MacCalman, 2009).

The studies above were all retrospective cohort or case-control studies that demonstrated positive, statistically significant exposure-response relationships between exposure to crystalline silica and lung cancer mortality. Except for the British pottery studies, where exposure-response trends were noted for average exposure only, lung cancer risk was found to be related to cumulative exposure. OSHA credits these studies because in general, they are of sufficient size and have adequate years of follow up, and have sufficient quantitative exposure data to reliably estimate exposures of cohort members. As part of their analyses, the authors of these studies also found positive exposure-response relationships for silicosis, indicating that underlying estimates of worker exposures were not likely to be substantially misclassified. Furthermore, the authors of these studies addressed potential confounding due to other carcinogenic exposures through study design or data analysis.

A series of studies of the diatomaceous earth industry (Checkoway et al., 1993, 1996, 1997, 1999) demonstrated positive exposure-response trends between cristobalite exposures and lung cancer as well as non-malignant respiratory disease mortality (NMRD). Checkoway et al. (1993) developed a “semi-quantitative” cumulative exposure estimate that demonstrated a statistically significant positive exposure-response trend (p = 0.026) between duration of employment or cumulative exposure and lung cancer mortality. The quartile analysis showed a monotonic increase in lung cancer mortality, with the highest exposure quartile having a RR of 2.74 for lung cancer mortality. Checkoway et al. (1996) conducted a re-analysis to address criticisms of potential confounding due to asbestos and again demonstrated a positive exposure response risk gradient when controlling for asbestos exposure and other variables. Rice et al. (2001) conducted a re-analysis and quantitative risk assessment of the Checkoway et al. (1997) study, which OSHA has included as part of its assessment of lung cancer mortality risk (See Section II, Preliminary Quantitative Risk Assessment).

In the British pottery industry, excess lung cancer risk was found to be associated with crystalline silica exposure among workers in a PMR study (McDonald et al., 1995) and in a cohort and nested case-control study (Cherry et al., 1998). In the PMR study, elevated PMRs for lung cancer were found after adjusting for potential confounding by asbestos exposure. In the study by Cherry et al., odds ratios for lung cancer mortality were statistically significantly elevated after adjusting for smoking. Odds ratios were related to average, but not cumulative, exposure to crystalline silica. The findings of the British pottery studies are supported by other studies within their industrial sector. Studies by Winter et al. (1990) of British pottery workers and by McLaughlin et al. (1992) both reported finding suggestive trends of increased lung cancer mortality with increasing exposure to respirable crystalline silica.

Costello and Graham (1988) and Graham et al. (2004) in a follow-up study found that Vermont granite workers employed prior to 1930 had an excess risk of lung cancer, but lung cancer mortality among granite workers hired after 1940 (post-implementation of controls) was not elevated in the Costello and Graham (1988) study and was only somewhat elevated (not statistically significant) in the Graham et al. (2004) study. Graham et al. (2004) concluded that their results did not support a causal relationship between granite dust exposure and lung cancer mortality. Looking at the same population, Attfield and Costello (2004) developed a quantitative estimate of cumulative exposure (8 exposure categories) adapted from a job exposure matrix developed by Davis et al. (1983). They found a statistically significant trend with log-transformed cumulative exposure. Lung cancer mortality rose reasonably consistently through the first seven increasing exposure groups, but fell in the highest cumulative exposure group. With the highest exposure group omitted, a strong positive dose-response trend was found for both untransformed and log-transformed cumulative exposures. Attfield and Costello (2004) concluded that exposure to crystalline silica in the range of cumulative exposures typically experienced by contemporarily exposed workers causes an increased risk of lung cancer mortality. The authors explained that the highest exposure group would have included the most unreliable exposure estimates being reconstructed from exposures 20 years prior to study initiation when exposure estimation was less precise. Also, even though the highest exposure group consisted of only 15 percent of the study population, it had a disproportionate effect on dampening the exposure-response relationship.

OSHA believes that the study by Attfield and Costello (2004) is of superior design in that it was a categorical analysis that used quantitative estimates of exposure and evaluated lung cancer mortality rates by exposure group. In contrast, the findings by Graham et al. (2004) are based on a dichotomous comparison of risk among high- versus low-exposure groups, where date-of-hire before and after implementation of ventilation controls is used as a surrogate for exposure. Consequently, OSHA believes that the study by Attfield and Costello is the more convincing study, and is one of the studies used by OSHA for quantitative risk assessment of lung cancer mortality due to crystalline silica exposure.

The conclusions of the Vermont granite worker study (Attfield and Costello, 2004) are supported by the findings in studies of workers in the U.S. crushed stone industry (Costello et al., 1995) and Danish stone industry (Guénel et al., 1989a, 1989b). Costello et al. (1995) found a non-statistically significant increase in lung cancer mortality among limestone quarry workers and a statistically significant increased lung cancer mortality in granite quarry workers who worked 20 years or more since first exposure. Guénel et al. (1989b), in a Danish cohort study, found statistically significant increases in lung cancer incidence among skilled stone workers and skilled granite stone cutters. A study of Finnish granite workers that initially showed increasing risk of lung cancer with increasing silica exposure, upon extended follow-up, did not show an association and is therefore considered a negative study (Toxichemica, Inc., 2004).

Studies of two overlapping cohorts in the industrial sand industry (Hughes et al., 2001; McDonald et al., 2001, 2005; Rando et al., 2001; Sanderson et al., 2000; Steenland and Sanderson, 2001) reported comparable results. These studies found a statistically significantly increased risk of lung cancer mortality with increased cumulative exposure in both categorical and continuous analyses. McDonald et al. (2001) examined a cohort that entered the workforce, on average, a decade earlier than the cohorts that Steenland and Sanderson (2001) examined. The McDonald cohort, drawn from eight plants, had more years of exposure in the industry (19 versus 8.8 years). The Steenland and Sanderson (2001) cohort worked in 16 plants, 7 of which overlapped with the McDonald, et al. (2001) cohort. McDonald et al. (2001), Hughes et al. (2001), and Rando et al. (2001) had access to smoking histories, plant records, and exposure measurements that allowed for historical reconstruction and the development of a job exposure matrix. Steenland and Sanderson (2001) had limited access to plant facilities, less detailed historic exposure data, and used MSHA enforcement records for estimates of recent exposure. These studies (Hughes et al., 2001; McDonald et al., 2005; Steenland and Sanderson, 2001) show very similar exposure response patterns of increased lung cancer mortality with increased exposure. OSHA included the quantitative exposure-response analysis from the Hughes et al. (2001) study in its Preliminary Quantitative Risk Assessment (Section II).

Brown and Rushton (2005a, 2005b) found no association between risk of lung cancer mortality and exposure to respirable crystalline silica among British industrial sand workers. However, the small sample size and number of years of follow-up limited the statistical power of the analysis. Additionally, as Steenland noted in a letter review (2005a), the cumulative exposures of workers in the Brown and Ruston (2005b) study were over 10 times lower than the cumulative exposures experienced by the cohorts in the pooled analysis that Steenland et al. (2001b) performed. The low exposures experienced by this cohort would have made detecting a positive association with lung cancer mortality even more difficult.

Excess lung cancer mortality was reported in a large cohort study of British coal miners (Miller et al., 2007; Miller and MacCalman, 2009). These studies examined the mortality experience of 17,800 miners through the end of 2005. By that time, the cohort had accumulated 516,431 person years of observation (an average of 29 years per miner), with 10,698 deaths from all causes. Overall lung cancer mortality was elevated (SMR=115.7, 95% C.I. 104.8-127.7), and a positive exposure-response relationship with crystalline silica exposure was determined from Cox regression after adjusting for smoking history. Three of the strengths of this study are the detailed time-exposure measurements of both quartz and total mine dust, detailed individual work histories, and individual smoking histories. For lung cancer, analyses based on the Cox regression provide strong evidence that, for these coal miners, quartz exposures were associated with increased lung cancer risk but that simultaneous exposures to coal dust did not cause increased lung cancer risk. Because of these strengths, OSHA included the quantitative analysis from this study in its Preliminary Quantitative Risk Assessment (Section II).

Studies of lung cancer mortality in metal ore mining populations reflect mixed results. Many of these mining studies were subject to confounding due to exposure to other potential carcinogens such as radon and arsenic. IARC (1997) noted that in only a few ore mining studies was confounding from other occupational carcinogens taken into account. IARC (1997) also noted that, where confounding was absent or accounted for in the analysis (gold miners in the U.S., tungsten miners in China, and zinc and lead miners in Sardinia, Italy), an association between silica exposure and lung cancer was absent. Many of the studies conducted since IARC's (1997) review more strongly implicate crystalline silica as a human carcinogen. Pelucchi et al. (2006), in a meta-analysis of studies conducted since IARC's (1997) review, reported statistically significantly elevated relative risks of lung cancer mortality in underground and surface miners in three cohort and four case-control studies (See Table I-15). Cassidy et al. (2007), in a pooled case-control analysis, showed a statistically significant increased risk of lung cancer mortality among miners (OR = 1.48). Cassidy et al. (2007) also demonstrated a clear linear trend of increasing odds ratios for lung cancer with increasing exposures.

Among workers in Chinese tungsten and iron mines, mortality from lung cancer was not found to be statistically significantly increased (Chen et al., 1992; McLaughlin et al., 1992). In contrast, studies of Chinese tin miners found increased lung cancer mortality rates and positive exposure-response associations with increased silica exposure (Chen et al., 1992). Unfortunately, in many of these Chinese tin mines, there was potential confounding from arsenic exposure, which was highly correlated with exposure to crystalline silica (Chen and Chen, 2002; Chen et al., 2006). Two other studies (Carta et al. (2001) of Sardinian miners and stone quarrymen; Finkelstein (1998) primarily of Canadian miners) were limited to silicotics. The Sardinian study found a non-statistically significant association between crystalline silica exposure and lung cancer mortality but no apparent exposure-response trend with silica exposure. The authors attributed the increased lung cancer to increased radon exposure and smoking among cases as compared to controls. Finkelstein (1998) found a positive association between silica exposure and lung cancer.

Gold mining has been extensively studied in the United States, South Africa, and Australia in four cohort and associated nested case-control studies, and in two separate case-control studies conducted in South Africa. As with metal ore mining, gold mining involves exposure to radon and other carcinogenic agents, which may confound the relationship between silica exposure and lung cancer. The U.S. gold miner study (Steenland and Brown, 1995a) did not find an increased risk of lung cancer, while the western Australian gold miner study (de Klerk and Musk, 1998) showed a SMR of 149 (95% CI 1.26-1.76) for lung cancer. Logistic regression analysis of the western Australian case control data showed that lung cancer mortality was statistically significantly associated with log cumulative silica exposure after adjusting for smoking and bronchitis. After additionally adjusting for silicosis, the relative risk remained elevated but was no longer statistically significant. The authors concluded that their findings showed statistically significantly increased lung cancer mortality in this cohort but that the increase in lung cancer mortality was restricted to silicotic members of the cohort.

Four studies of gold miners were conducted in South Africa. Two case control studies (Hessel et al., 1986, 1990) reported no significant association between silica exposure and lung cancer, but these two studies may have underestimated risk, according to Hnizdo and Sluis-Cremer (1991). Two cohort studies (Reid and Sluis-Cremer, 1996; Hnizdo and Sluis-Cremer, 1991) and their associated nested case-control studies found elevated SMRs and odds ratios, respectively, for lung cancer. Reid and Sluis-Cremer (1996) attributed the increased mortality due to lung cancer and other non-malignant respiratory diseases to cohort members' lifestyle choices (particularly smoking and alcohol consumption). However, OSHA notes that the study reported finding a positive, though not statistically significant, association between cumulative crystalline silica exposure and lung cancer, as well as statistically significant association with renal failure, COPD, and other respiratory diseases that have been implicated with silica exposure.

In contrast, Hnizdo and Sluis-Cremer (1991) found a positive exposure-response relationship between cumulative exposure and lung cancer mortality among South African gold miners after accounting for smoking. In a nested case-control study from the same cohort, Hnizdo et al. (1997) found a statistically significant increase in lung cancer mortality that was associated with increased cumulative dust exposure and time spent underground. Of the studies examining silica and lung cancer among South African gold miners, these two studies were the least likely to have been affected by exposure misclassification, given their rigorous methodologies and exposure measurements. Although not conclusive in isolation, OSHA considers the mining study results, particularly the gold mining and the newer mining studies, as supporting evidence of a causal relationship between exposure to silica and lung cancer risk.

OSHA has preliminarily determined that the results of the studies conducted in three industry sectors (foundry, silicon carbide, and construction sectors) were confounded by the presence of exposures to other carcinogens. Exposure data from these studies were not sufficient to distinguish between exposure to silica dust and exposure to other occupational carcinogens. Thus, elevated rates of lung cancer found in these industries could not be attributed to silica. IARC previously made a similar determination in reference to the foundry industry. However, with respect to the construction industry, Cassidy et al. (2007), in a large, European community-based case-control study, reported finding a clear linear trend of increasing odds ratio with increasing cumulative exposure to crystalline silica (estimated semi-quantitatively) after adjusting for smoking and exposure to insulation and wood dusts. Similar trends were found for workers in the manufacturing and mining industries as well. This study was a very large multi-national study that utilized information on smoking histories and exposure to silica and other occupational carcinogens. OSHA believes that this study provides further evidence that exposure to crystalline silica increases the risk of lung cancer mortality and, in particular, in the construction industry.

In addition, a recent analysis of 4.8 million death certificates from 27 states within the U.S. for the years 1982 to 1995 showed statistically significant excesses in lung cancer mortality, silicosis mortality, tuberculosis, and NMRD among persons with occupations involving medium and high exposure to respirable crystalline silica (Calvert et al., 2003). A national records and death certificate study was also conducted in Finland by Pukkala et al. (2005), who found a statistically significant excess of lung cancer incidence among men and women with estimated medium and heavy exposures. OSHA believes that these large national death certificate studies and the pooled European community-based case-control study are strongly supportive of the previously reviewed epidemiologic data and supports the conclusion that occupational exposure to crystalline silica is a risk factor for lung cancer mortality.

One of the more compelling studies evaluated by OSHA is the pooled analysis of 10 occupational cohorts (5 mines and 5 industrial facilities) conducted by Steenland et al. (2001a), which demonstrated an overall positive exposure-response relationship between cumulative exposure to silica and lung cancer mortality. These ten cohorts included 65,980 workers and 1,072 lung cancer deaths, and were selected because of the availability of raw data on exposure to crystalline silica and health outcomes. The investigators used a nested case control design and found lung cancer risk increased with increasing cumulative exposure, log cumulative exposure, and average exposure. Exposure-response trends were similar between mining and non-mining cohorts. From their analysis, the authors concluded that “[d]espite this relatively shallow exposure-response trend, overall our results tend to support the recent conclusion by IARC (1997) that inhaled crystalline silica in occupational settings is a human carcinogen, and suggest that existing permissible exposure limits for silica need to be lowered (Steenland et al., 2001a). To evaluate the potential effect of random and systematic errors in the underlying exposure data from these 10 cohort studies, Steenland and Bartell (Toxichemica, Inc., 2004) conducted a series of sensitivity analyses at OSHA's request. OSHA's Preliminary Quantitative Risk Assessment (Section II) presents additional information on the Steenland et al. (2001a) pooled cohort study and the sensitivity analysis performed by Steenland and Bartell (Toxichemica, Inc., 2004).

2. Smoking, Silica Exposure, and Lung Cancer

Smoking is known to be a major risk factor for lung cancer. However, OSHA believes it is unlikely that smoking explains the observed exposure-response trends in the studies described above, particularly the retrospective cohort or nested case-control studies of diatomaceous earth, British pottery, Vermont granite, British coal, South African gold, and industrial sand workers. Also, the positive associations between silica exposure and lung cancer in multiple studies in multiple sectors indicates that exposure to crystalline silica independently increases the risk of lung cancer.

Studies by Hnizdo et al. (1997), McLaughlin et al. (1992), Hughes et al. (2001), McDonald et al. (2001, 2005), Miller and MacCalman (2009), and Cassidy et al. (2007) had detailed smoking histories with sufficiently large populations and a sufficient number of years of follow-up time to quantify the interaction between crystalline silica exposure and cigarette smoking. In a cohort of white South African gold miners (Hnizdo and Sluis-Cremer, 1991) and in the follow-up nested case-control study (Hnizdo et al., 1997) found that the combined effect of exposure to respirable crystalline silica and smoking was greater than additive, suggesting a multiplicative effect. This synergy appeared to be greatest for miners with greater than 35 pack-years of smoking and higher cumulative exposure to silica. In the Chinese nested case-control studies reported by McLaughlin et al. (1992), cigarette smoking was associated with lung cancer, but control for smoking did not influence the association between silica and lung cancer in the mining and pottery cohorts studied. The studies of industrial sand workers by Hughes et al. (2001) and British coal workers by Miller and MacCalman (2009) found positive exposure-response trends after adjusting for smoking histories, as did Cassidy et al. (2007) in their community-based case-control study of exposed European workers.

In reference to control of potential confounding by cigarette smoking in crystalline silica studies, Stayner (2007), in an invited journal commentary, stated:

Of particular concern in occupational cohort studies is the difficulty in adequately controlling for confounding by cigarette smoking. Several of the cohort studies that adjusted for smoking have demonstrated an excess of lung cancer, although the control for smoking in many of these studies was less than optimal. The results of the article by Cassidy et al. presented in this journal appear to have been well controlled for smoking and other workplace exposures. It is quite implausible that residual confounding by smoking or other risk factors for lung cancer in this or other studies could explain the observed excess of lung cancer in the wide variety of populations and study designs that have been used. Also, it is generally considered very unlikely that confounding by smoking could explain the positive exposure-response relationships observed in these studies, which largely rely on comparisons between workers with similar socioeconomic backgrounds.

Given the findings of investigators who have accounted for the impact of smoking, the weight of the evidence reviewed here implicates respirable crystalline silica as an independent risk factor for lung cancer mortality. This finding is further supported by animal studies demonstrating that exposure to silica alone can cause lung cancer (e.g., Muhle et al., 1995).

3. Silicosis and Lung Cancer Risk

In general, studies of workers with silicosis, as well as meta-analyses that include these studies, have shown that workers with radiologic evidence of silicosis have higher lung cancer risk than those without radiologic abnormalities or mixed cohorts. Three meta-analyses attempted to look at the association of increasing ILO radiographic categories of silicosis with increasing lung cancer mortality. Two of these analyses (Kurihara and Wada, 2004; Tsuda et al., 1997) showed no association with increasing lung cancer mortality, while Lacasse et al. (2005) demonstrated a positive dose-response for lung cancer with increasing ILO radiographic category. A number of other studies, discussed above, found increased lung cancer risk among exposed workers absent radiological evidence of silicosis (Cassidy et al., 2007; Checkoway et al., 1999; Cherry et al., 1998; Hnizdo et al., 1997; McLaughlin et al., 1992). For example, the diatomaceous earth study by Checkoway et al. (1999) showed a statistically significant exposure-response for lung cancer among non-silicotics. Checkoway and Franzblau (2000), reviewing the international literature, found all epidemiological studies conducted to that date were insufficient to conclusively determine the role of silicosis in the etiology of lung cancer. OSHA preliminarily concludes that the more recent pooled and meta-analyses do not provide compelling evidence that silicosis is a necessary precursor to lung cancer. The analyses that do suggest an association between silicosis and lung cancer may simply reflect that more highly exposed individuals are at a higher risk for lung cancer.

Animal and in vitro studies have demonstrated that the early steps in the proposed mechanistic pathways that lead to silicosis and lung cancer seem to share some common features. This has led some of these researchers to also suggest that silicosis is a prerequisite to lung cancer. Some have suggested that any increased lung cancer risk associated with silica may be a consequence of the inflammation (and concomitant oxidative stress) and increased epithelial cell proliferation associated with the development of silicosis. However, other researchers have noted that other key factors and proposed mechanisms, such as direct damage to DNA by silica, inhibition of p53, loss of cell cycle regulation, stimulation of growth factors, and production of oncogenes, may also be involved in carcinogenesis induced by silica (see Section II.F of the background document for more information on these studies). Thus, OSHA preliminarily concludes that available animal and in vitro studies do not support the hypothesis that development of silicosis is necessary for silica exposure to cause lung cancer.

4. Relationship Between Silica Polymorphs and Lung Cancer Risk

OSHA's current PELs for respirable crystalline silica reflects a once-held belief that cristobalite is more toxic than quartz (i.e., the existing general industry PEL for cristobalite is one-half the general industry PEL for quartz). Available evidence indicates that this does not appear to be the case with respect to the carcinogenicity of crystalline silica. A comparison between cohorts having principally been exposed to cristobalite (the diatomaceous earth study and the Italian refractory brick study) with other well conducted studies of quartz-exposed cohorts suggests no difference in the toxicity of cristobalite versus quartz. The data indicates that the SMRs for lung cancer mortality among workers in the diatomaceous earth (SMR = 141) and refractory brick (SMR=151) cohort studies are within the range of the SMR point estimates of other cohort studies with principally quartz exposures (quartz exposure of Vermont granite workers yielding an SMR of 117; quartz and possible post-firing cristobalite exposure of British pottery workers yielding an SMR of 129; quartz exposure among industrial sand workers yielding SMRs of 129, (McDonald et al., 2001) and 160 (Steenland and Sanderson, 2001)). Also, the SMR point estimates for the diatomaceous earth and refractory brick studies are similar to, and fall within the 95 percent confidence interval of, the odds ratio (OR=1.37, 95% CI 1.14-1.65) of the recently conducted multi-center case-control study in Europe (Cassidy et al., 2007).

OSHA believes that the current epidemiological literature provides little, if any, support for treating cristobalite as presenting a greater lung cancer risk than comparable exposure to respirable quartz. Furthermore, the weight of the available toxicological literature no longer supports the hypothesis that cristobalite has a higher toxicity than quartz, and quantitative estimates of lung cancer risk do not suggest that cristobalite is more carcinogenic than quartz. (See Section I.F of the background document, Physical Factors that May Influence Toxicity of Crystalline Silica, for a fuller discussion of this issue.) OSHA preliminary concludes that respirable cristobalite and quartz dust have similar potencies for increasing lung cancer risk. Both IARC (1997) and NIOSH (2002) reached similar conclusions.

5. Cancers of Other Sites

Respirable crystalline silica exposure has also been investigated as a potential risk factor for cancer at other sites such as the larynx, nasopharynx and the digestive system including the esophagus and stomach. Although many of these studies suggest an association between exposure to crystalline silica and an excess risk of cancer mortality, most are too limited in terms of size, study design, or potential for confounding to be conclusive. Other than for lung cancer, cancer mortality studies demonstrating a dose-response relationship are quite limited. In their silica hazard review, NIOSH (2002) concluded that, exclusive of the lung, an association has not been established between silica exposure and excess mortality from cancer at other sites. A brief summary of the relevant literature is presented below.

a. Cancer of the Larynx and Nasopharynx

Several studies, including three of the better-quality lung cancer studies (Checkoway et al., 1997; Davis et al., 1983; McDonald et al., 2001) suggest an association between exposure to crystalline silica and increased mortality from laryngeal cancer. However, the evidence for an association is not strong due to the small number of cases reported and lack of statistical significance of most of the findings.

b. Gastric (Stomach) Cancer

In their 2002 hazard review of respirable crystalline silica, NIOSH identified numerous epidemiological studies and reported statistically significant increases in death rates due to gastric or stomach cancer. OSHA preliminarily concurs with observations made previously by Cocco et al. (1996) and the NIOSH (2002) crystalline silica hazard review that the vast majority of epidemiology studies of silica and stomach cancer have not sufficiently adjusted for the effects of confounding factors or have not been sufficiently designed to assess a dose-response relationship (e.g., Finkelstein and Verma, 2005; Moshammer and Neuberger, 2004; Selikoff, 1978, Stern et al., 2001). Other studies did not demonstrate a statistically significant dose-response relationship (e.g., Calvert et al., 2003; Tsuda et al., 2001). Therefore, OSHA believes the evidence is insufficient to conclude that silica is a gastric carcinogen.

c. Esophageal Cancer

Three well-conducted nested case-control studies of Chinese workers indicated an increased risk of esophageal cancer mortality attributed by the study's authors to respirable crystalline silica exposure in refractory brick production, boiler repair, and foundry workers (Pan et al., 1999; Wernli et al., 2006) and caisson construction work (Yu et al., 2005). Each study demonstrated a dose-response association with some surrogate measure of exposure, but confounding due to other occupational exposures is possible in all three work settings (heavy metal exposure in the repair of boilers in steel plants, PAH exposure in foundry workers, radon and radon daughter exposure in Hong Kong caisson workers). Other less well-constructed studies also indicated elevated rates of esophageal cancer mortality with silica exposure (Tsuda et al., 2001; Xu et al., 1996a).

In contrast, two large national mortality studies in Finland and the United States, using qualitatively ranked exposure estimates, did not show a positive association between silica exposure and esophageal cancer mortality (Calvert et al., 2003; Weiderpass et al., 2003). OSHA preliminarily concludes that the epidemiological literature is not sufficiently robust to attribute increased esophageal cancer mortality to exposure to respirable crystalline silica.

d. Other Miscellaneous Cancers

In 2002, NIOSH conducted a thorough literature review of the health effects potentially associated with crystalline silica exposure including a review of lung cancer and other carcinogens. NIOSH noted that for workers who may have been exposed to crystalline silica, there have been infrequent reports of statistically significant excesses of deaths for other cancers. A summary of these cancer studies as cited in NIOSH (2002) have been reported in the following organ systems (see NIOSH, 2002 for full bibliographic references): salivary gland; liver; bone; pancreatic; skin; lymphopoetic or hematopoietic; brain; and bladder.

According to NIOSH (2002), an association has not been established between these cancers and exposure to crystalline silica. OSHA believes that these isolated reports of excess cancer mortality at these sites are not sufficient to draw any inferences about the role of silica exposure. The findings have not been consistently seen among epidemiological studies and there is no evidence of an exposure response relationship.

C. Other Nonmalignant Respiratory Disease

In addition to causing silicosis, exposure to crystalline silica has been associated with increased risks of other non-malignant respiratory diseases (NMRD), primarily chronic obstructive pulmonary disease (COPD). COPD is a disease state characterized by airflow limitation that is not fully reversible. The airflow limitation is usually progressive and is associated with an abnormal inflammatory response of the lungs to noxious particles or gases. In patients with COPD, either chronic bronchitis or emphysema may be present or both conditions may be present together. The following presents OSHA's discussion of the literature describing the relationships between silica exposure and non-malignant respiratory disease.

1. Emphysema

OSHA has considered a series of longitudinal studies of white South African gold miners conducted by Hnizdo and co-workers. Hnizdo et al. (1991) found a significant association between emphysema (both panacinar and centriacinar) and years of employment in a high dust occupation (respirable dust was estimated to contain 30 percent free silica). There was no such association found for non-smokers, as there were only four non-smokers with a significant degree of emphysema found in the cohort. A further study by Hnizdo et al. (1994) looked at only life-long non-smoking South African gold miners. In this population, no significant degree of emphysema or association with years of exposure or cumulative dust exposure was found. However, the degree of emphysema was significantly associated with the degree of hilar gland nodules, which the authors suggested might act as a surrogate for exposure to silica. The authors concluded that the minimal degree of emphysema seen in non-smoking miners exposed to the cumulative dust levels found in this study (mean 6.8 mg/m³, SD 2.4, range 0.5 to 20.2, 30 percent crystalline silica) was unlikely to cause meaningful impairment of lung function.

From the two studies above, Hnizdo et al. (1994) concluded that the statistically significant association between exposure to silica dust and the degree of emphysema in smokers suggests that tobacco smoking potentiates the effect of silica dust. In contrast to their previous studies, a later study by Hnizdo et al. (2000) of South African gold miners found that emphysema prevalence was decreased in relation to dust exposure. The authors suggested that selection bias was responsible for this finding.

The findings of several cross-sectional and case-control studies were more mixed. Becklake et al. (1987), in an unmatched case-control study of white South African gold miners, determined that a miner who had worked in high dust for 20 years had a greater chance of getting emphysema than a miner who had never worked in high dust. A reanalysis of this data (de Beer et al., 1992) including added-back cases and controls (because of possible selection bias in the original study), still found an increased risk for emphysema, although the reported odds ratio was smaller than previously reported by Becklake et al. (1987). Begin et al. (1995), in a study of the prevalence of emphysema in silica-exposed workers with and without silicosis, found that silica-exposed smokers without silicosis had a higher prevalence of emphysema than a group of asbestos-exposed workers with similar smoking history. In non-smokers, the prevalence of emphysema was much higher in those with silicosis than in those without silicosis. A study of black underground gold miners found that the presence and grade of emphysema were statistically significantly associated with the presence of silicosis but not with years of mining (Cowie et al., 1993).

Several of the above studies (Becklake et al., 1987; Begin et al., 1995; Hnizdo et al., 1994) found that emphysema can occur in silica-exposed workers who do not have silicosis and suggest that a causal relationship may exist between exposure to silica and emphysema. The findings of experimental (animal) studies that emphysema occurs at lower silica doses than does fibrosis in the airways or the appearance of early silicotic nodules (e.g., Wright et al., 1988) tend to support the findings in human studies that silica-induced emphysema can occur absent signs of silicosis.

Others have also concluded that there is a relationship between emphysema and exposure to crystalline silica. Green and Vallyathan (1996) reviewed several studies of emphysema in workers exposed to silica. The authors stated that these studies show an association between cumulative dust exposure and death from emphysema. IARC (1997) has also briefly reviewed studies on emphysema in its monograph on crystalline silica carcinogenicity and concluded that exposure to crystalline silica increases the risk of emphysema. In their 2002 Hazard Review, NIOSH concluded that occupational exposure to respirable crystalline silica is associated with emphysema but that some epidemiologic studies suggested that this effect may be less frequent or absent in non-smokers.

Hnizdo and Vallyathan (2003) also conducted a review of studies addressing COPD due to occupational silica exposure and concluded that chronic exposure to silica dust at levels that do not cause silicosis may cause emphysema.

Based on these findings, OSHA preliminarily concludes that exposure to respirable crystalline silica or silica-containing dust can increase the risk of emphysema, regardless of whether silicosis is present. This appears to be clearly the case for smokers. It is less clear whether nonsmokers exposed to silica would also be at higher risk and if so, at what levels of exposure. It is also possible that smoking potentiates the effect of silica dust in increasing emphysema risk.

2. Chronic Bronchitis

There were no longitudinal studies available designed to investigate the relationship between silica exposure and bronchitis. However, several cross-sectional studies provide useful information. Studies are about equally divided between those that have reported a relationship between silica exposure and bronchitis and those that have not. Several studies demonstrated a qualitative or semiquantitative relationship between silica exposure and chronic bronchitis. Sluis-Cremer et al. (1967) found a significant difference between the prevalence of chronic bronchitis in dust-exposed and non-dust exposed male residents of a South African gold mining town who smoked, but found no increased prevalence among non-smokers. In contrast, a different study of South African gold miners found that the prevalence of chronic bronchitis increased significantly with increasing dust concentration and cumulative dust exposure in smokers, nonsmokers, and ex-smokers (Wiles and Faure, 1977). Similarly, a study of Western Australia gold miners found that the prevalence of chronic bronchitis, as indicated by odds ratios (controlled for age and smoking), was significantly increased in those that had worked in the mines for 1 to 9 years, 10 to 19 years, and more than 20 years, as compared to lifetime non-miners (Holman et al., 1987). Chronic bronchitis was present in 62 percent of black South African gold miners and 45 percent of those who had never smoked in a study by Cowie and Mabena (1991). The prevalence of what the researchers called “chronic bronchitic symptom complex” reflected the intensity of dust exposure. A higher prevalence of respiratory symptoms, independent of smoking and age, was also found for granite quarry workers in Singapore in a high exposure group as compared to low exposure and control groups, even after excluding those with silicosis from the analysis (Ng et al., 1992b).

Other studies found no relationship between silica exposure and the prevalence of chronic bronchitis. Irwig and Rocks (1978) compared silicotic and non-silicotic South African gold miners and found no significant difference in symptoms of chronic bronchitis. The prevalence of symptoms of chronic bronchitis were also not found to be associated with years of mining, after adjusting for smoking, in a population of current underground uranium miners (Samet et al., 1984). Silica exposure was described in the study to be “on occasion” above the TLV. It was not possible to determine, however, whether miners with respiratory diseases had left the workforce, making the remaining population unrepresentative. Hard-rock (molybdenum) miners, with 27 and 49 percent of personal silica samples greater than 100 and 55 μg/m³, respectively, also showed no increase in prevalence of chronic bronchitis in association with work in that industry (Kreiss et al., 1989). However, the authors thought that differential out-migration of symptomatic miners and retired miners from the industry and town might explain that finding. Finally, grinders of agate stones (with resulting dust containing 70.4 percent silica) in India also had no increase in the prevalence of chronic bronchitis compared to controls matched by socioeconomic status, age and smoking, although there was a significantly higher prevalence of acute bronchitis in female grinders. A significantly higher prevalence and increasing trend with exposure duration for pneumoconiosis in the agate workers indicated that had an increased prevalence in chronic bronchitis been present, it would have been detected (Rastogi et al., 1991). However, control workers in this study may also have been exposed to silica and the study and control workers both had high tuberculosis prevalence, possibly masking an association of exposure with bronchitis (NIOSH, 2002). Furthermore, exposure durations were very short.

Thus, some prevalence studies supported a finding of increased bronchitis in workers exposed to silica-containing dust, while other studies did not support such a finding. However, OSHA believes that many of the studies that did not find such a relationship were likely to be biased towards the null. For example, some of the molybdenum miners studied by Kreiss et al. (1989), particularly retired and symptomatic miners, may have left the town and the industry before the time that the cross-sectional study was conducted, resulting in a survivor effect that could have interfered with detection of a possible association between silica exposure and bronchitis. This survivor effect may also have been operating in the study of uranium miners in New Mexico (Samet et al., 1984). In two of the negative studies, members of comparison and control groups were also exposed to crystalline silica (Irwig and Rocks, 1978; Rastogi et al., 1991), creating a potential bias toward the null. Additionally, tuberculosis in both exposed and control groups in the agate worker study (Rastogi et al., 1991)) may have masked an effect (NIOSH, 2002), and the exposure durations were very short. Several of the positive studies demonstrated a qualitative or semi-quantitative relationship between silica exposure and chronic bronchitis.

Others have reviewed relevant studies and also concluded that there is a relationship between exposure to crystalline silica and the development of bronchitis. The American Thoracic Society (ATS) (1997) published an official statement on the adverse effects of crystalline silica exposure that included a section that discussed studies on chronic bronchitis (defined by chronic sputum production). According to the ATS review, chronic bronchitis was found to be common among worker groups exposed to dusty environments contaminated with silica. In support of this conclusion, ATS cited studies with what they viewed as positive findings of South African (Hnizdo et al., 1990) and Australian (Holman et al., 1987) gold miners, Indonesian granite workers (Ng et al., 1992b), and Indian agate workers (Rastogi et al., 1991). ATS did not mention studies with negative findings.

A review published by NIOSH in 2002 discussed studies related to silica exposure and development of chronic bronchitis. NIOSH concluded, based on the same studies reviewed by OSHA, that occupational exposure to respirable crystalline silica is associated with bronchitis, but that some epidemiologic studies suggested that this effect may be less frequent or absent in non-smokers.

Hnizdo and Vallyathan (2003) also reviewed studies addressing COPD due to occupational silica exposure and concluded that chronic exposure to silica dust at levels that do not cause silicosis may cause chronic bronchitis. They based this conclusion on studies that they cited as showing that the prevalence of chronic bronchitis increases with intensity of exposure. The cited studies were also reviewed by OSHA (Cowie and Mabena, 1991; Holman et al., 1987; Kreiss et al., 1989; Sluis-Cremer et al., 1967; Wiles and Faure, 1977).

OSHA preliminarily concludes that exposure to respirable crystalline silica may cause chronic bronchitis and an exposure-response relationship may exist. Smokers may be at increased risk as compared to non-smokers. Chronic bronchitis may occur in silica-exposed workers who do not have silicosis.

3. Pulmonary Function Impairment

OSHA has reviewed numerous studies on the relationship of silica exposure to pulmonary function impairment as measured by spirometry. There were several longitudinal studies available. Two groups of researchers conducted longitudinal studies of lung function impairment in Vermont granite workers and reached opposite conclusions. Graham et al (1981, 1994) examined stone shed workers, who had the highest exposures to respirable crystalline silica (between 50 and 100 μg/m³), along with quarry workers (presumed to have lower exposure) and office workers (expected to have negligible exposure). The longitudinal losses of FVC and FEV₁ were not correlated with years employed, did not differ among shed, quarry, and office workers, and were similar, according to the authors, to other blue collar workers not exposed to occupational dust.

Eisen et al. (1983, 1995) found the opposite. They looked at lung function in two groups of granite workers: “survivors”, who participated in each of five annual physical exams, and “dropouts”, who did not participate in the final exam. There was a significant exposure-response relationship between exposure to crystalline silica and FEV₁ decline among the dropouts but not among the survivors. The dropout group had a steeper FEV₁ loss, and this was true for each smoking category. The authors concluded that exposures of about 50 ug/m³ produced a measurable effect on pulmonary function in the dropouts. Eisen et al. (1995) felt that the “healthy worker effect” was apparent in this study and that studies that only looked at “survivors” would be less likely to see any effect of silica on pulmonary function.

A 12-year follow-up of age- and smoking-matched granite crushers and referents in Sweden found that over the follow-up period, the granite crushers had significantly greater decreases in FEV₁, FEV₁/FVC, maximum expiratory flow, and FEF₅₀ than the referents (Malmberg et al., 1993). A longitudinal study of South African gold miners conducted by Hnizdo (1992) found that cumulative dust exposure was a significant predictor of most indices of decreases in lung function, including FEV₁ and FVC. A multiple linear regression analysis showed that the effects of silica exposure and smoking were additive. Another study of South African gold miners (Cowie, 1998) also found a loss of FEV₁ in those without silicosis. Finally, a study of U.S. automotive foundry workers (Hertzberg et al., 2002) found a consistent association with increased pulmonary function abnormalities and estimated measures of cumulative silica exposure within 0.1 mg/m³. The Hnizdo (1992), Cowie et al. (1993), and Cowie (1998) studies of South African gold miners and the Malmberg et al. (1993) study of Swedish granite workers found very similar reductions in FEV₁ attributable to silica dust exposure.

A number of prevalence studies have described relationships between lung function loss and silica exposure or exposure measurement surrogates (e.g., duration of exposure). These findings support those of the longitudinal studies. Such results have been found in studies of white South African gold miners (Hnizdo et al., 1990; Irwig and Rocks, 1978), black South African gold miners (Cowie and Mabena, 1991), Quebec silica-exposed workers (Begin, et al., 1995), Singapore rock drilling and crushing workers (Ng et al., 1992b), Vermont granite shed workers (Theriault et al., 1974a, 1974b), aggregate quarry workers and coal miners in Spain (Montes et al., 2004a, 2004b), concrete workers in The Netherlands (Meijer et al., 2001), Chinese refractory brick manufacturing workers in an iron-steel plant (Wang et al., 1997), Chinese gemstone workers (Ng et al., 1987b), hard-rock miners in Manitoba, Canada (Manfreda et al., 1982) and Colorado (Kreiss et al., 1989), pottery workers in France (Neukirch et al., 1994), potato sorters exposed to diatomaceous earth containing crystalline silica in The Netherlands (Jorna et al., 1994), slate workers in Norway (Suhr et al., 2003), and men in a Norwegian community (Humerfelt et al., 1998). Two of these prevalence studies also addressed the role of smoking in lung function impairment associated with silica exposure. In contrast to the longitudinal study of South African gold miners discussed above (Hnizdo, 1992), another study of South African gold miners (Hnizdo et al., 1990) found that the joint effect of dust and tobacco smoking on lung function impairment was synergistic, rather than additive. Also, Montes et al. (2004b) found that the criteria for dust-tobacco interactions were satisfied for FEV₁ decline in a study of Spanish aggregate quarry workers.

One of the longitudinal studies and many of the prevalence studies discussed above directly addressed the question of whether silica-exposed workers can develop pulmonary function impairment in the absence of silicosis. These studies found that pulmonary function impairment: (1) Can occur in silica-exposed workers in the absence of silicosis, (2) was still evident when silicosis was controlled for in the analysis, and (3) was related to the magnitude and duration of silica exposure rather than to the presence or severity of silicosis.

Many researchers have concluded that a relationship exists between exposure to silica and lung function impairment. IARC (1997) has briefly reviewed studies on airways disease (i.e., chronic airflow limitation and obstructive impairment of lung function) in its monograph on crystalline silica carcinogenicity and concluded that exposure to crystalline silica causes these effects. In its official statement on the adverse effects of crystalline silica exposure, the American Thoracic Society (ATS) (1997) included a section on airflow obstruction. The ATS noted that, in most of the studies reviewed, airflow limitation was associated with chronic bronchitis. The review of Hnizdo and Vallyathan (2003) also addressed COPD due to occupational silica exposure. They examined the epidemiological evidence for an exposure-response relationship for airflow obstruction in studies where silicosis was present or absent. Hnizdo and Vallyathan (2003) concluded that chronic exposure to silica dust at levels that do not cause silicosis may cause airflow obstruction.

Based on the evidence discussed above from a number of longitudinal studies and numerous cross-sectional studies, OSHA preliminarily concludes that there is an exposure-response relationship between exposure to respirable crystalline silica and the development of impaired lung function. The effect of tobacco smoking on this relationship may be additive or synergistic. Also, pulmonary function impairment has been shown to occur among silica-exposed workers who do not show signs of silicosis.

4. Non-malignant Respiratory Disease Mortality

In this section, OSHA reviews studies on NMRD mortality that focused on causes of death other than from silicosis. Two studies of gold miners, a study of diatomaceous earth workers, and a case-control analysis of death certificate data provide useful information.

Wyndham et al. (1986) found a significant excess mortality for chronic respiratory diseases in a cohort of white South African gold miners. Although these data did include silicosis mortality, the authors found evidence demonstrating that none of the miners certified on the death certificate as dying from silicosis actually died from that disease. Instead, pneumoconiosis was always an incidental finding in those dying from some other cause, the most common of which was chronic obstructive lung disease. A case-referent analysis found that, although the major risk factor for chronic respiratory disease was smoking, there was a statistically significant additional effect of cumulative dust exposure, with the relative risk estimated to be 2.48 per ten units of 1000 particle years of exposure.

A synergistic effect of smoking and cumulative dust exposure on mortality from COPD was found in another study of white South African gold miners (Hnizdo, 1990). Analysis of various combinations of dust exposure and smoking found a trend in odds ratios that indicated this synergism. There was a statistically significant increasing trend for dust particle-years and for cigarette-years of smoking. For cumulative dust exposure, an exposure-response relationship was found, with the analysis estimating that those with exposures of 10,000, 17,500, or 20,000 particle-years of exposure had a 2.5-, 5.06-, or 6.4-times higher mortality risk for COPD, respectively, than those with the lowest dust exposure of less than 5000 particle-years. The authors concluded that dust alone would not lead to increased COPD mortality but that dust and smoking act synergistically to cause COPD and were thus the main risk factor for death from COPD in their study.

Park et al. (2002) analyzed the California diatomaceous earth cohort data originally studied by Checkoway et al. (1997), consisting of 2,570 diatomaceous earth workers employed for 12 months or more from 1942 to 1994, to quantify the relationship between exposure to cristobalite and mortality from chronic lung disease other than cancer (LDOC). Diseases in this category included pneumoconiosis (which included silicosis), chronic bronchitis, and emphysema, but excluded pneumonia and other infectious diseases. Smoking information was available for about 50 percent of the cohort and for 22 of the 67 LDOC deaths available for analysis, permitting Park et al. (2002) to at least partially adjust for smoking. Using the exposure estimates developed for the cohort by Rice et al. (2001) in their exposure-response study of lung cancer risks, Park et al. (2002) evaluated the quantitative exposure-response relationship for LDOC mortality and found a strong positive relationship with exposure to respirable crystalline silica. OSHA finds this study particularly compelling because of the strengths of the study design and availability of smoking history data on part of the cohort and high-quality exposure and job history data; consequently, OSHA has included this study in its Preliminary Quantitative Risk Assessment.

In a case-control analysis of death certificate data drawn from 27 U.S. states, Calvert et al. (2003) found increased mortality odds ratios among those in the medium and higher crystalline silica exposure categories, a significant trend of increased risk for COPD mortality with increasing silica exposures, and a significantly increased odds ratio for COPD mortality in silicotics as compared to those without silicosis.

Green and Vallyathan (1996) also reviewed several studies of NMRD mortality in workers exposed to silica. The authors stated that these studies showed an association between cumulative dust exposure and death from the chronic respiratory diseases.

Based on the evidence presented in the studies above, OSHA preliminarily concludes that respirable crystalline silica increases the risk for mortality from non-malignant respiratory disease (not including silicosis) in an exposure-related manner. However, it appears that the risk is strongly influenced by smoking, and the effects of smoking and silica exposure may be synergistic.

D. Renal and Autoimmune Effects

In recent years, evidence has accumulated that suggests an association between exposure to crystalline silica and an increased risk of renal disease. Over the past 10 years, epidemiologic studies have been conducted that provide evidence of exposure-response trends to support this association. There is also suggestive evidence that silica can increase the risk of rheumatoid arthritis and other autoimmune diseases (Steenland, 2005b). In fact, an autoimmune mechanism has been postulated for some silica-associated renal disease (Calvert et al., 1997). This section will discuss the evidence supporting an association of silica exposure with renal and autoimmune diseases.

Overall, there is substantial evidence suggesting an association between exposure to crystalline silica and increased risks of renal and autoimmune diseases. In addition to a number of case reports, epidemiologic studies have found statistically significant associations between occupational exposure to silica dust and chronic renal disease (e.g., Calvert et al., 1997), subclinical renal changes (e.g., Ng et al., 1992c), end-stage renal disease morbidity (e.g., Steenland et al., 1990), chronic renal disease mortality (Steenland et al., 2001b, 2002a), and Wegener's granulomatosis (Nuyts et al., 1995). In other findings, silica-exposed individuals, both with and without silicosis, had an increased prevalence of abnormal renal function (Hotz et al., 1995), and renal effects have been reported to persist after cessation of silica exposure (Ng et al., 1992c). Possible mechanisms suggested for silica-induced renal disease include a direct toxic effect on the kidney, deposition in the kidney of immune complexes (IgA) following silica-related pulmonary inflammation, or an autoimmune mechanism (Calvert et al., 1997; Gregorini et al., 1993).

Several studies of exposed worker populations reported finding excess renal disease mortality and morbidity. Wyndham et al. (1986) reported finding excess mortality from acute and chronic nephritis among South African goldminers that had been followed for 9 years. Italian ceramic workers experienced an overall increase in the prevalence of end-stage renal disease (ESRD) cases compared to regional rates; the six cases that occurred among the workers had cumulative exposures to crystalline silica of between 0.2 and 3.8 mg/m³-years (Rapiti et al., 1999).

Calvert et al. (1997) found an increased incidence of non-systemic ESRD cases among 2,412 South Dakota gold miners exposed to a median crystalline silica concentration of 0.09 mg/m³. In another study of South Dakota gold miners, Steenland and Brown (1995a) reported a positive trend of chronic renal disease mortality risk and cumulative exposure to respirable crystalline silica, but most of the excess deaths were concentrated among workers hired before 1930 when exposures were likely higher than in more recent years.

Excess renal disease mortality has also been described among North American industrial sand workers. McDonald et al., (2001, 2005) found that nephritis/nephrosis mortality was elevated overall among 2,670 industrial sand workers hired 20 or more years prior to follow-up, but there was no apparent relationship with either cumulative or average exposure to crystalline silica. However, Steenland et al. (2001b) did find that increased mortality from acute and chronic renal disease was related to increasing quartiles of cumulative exposure among a larger cohort of 4,626 industrial sand workers. In addition, they also found a positive trend for ESRD case incidence and quartiles of cumulative exposure.

In a pooled cohort analysis, Steenland et al. (2002a) combined the industrial sand cohort from Steenland et al. (2001b), gold mining cohort from Steenland and Brown (1995a), and the Vermont granite cohort studies by Costello and Graham (1988). In all, the combined cohort consisted of 13,382 workers with exposure information available for 12,783. The exposure estimates were validated by the monotonically increasing exposure-response trends seen in analyses of silicosis, since cumulative silica levels are known to predict silicosis risk. The mean duration of exposure, cumulative exposure, and concentration of respirable silica for the cohort were 13.6 years, 1.2 mg/m³-years, and 0.07 mg/m³, respectively.

The analysis demonstrated statistically significant exposure-response trends for acute and chronic renal disease mortality with quartiles of cumulative exposure to respirable crystalline silica. In a nested case-control study design, a positive exposure-response relationship was found across the three cohorts for both multiple-cause mortality (i.e., any mention of renal disease on the death certificate) and underlying cause mortality. Renal disease risk was most prevalent among workers with cumulative exposures of 0.5 mg/m³ or more (Steenland et al., 2002a).

Other studies failed to find an excess renal disease risk among silica-exposed workers. Davis et al. (1983) found an elevated, but not a statistically significant increase, in mortality from diseases of the genitourinary system among Vermont granite shed workers. There was no observed relationship between mortality from this cause and cumulative exposure. A similar finding was reported by Koskela et al. (1987) among Finnish granite workers, where there were 4 deaths due to urinary tract disease compared to 1.8 expected. Both Carta et al. (1994) and Cocco et al. (1994) reported finding no increased mortality from urinary tract disease among workers in an Italian lead mine and a zinc mine. However, Cocco et al. (1994) commented that exposures to respirable crystalline silica were low, averaging 0.007 and 0.09 mg/m³ in the two mines, respectively, and that their study in particular had low statistical power to detect excess mortality.

There are many case series, case-control, and cohort studies that provide support for a causal relationship between exposure to respirable crystalline silica and an increased renal disease risk (Kolev et al., 1970; Osorio et al., 1987; Steenland et al., 1990; Gregorini et al., 1993; Nuyts et al., 1995). In addition, a number of studies have demonstrated early clinical signs of renal dysfunction (i.e., urinary excretion of low- and high-molecular weight proteins and other markers of renal glomerular and tubular disruption) in workers exposed to crystalline silica, both with and without silicosis (Ng et al., 1992c; Hotz et al., 1995; Boujemaa, 1994; Rosenman et al., 2000).

OSHA believes that there is substantial evidence on which to base a finding that exposure to respirable crystalline silica increases the risk of renal disease mortality and morbidity. In particular, OSHA believes that the 3-cohort pooled analysis conducted by Steenland et al. (2002a) is particularly convincing. OSHA believes that the findings of this pooled analysis seem credible because the analysis involved a large number of workers from three cohorts with well-documented, validated job-exposure matrices and found a positive and monotonic increase in renal disease risk with increasing exposure for both underlying and multiple cause data. However, there are considerably less data, and thus the findings based on them are less robust, than what is available for silicosis mortality or lung cancer mortality. Nevertheless, OSHA preliminarily concludes that the underlying data are sufficient to provide useful estimates of risk and has included the Steenland et al. (2002a) analysis in its Preliminary Quantitative Risk Assessment.

Several studies of different designs, including case series, cohort, registry linkage and case-control, conducted in a variety of exposed groups suggest an association between silica exposure and increased risk of systemic autoimmune disease (Parks et al., 1999). Studies have found that the most common autoimmune diseases associated with silica exposure are scleroderma (e.g., Sluis-Cremer et al., 1985); rheumatoid arthritis (e.g. Klockars et al., 1987; Rosenman and Zhu, 1995); and systemic lupus erythematosus (e.g., Brown et al., 1997). Mechanisms suggested for silica-related autoimmune disease include an adjuvant effect of silica (Parks et al., 1999), activation of the immune system by the fibrogenic proteins and growth factors released as a result of the interaction of silica particles with macrophages (e.g., Haustein and Anderegg, 1998), and a direct local effect of non-respirable silica particles penetrating the skin and producing scleroderma (Green and Vallyathan, 1996). However, there are no quantitative exposure-response data available at this time on which to base a quantitative risk assessment for autoimmune diseases.

Therefore, OSHA preliminarily concludes that there is substantial evidence that silica exposure increases the risks of renal and autoimmune disease. The positive and monotonic exposure-response trends demonstrated for silica exposure and renal disease risk more strongly suggest a causal link. The studies by Steenland et al. (2001b, 2002a) and Steenland and Brown (1995a) provide evidence of a positive exposure-response relationship. For autoimmune diseases, the available data did not provide an adequate basis for assessing exposure-response relationships. However, OSHA believes that the available exposure-response data on silica exposure and renal disease is sufficient to allow for quantitative estimates of risk.

E. Physical Factors That May Influence Toxicity of Crystalline Silica

Much research has been conducted to investigate the influence of various physical factors on the toxicologic potency of crystalline silica. Such factors examined include crystal polymorphism; the age of fractured surfaces of the crystal particle; the presence of impurities, particularly metals, on particle surfaces; and clay occlusion of the particle. These factors likely vary among different workplace settings suggesting that the risk to workers exposed to a given level of respirable crystalline silica may not be equivalent in different work environments. In this section, OSHA examines the research demonstrating the effects of these factors on the toxicologic potency of silica.

The modification of surface characteristics by the physical factors noted above may alter the toxicity of silica by affecting the physical and biochemical pathways of the mechanistic process. Thus, OSHA has reviewed the proposed mechanisms by which silica exposure leads to silicosis and lung cancer. It has been proposed that silicosis results from a cycle of cell damage, oxidant generation, inflammation, scarring and fibrosis. A silica particle entering the lung can cause lung damage by two major mechanisms: direct damage to lung cells due to the silica particle's unique surface properties or by the activation or stimulation of alveolar macrophages (after phagocytosis) and/or alveolar epithelial cells. In either case, an elevated production of reactive oxygen and nitrogen species (ROS/RNS) results in oxidant damage to lung cells. The oxidative stress and lung injury stimulates alveolar macrophages and/or alveolar epithelial cells to produce growth factors and fibrogenic mediators, resulting in fibroblast activation and pulmonary fibrosis. A continuous ingestion-reingestion cycle, with cell activation and death, is established.

OSHA has examined evidence on the comparative toxicity of the silica polymorphs (quartz, cristobalite, and tridymite). A number of animal studies appear to suggest that cristobalite and tridymite are more toxic to the lung than quartz and more tumorigenic (e.g., King et al., 1953; Wagner et al., 1980). However, in contrast to these findings, several authors have reviewed the studies done in this area and concluded that cristobalite and tridymite are not more toxic than quartz (e.g., Bolsaitis and Wallace, 1996; Guthrie and Heaney, 1995). Furthermore, a difference in toxicity between cristobalite and quartz has not been observed in epidemiologic studies (tridymite has not been studied) (NIOSH, 2002). In an analysis of exposure-response for lung cancer, Steenland et al. (2001a) found similar exposure-response trends between cristobalite-exposed workers and other cohorts exposed to quartz.

A number of studies have compared the toxicity of freshly fractured versus aged silica. Although animal studies have demonstrated that freshly fractured silica is more toxic than aged silica, aged silica still retains significant toxicity (Porter et al., 2002; Shoemaker et al., 1995; Vallyathan et al., 1995). Studies of workers exposed to freshly fractured silica have demonstrated that these workers exhibit the same cellular effects as seen in animals exposed to freshly fractured silica (Castranova et al., 1998; Goodman et al., 1992). There have been no studies, however, comparing workers exposed to freshly fractured silica to those exposed to aged silica. Animal studies also suggest that pulmonary reactions of rats to short-duration exposure to freshly fractured silica mimic those seen in acute silicosis in humans (Vallyathan et al., 1995).

Surface impurities, particularly metals, have been shown to alter silica toxicity. Iron, depending on its state and quantity, has been shown to either increase or decrease toxicity. Aluminum has been shown to decrease toxicity (Castranova et al., 1997; Donaldson and Borm, 1998; Fubini, 1998). Silica coated with aluminosilicate clay exhibits lower toxicity, possibly as a result of reduced bioavailability of the silica particle surface (Donaldson and Borm, 1998; Fubini, 1998). This reduced bioavailability may be due to aluminum ions left on the silica surface by the clay (Bruch et al., 2004; Cakmak et al., 2004; Fubini et al., 2004). Aluminum and other metal ions are thought to modify silanol groups on the silica surface, thus decreasing the membranolytic and cytotoxic potency and resulting in enhanced particle clearance from the lung before damage can take place (Fubini, 1998). An epidemiologic study found that the risk of silicosis was less in pottery workers than in tin and tungsten miners (Chen et al., 2005; Harrison et al., 2005), possibly reflecting that pottery workers were exposed to silica particles having less biologically available, non-clay-occluded surface area than was the case for miners. The authors concluded that clay occlusion of silica particles can be a factor in reducing disease risk.

Although it is evident that a number of factors can act to mediate the toxicological potency of crystalline silica, it is not clear how such considerations should be taken into account to evaluate lung cancer and silicosis risks to exposed workers. After evaluating many in vitro studies that had been conducted to investigate the surface characteristics of crystalline silica particles and their influence on fibrogenic activity, NIOSH (2002) concluded that further research is needed to associate specific surface characteristics that can affect toxicity with specific occupational exposure situations and consequent health risks to workers. According to NIOSH (2002), such exposures may include work processes that produce freshly fractured silica surfaces or that involve quartz contaminated with trace elements such as iron. NIOSH called for further in vitro and in vivo studies of the toxicity and pathogenicity of alpha quartz compared with its polymorphs, quartz contaminated with trace elements, and further research on the association of surface properties with specific work practices and health effects.

In discussing the “considerable” heterogeneity shown across the 10 studies used in the pooled lung cancer risk analysis, Steenland et al. (2001a) pointed to hypotheses that physical differences in silica exposure (e.g., freshness of particle cleavage) between cohorts may be a partial explanation of observed differences in exposure-response coefficients derived from those cohort studies. However, the authors did not have specific information on whether or how these factors might have actually influenced the observed differences. Similarly, in the pooled analysis and risk assessments for silicosis mortality conducted by Mannetje et al. (2002b), differences in biological activity of different types of silica dust could not be specifically taken into account. Mannetje et al. (2002b) determined that the exposure-response relationship between silicosis and log-transformed cumulative exposure to crystalline silica was comparable between studies and no significant heterogeneity was found. The authors therefore concluded that their findings were relevant for different circumstances of occupational exposure to crystalline silica. Both the Steenland et al. (2001a) and Mannetje et al. (2002b) studies are discussed in detail in OSHA's Preliminary Quantitative Risk Assessment (section II of the background document and summarized in section VI of this preamble).

OSHA preliminarily concludes that there is considerable evidence to support the hypothesis that surface activity of crystalline silica particles plays an important role in producing disease, and that several environmental influences can modify surface activity to either enhance or diminish the toxicity of silica. However, OSHA believes that the available information is insufficient to determine in any quantitative way how these influences may affect disease risk to workers in any particular workplace setting.

VI. Summary of OSHA's Preliminary Quantitative Risk Assessment

A. Introduction

The Occupational Safety and Health Act (OSH Act or Act) and some landmark court cases have led OSHA to rely on quantitative risk assessment, to the extent possible, to support the risk determinations required to set a permissible exposure limit (PEL) for a toxic substance in standards under the OSH Act. A determining factor in the decision to perform a quantitative risk assessment is the availability of suitable data for such an assessment. In the case of crystalline silica, there has been extensive research on its health effects, and several quantitative risk assessments have been published in the peer-reviewed scientific literature that describe the risk to exposed workers of lung cancer mortality, silicosis mortality and morbidity, non-malignant respiratory disease mortality, and renal disease mortality. These assessments were based on several studies of occupational cohorts in a variety of industry sectors, the underlying studies of which are described in OSHA's review of the health effects literature (see section V of this preamble). In this section, OSHA summarizes its Preliminary Quantitative Risk Assessment (QRA) for crystalline silica, which is presented in Section II of the background document entitled “Respirable Crystalline Silica—Health Effects Literature Review and Preliminary Quantitative Risk Assessment” (placed in Docket OSHA-2010-0034).

OSHA has done what it believes to be a comprehensive review of the literature to provide quantitative estimates of risk for crystalline silica-related diseases. Quantitative risk assessments for lung cancer and silicosis mortality were published after the International Agency for Research on Cancer (IARC) determined more than a decade ago that there was sufficient evidence to regard crystalline silica as a human carcinogen (IARC, 1997). This finding was based on several studies of worker cohorts demonstrating associations between exposure to crystalline silica and an increased risk of lung cancer. Although IARC judged the overall evidence as being sufficient to support this conclusion, IARC also noted that some studies of crystalline silica-exposed workers did not demonstrate an excess risk of lung cancer and that exposure-response trends were not always consistent among studies that were able to describe such trends. These findings led Steenland et al. (2001a) and Mannetje et al. (2002b) to conduct comprehensive exposure-response analyses of the risk of lung cancer and silicosis mortality associated with exposure to crystalline silica. These studies, referred to as the IARC multi-center studies of lung cancer and silicosis mortality, relied on all available cohort data from previously published epidemiological studies for which there were adequate quantitative data on worker exposures to crystalline silica to derive pooled estimates of disease risk. In addition, OSHA identified four single-cohort studies of lung cancer mortality that it judged suitable for quantitative risk assessment; two of these cohorts (Attfield and Costello, 2004; Rice et al., 2001) were included among the 10 used in the IARC multi-center study and studies of two other cohorts appeared later (Hughes et al., 2001; McDonald et al., 2001, 2005; Miller and MacCalman, 2009). For non-malignant respiratory disease mortality, in addition to the silicosis mortality study by Mannetje et al. (2002b), Park et al. (2002) conducted an exposure-response analysis of non-malignant respiratory disease mortality (including silicosis and other chronic obstructive pulmonary diseases) among diatomaceous earth workers. Exposure-response analyses for silicosis morbidity have been published in several single-cohort studies (Chen et al., 2005; Hnizdo and Sluis-Cremer, 1993; Steenland and Brown, 1995b; Miller et al., 1998; Buchanan et al., 2003). Finally, a quantitative assessment of end-stage renal disease mortality based on data from three worker cohorts was developed by Steenland et al. (2002a).

In addition to these published studies, OSHA's contractor, Toxichemica, Inc., commissioned Drs. Kyle Steenland and Scott Bartell of Emory University to perform an uncertainty analysis to examine the effect on lung cancer and silicosis mortality risk estimates of uncertainties that exist in the exposure assessments underlying the two IARC multi-center analyses (Toxichemica, Inc., 2004).

OSHA's Preliminary QRA presents estimates of the risk of silica-related diseases assuming exposure over a working life (45 years) to the proposed 8-hour time-weighted average (TWA) PEL and action level of 0.05 and 0.025 mg/m³, respectively, of respirable crystalline silica, as well as to OSHA's current PELs. OSHA's current general industry PEL for respirable quartz is expressed both in terms of a particle count formula and a gravimetric concentration formula, while the current construction and shipyard employment PELs for respirable quartz are only expressed in terms of a particle count formula. The current PELs limit exposure to respirable dust; the specific limit in any given instance depends on the concentration of crystalline silica in the dust. For quartz, the gravimetric general industry PEL approaches a limit of 0.1 mg/m³ as respirable quartz as the quartz content increases (see discussion in Section XVI of this preamble, Summary and Explanation for paragraph (c)). OSHA's Preliminary QRA presents risk estimates for exposure over a working lifetime to 0.1 mg/m³ to represent the risk associated with exposure to the current general industry PEL. OSHA's current PEL for construction and shipyard employment is a formula PEL that limits exposure to respirable dust expressed as a respirable particle count concentration. As with the gravimetric general industry PEL, the limit varies depending on quartz content of the dust. There is no single mass concentration equivalent for the construction and shipyard PELs; OSHA's Preliminary QRA reviews several studies that suggest that the current construction/shipyard PEL likely lies in the range between 0.25 and 0.5 mg/m³ respirable quartz, and OSHA presents risk estimates for this range of exposure to represent the risks associated with exposure to the current construction/shipyard PEL. In general industry, for both the gravimetric and particle count PELs, OSHA's current PEL for cristobalite and tridymite are half the value for quartz. Thus, OSHA's Preliminary QRA presents risk estimates associated with exposure over a working lifetime to 0.025, 0.05, 0.1, 0.25, and 0.5 mg/m³ respirable silica (corresponding to cumulative exposures over 45 years to 1.125, 2.25, 4.5, 11.25, and 22.5 mg/m³-years).

Risk estimates for lung cancer mortality, silicosis and non-malignant respiratory disease mortality, and renal disease mortality are presented in terms of lifetime (up to age 85) excess risk per 1,000 workers for exposure over an 8-hour working day, 250 days per year, and a 45-year working life. For silicosis morbidity, OSHA based its risk estimates on cumulative risk models used by the various investigators to develop quantitative exposure-response relationships. These models characterized the risk of developing silicosis (as detected by chest radiography) up to the time that cohort members (including both active and retired workers) were last examined. Thus, risk estimates derived from these studies represent less-than-lifetime risks of developing radiographic silicosis. OSHA did not attempt to estimate lifetime risk (i.e., up to age 85) for silicosis morbidity because the relationships between age, time, and disease onset post-exposure have not been well characterized.

A draft preliminary quantitative risk assessment document was submitted for external scientific peer review in accordance with the Office of Management and Budget's “Final Information Quality Bulletin for Peer Review” (OMB, 2004). A summary of OSHA's responses to the peer reviewers' comments appears in Section III of the background document.

In the sections below, OSHA describes the studies and the published risk assessments it uses to estimate the occupational risk of crystalline silica-related disease. (The Preliminary QRA itself also discusses several other available studies that OSHA does not include and OSHA's reasons for not including these studies.)

B. Lung Cancer Mortality

1. Summary of Studies

In its Preliminary QRA, OSHA discusses risk assessments from six published studies that quantitatively analyzed exposure-response relationships for crystalline silica and lung cancer; some of these also provided estimates of risks associated with exposure to OSHA's current PEL or NIOSH's Recommended Exposure Limit (REL) of 0.05 mg/m³. These studies include: (1) A quantitative analysis by Steenland et al. (2001a) of worker cohort data pooled from ten studies; (2) an exposure-response analysis by Rice et al. (2001) of a cohort of diatomaceous earth workers primarily exposed to cristobalite; (3) an analysis by Attfield and Costello (2004) of U.S. granite workers; (4) a risk assessment by Kuempel et al. (2001), who employed a kinetic rat lung model to describe the relationship between quartz lung burden and cancer risk, then calibrated and validated that model using the diatomaceous earth worker and granite worker cohort mortality data; (5) an exposure-response analysis by Hughes et al., (2001) of U.S. industrial sand workers; and (6) a risk analysis by Miller et al. (2007) and Miller and MacCalman (2009) of British coal miners. These six studies are described briefly below and are followed by a summary of the lung cancer risk estimates derived from these studies.

a. Steenland et al. (2001a) Pooled Cohort Analysis

OSHA considers the lung cancer analysis conducted by Steenland et al. (2001a) to be of prime importance for risk estimation because of its size, incorporation of data from multiple cohorts, and availability of detailed exposure and job history data. Subsequent to its publication, Steenland and Bartell (Toxichemica, Inc., 2004) conducted a quantitative uncertainty analysis on the pooled data set to evaluate the potential impact on the risk estimates of random and systematic exposure misclassification, and Steenland (personal communication, 2010) conducted additional exposure-response modeling.

The original study consisted of a pooled exposure-response analysis and risk assessment based on raw data obtained from ten cohorts of silica-exposed workers (65,980 workers, 1,072 lung cancer deaths). Steenland et al. (2001a) initially identified 13 cohort studies as containing exposure information sufficient to develop a quantitative exposure assessment; the 10 studies included in the pooled analysis were those for which data on exposure and health outcome could be obtained for individual workers. The cohorts in the pooled analysis included U.S. gold miners (Steenland and Brown, 1995a), U.S. diatomaceous earth workers (Checkoway et al., 1997), Australian gold miners (de Klerk and Musk, 1998), Finnish granite workers (Koskela et al., 1994), U.S. industrial sand employees (Steenland and Sanderson, 2001), Vermont granite workers (Costello and Graham, 1988), South African gold miners (Hnizdo and Sluis-Cremer, 1991; Hnizdo et al., 1997), and Chinese pottery workers, tin miners, and tungsten miners (Chen et al., 1992).

The exposure assessments developed for the pooled analysis are described by Mannetje et al. (2002a). The exposure information and measurement methods used to assess exposure from each of the 10 cohort studies varied by cohort and by time and included dust measurements representing particle counts, mass of total dust, and respirable dust mass. All exposure information was converted to units of mg/m³ respirable crystalline silica by generating cohort-specific conversion factors based on the silica content of the dust to which workers were exposed.

A case-control study design was employed for which cases and controls were matched for race, sex, age (within 5 years) and study; 100 controls were matched to each case. To test the reasonableness of the cumulative exposure estimates for cohort members, Mannetje et al. (2002a) examined exposure-response relationships for silicosis mortality by performing a nested case-control analysis for silicosis or unspecified pneumoconiosis using conditional logistic regression. Each cohort was stratified into quartiles by cumulative exposure, and standardized rate ratios (SRR) for silicosis were calculated using the lowest-exposure quartile as the baseline. Odds ratios (OR) for silicosis were also calculated for the pooled data set overall, which was stratified into quintiles based on cumulative exposure.

For the pooled data set, the relationship between odds ratio for silicosis mortality and increasing cumulative exposure was “positive and reasonably monotonic”, ranging from 3.1 for the lowest quartile of exposure to 4.8 for the highest. In addition, in seven of the ten individual cohorts, there were statistically significant trends between silicosis mortality rate ratios (SRR) and cumulative exposure. For two of the cohorts (U.S. granite workers and U.S. gold miners), the trend test was not statistically significant (p=0.10). A trend analysis could not be performed on the South African gold miner cohort since silicosis was not coded as an underlying cause of death in that country. A more rigorous analysis of silicosis mortality on pooled data from six of these cohorts also showed a strong, statistically significant increasing trend with increasing decile of cumulative exposure (Mannetje et al., 2002b), providing additional evidence for the reasonableness of the exposure assessment used for the Steenland et al (2001a) lung cancer analysis.

For the pooled lung cancer mortality analysis, Steenland et al. (2001a) conducted a nested case-control analysis via Cox regression, in which there were 100 controls chosen for each case randomly selected from among cohort members who survived past the age at which the case died, and matched on age (the time variable in Cox regression), study, race/ethnicity, sex, and date of birth within 5 years (which, in effect, matched on calendar time given the matching on age). Using alternative continuous exposure variables in a log-linear relative risk model (log RR=βx, where x represents the exposure variable and β the coefficient to be estimated), Steenland et al. (2001a) found that the use of either 1) cumulative exposure with a 15-year lag, 2) the log of cumulative exposure with a 15-year lag, or 3) average exposure resulted in positive statistically significant (p≤0.05) exposure-response coefficients. The models that provided the best fit to the data were those that used cumulative exposure and log-transformed cumulative exposure. The fit of the log-linear model with average exposure was clearly inferior to those using cumulative and log-cumulative exposure metrics.

There was significant heterogeneity among studies (cohorts) using either cumulative exposure or average exposure. The authors suggested a number of possible reasons for such heterogeneity, including errors in measurement of high exposures (which tends to have strong influence on the exposure-response curve when untransformed exposure measures are used), the differential toxicity of silica depending on the crystalline polymorph, the presence of coatings or trace minerals that alter the reactivity of the crystal surfaces, and the age of the fractured surfaces. Models that used the log transform of cumulative exposure showed no statistically significant heterogeneity among cohorts (p=0.36), possibly because they are less influenced by very high exposures than models using untransformed cumulative exposure. For this reason, as well as the good fit of the model using log-cumulative exposure, Steenland et al. (2001a) conducted much of their analysis using log-transformed cumulative exposure. The sensitivity analysis by Toxichemica, Inc. (2004) repeated this analysis after correcting some errors in the original coding of the data set. At OSHA's request, Steenland (2010) also conducted a categorical analysis of the pooled data set and additional analyses using linear relative risk models (with and without log-transformation of cumulative exposure) as well as a 2-piece spline model.

The cohort studies included in the pooled analysis relied in part on particle count data and the use of conversion factors to estimate exposures of workers to mass respirable quartz. A few studies were able to include at least some respirable mass sampling data. OSHA believes that uncertainty in the exposure assessments that underlie each of the 10 studies included in the pooled analysis is likely to represent one of the most important sources of uncertainty in the risk estimates. To evaluate the potential impact of uncertainties in the underlying exposure assessments on estimates of the risk, OSHA's contractor, Toxichemica, Inc. (2004), commissioned Drs. Kyle Steenland and Scott Bartell of Emory University to conduct an uncertainty analysis using the raw data from the pooled cancer risk assessment. The uncertainty analysis employed a Monte Carlo technique in which two kinds of random exposure measurement error were considered; these were (1) random variation in respirable dust measurements and (2) random error in estimating respirable quartz exposures from historical data on particle count concentration, total dust mass concentration, and respirable dust mass concentration measurements. Based on the results of this uncertainty analysis, OSHA does not have reason to believe that random error in the underlying exposure estimates in the Steenland et al. (2001a) pooled cohort study of lung cancer is likely to have substantially influenced the original findings, although a few individual cohorts (particularly the South African and Australian gold miner cohorts) appeared to be sensitive to measurement errors.

The sensitivity analysis also examined the potential effect of systematic bias in the use of conversion factors to estimate respirable crystalline silica exposures from historical data. Absent a priori reasons to suspect bias in a specific direction (with the possible exception of the South African cohort), Toxichemica, Inc. (2004) considered possible biases in either direction by assuming that exposure was under-estimated by 100% (i.e., the true exposure was twice the estimated) or over-estimated by 100% (i.e., the true exposure was half the estimated) for any given cohort in the original pooled dataset. For the conditional logistic regression model using log cumulative exposure with a 15-year lag, doubling or halving the exposure for a specific study resulted in virtually no change in the exposure-response coefficient for that study or for the pooled analysis overall. Therefore, based on the results of the uncertainty analysis, OSHA believes that misclassification errors of a reasonable magnitude in the estimation of historical exposures for the 10 cohort studies were not likely to have substantially biased risk estimates derived from the exposure-response model used by Steenland et al. (2001a).

b. Rice et al. (2001) Analysis of Diatomaceous Earth Workers

Rice et al. (2001) applied a variety of exposure-response models to the same California diatomaceous earth cohort data originally reported on by Checkoway et al. (1993, 1996, 1997) and included in the pooled analysis conducted by Steenland et al. (2001a) described above. The cohort consisted of 2,342 white males employed for at least one year between 1942 and 1987 in a California diatomaceous earth mining and processing plant. The cohort was followed until 1994, and included 77 lung cancer deaths. Rice et al. (2001) relied on the dust exposure assessment developed by Seixas et al. (1997) from company records of over 6,000 samples collected from 1948 to 1988; cristobalite was the predominate form of crystalline silica to which the cohort was exposed. Analysis was based on both Poisson regression models Cox's proportional hazards models with various functions of cumulative silica exposure in mg/m³-years to estimate the relationship between silica exposure and lung cancer mortality rate. Rice et al. (2001) reported that exposure to crystalline silica was a significant predictor of lung cancer mortality for nearly all of the models employed, with the linear relative risk model providing the best fit to the data in the Poisson regression analysis.

c. Attfield and Costello (2004) Analysis of Granite Workers

Attfield and Costello (2004) analyzed the same U.S. granite cohort originally studied by Costello and Graham (1988) and Davis et al. (1983) and included in the Steenland et al. (2001a) pooled analysis, consisting of 5,414 male granite workers who were employed in the Vermont granite industry between 1950 and 1982 and who had received at least one chest x-ray from the surveillance program of the Vermont Department of Industrial Hygiene. Their 2004 report extended follow-up from 1982 to 1994, and found 201 deaths. Workers' cumulative exposures were estimated by Davis et al. (1983) based on historical exposure data collected in six environmental surveys conducted between 1924 and 1977, plus work history information.

Using Poisson regression models and seven cumulative exposure categories, the authors reported that the results of the categorical analysis showed a generally increasing trend of lung cancer rate ratios with increasing cumulative exposure, with seven lung cancer death rate ratios ranging from 1.18 to 2.6. A complication of this analysis was that the rate ratio for the highest exposure group in the analysis (cumulative exposures of 6.0 mg/m³-years or higher) was substantially lower than those for other exposure groups. Attfield and Costello (2004) reported that the best-fitting model was based on a 15-year lag, use of untransformed cumulative exposure, and omission of the highest exposure group.

The authors argued that it was appropriate to base their risk estimates on a model that was fitted without the highest exposure group for several reasons. They believed the underlying exposure data for the high-exposure group was weaker than for the others, and that there was a greater likelihood that competing causes of death and misdiagnoses of causes of death attenuated the lung cancer death rate. Second, all of the remaining groups comprised 85 percent of the deaths in the cohort and showed a strong linear increase in lung cancer mortality with increasing exposure. Third, Attfield and Costello (2004) believed that the exposure-response relationship seen in the lower exposure groups was more relevant given that the exposures of these groups were within the range of current occupational standards. Finally, the authors stated that risk estimates derived from the model after excluding the highest exposure group were more consistent with other published risk estimates than was the case for estimates derived from the model using all exposure groups. Because of these reasons, OSHA believes it is appropriate to rely on the model employed by Attfield and Costello (2004) after omitting the highest exposure group.

d. Kuempel et al. (2001) Rat-Based Model for Human Lung Cancer

Kuempel et al. (2001) published a rat-based toxicokinetic/toxicodynamic model for silica exposure for predicting human lung cancer, based on lung burden concentrations necessary to cause the precursor events that can lead to adverse physiological effects in the lung. These adverse physiological effects can then lead to lung fibrosis and an indirect genotoxic cause of lung cancer. The hypothesized first step, or earliest expected response, in these disease processes is chronic lung inflammation, which the authors consider as a disease limiting step. Since the NOAEL of lung burden associated with this inflammation, based on the authors' rat-to-human lung model conversion, is the equivalent of exposure to 0.036 mg/m³ (M_crit) for 45 years, exposures below this level would presumably not lead to (based on an indirect genotoxic mechanism) lung cancer, at least in the “average individual.” Since silicosis also is inflammation mediated, this exposure could also be considered to be an average threshold level for that disease as well.

Kuempel et al. (2001) have used their rat-based lung cancer model with human data, both to validate their model and to estimate the lung cancer risk as a function of quartz lung burden. First they “calibrated” human lung burdens from those in rats based on exposure estimates and lung autopsy reports of U.S. coal miners. Then they validated these lung burden estimates using quartz exposure data from U.K. coal miners. Using these human lung burden/exposure concentration equivalence relationships, they then converted the cumulative exposure-lung cancer response slope estimates from both the California diatomaceous earth workers (Rice et al., 2001) and Vermont granite workers (Attfield and Costello, 2001) to lung burden-lung cancer response slope estimates. Finally, they used these latter slope estimates in a life table program to estimate lung cancer risk associated with their “threshold” exposure of 0.036 mg/m³ and to the OSHA PEL and NIOSH REL. Comparing the estimates from the two epidemiology studies with those based on a male rat chronic silica exposure study the authors found that, ” the lung cancer excess risk estimates based on male rat data are approximately three times higher than those based on the male human data.” Based on this modeling and validation exercise, Keumpel et al. concluded, “the rat-based estimates of excess lung cancer risk in humans exposed to crystalline silica are reasonably similar to those based on two human occupational epidemiology studies.”

Toxichemica, Inc. (2004) investigated whether use of the dosimetry model would substantially affect the results of the pooled lung cancer data analysis initially conducted by Steenland et al. (2001a). They replicated the lung dosimetry model using Kuempel et al.'s (2001) reported median fit parameter values, and compared the relationship between log cumulative exposure and 15-year lagged lung burden at the age of death in case subjects selected for the pooled case-control analysis. The two dose metrics were found to be highly correlated (r=0.99), and models based on either log silica lung burden or log cumulative exposure were similarly good predictors of lung cancer risk in the pooled analysis (nearly identical log-likelihoods of -4843.96 and—4843.996, respectively). OSHA believes that the Kuempel et al. (2001) analysis is a credible attempt to quantitatively describe the retention and accumulation of quartz in the lung, and to relate the external exposure and its associated lung burden to the inflammatory process. However, using the lung burden model to convert the cumulative exposure coefficients to a different exposure metric appears to add little additional information or insight to the risk assessments conducted on the diatomaceous earth and granite cohort studies. Therefore, for the purpose of quantitatively evaluating lung cancer risk in exposed workers, OSHA has chosen to rely on the epidemiology studies themselves and the cumulative exposure metrics used in those studies.

e. Hughes et al. (2001), McDonald et al. (2001), and McDonald et al. (2005) Study of North American Industrial Sand Workers

McDonald et al. (2001), Hughes et al. (2001) and McDonald et al. (2005) followed up on a cohort study of North American industrial sand workers that overlapped with the industrial sand cohort (18 plants, 4,626 workers) studied by Steenland and Sanderson (2001) and included in Steenland et al.'s (2001a) pooled cohort analysis. The McDonald et al. (2001) follow-up cohort included 2,670 men employed before 1980 for three years or more in one of nine North American (8 U.S. and 1 Canadian) sand-producing plants, including 1 large associated office complex. Information on cause of death was obtained, from 1960 through 1994, for 99 percent of the deceased workers for a total 1,025 deaths representing 38 percent of the cohort. A nested case-control study and analysis based on 90 lung cancer deaths from this cohort was also conducted by Hughes et al. (2001). A later update through 2000, of both the cohort and nested case-control studies by McDonald et al. (2005), eliminated the Canadian plant, following 2,452 men from the eight U.S. plants. For the lung cancer case-control part of the study the update included 105 lung cancer deaths. Both the initial and updated case control studies used up to two controls per case.

Although the cohort studies provided evidence of increased risk of lung cancer (SMR = 150, p = 0.001, based on U.S. rates) for deaths occurring 20 or more years from hire, the nested case-control studies, Hughes et al. (2001) and McDonald et al. (2005), allowed for individual job, exposure, and smoking histories to be taken into account in the exposure-response analysis for lung cancer. Both of these case-control analyses relied on an analysis of exposure information reported by Sanderson et al. (2000) and by Rando et al. (2001) to provide individual estimates of average and cumulative exposure. Statistically significant positive exposure-response trends for lung cancer were found for both cumulative exposure (lagged 15 years) and average exposure concentration, but not for duration of employment, after controlling for smoking. A monotonic increase was seen for both lagged and unlagged cumulative exposure when the four upper exposure categories were collapsed into two. With exposure lagged 15 years and after adjusting for smoking, increasing quartiles of cumulative silica exposure were associated with lung cancer mortality (odds ratios of 1.00, 0.84, 2.02 and 2.07, p-value for trend=0.04). There was no indication of an interaction effect of smoking and cumulative silica exposure (Hughes et al., 2001).

OSHA considers this Hughes et al. (2001) study and analysis to be of high enough quality to provide risk estimates for excess lung cancer for silica exposure to industrial sand workers. Using the median cumulative exposure levels of 0, 0.758, 2.229 and 6.183 mg/m³-years, Hughes et al. estimated lung cancer odds ratios, ORs (no. of deaths), for these categories of 1.00 (14), 0.84 (15), 2.02 (31), and 2.07 (30), respectively, on a 15-year lag basis (p-value for trend=0.04.) For the updated nested case control analysis, McDonald et al. (2005) found very similar results, with exposure lagged 15 years and, after adjusting for smoking, increasing quartiles of cumulative silica exposure were associated with lung cancer ORs (no. of deaths) of 1.00 (13), 0.94 (17), 2.24 (38), and 2.66 (37) (p-value for trend=0.006). Because the Hughes et al. (2001) report contained information that allowed OSHA to better calculate exposure-response estimates and because of otherwise very similar results in the two papers, OSHA has chosen to base its lifetime excess lung cancer risk estimate for these industrial sand workers on the Hughes et al. (2001) case-control study. Using the median exposure levels of 0, 0.758, 2.229 and 6.183 mg-years/m³, respectively, for each of the four categories described above, and using the model: ln OR = α + β × Cumulative Exposure, the coefficient for the exposure estimate was β = 0.13 per (mg/m³-years), with a standard error of β = 0.074 (calculated from the trend test p-value in the same paper). In this model, with background lung cancer risks of about 5 percent, the OR provides a suitable estimate of the relative risk.

f. Miller et al. (2007) and Miller and MacCalman (2009) Study of British Coal Workers Exposed to Respirable Quartz

Miller et al. (2007) and Miller and MacCalman (2009) continued a follow-up mortality study, begun in 1970, of 18,166 coalminers from 10 British coalmines initially followed through the end of 1992 (Miller et al., 1997). The two recent reports on mortality analyzed the cohort of 17,800 miners and extended the analysis through the end of 2005. By that time there were 516,431 person years of observation, an average of 29 years per miner, with 10,698 deaths from all causes. Causes of deaths of interest included pneumoconiosis, other non-malignant respiratory diseases (NMRD), lung cancer, stomach cancer, and tuberculosis. Three of the strengths of this study are its use of detailed time-exposure measurements of both quartz and total mine dust, detailed individual work histories, and individual smoking histories. However, the authors noted that no additional exposure measurements were included in the updated analysis, since all the mines had closed by the mid 1980's.

For this cohort mortality study there were analyses using both external (regional age-time and cause specific mortality rates) internal controls. For the analysis from external mortality rates, the all-cause mortality SMR from 1959 through 2005 was 100.9 (95% C.I., 99.0-102.8), based on all 10,698 deaths. However, these death ratios were not uniform over time. For the period from 1990 to 2005, the all-cause SMR was 109.6 (95% C.I., 106.5-112.8), while the ratios for previous periods were less than 100. This pattern of recent increasing SMRs was also seen in the recent cause-specific death rate for lung cancer, SMR=115.7 (95% C.I., 104.8-127.7). For the analysis based on internal rates and using Cox regression methods, the relative risk for lung cancer risk based on a cumulative quartz exposure equivalent to approximately 0.055 mg/m³ for 45 years was RR = 1.14 (95% C.I., 1.04 to 1.25). This risk is adjusted for concurrent coal dust exposure and smoking status, and incorporated a 15-year lag in quartz exposures. The analysis showed a strong effect for smoking (independent of quartz exposure) on lung cancer. For lung cancer, OSHA believes that the analyses based on the Cox regression method provides strong evidence that for these coal miners' quartz exposures were associated with increased lung cancer risk, but that simultaneous exposures to coal dust did not cause increased lung cancer risk. To estimate lung cancer risk from this study, OSHA estimated the regression slope for a log-linear relative risk model based on the Miller and MacCalman's (2009) finding of a relative risk of 1.14 for a cumulative exposure of 0.055 mg/m³-years.

2. Summary of OSHA's Estimates of Lung Cancer Mortality Risk

Tables VI-1 and VI-2 summarize the excess lung cancer risk estimates from occupational exposure to crystalline silica, based on five of the six lung cancer risk assessments discussed above. OSHA's estimates of lifetime excess lung cancer risk associated with 45 years of exposure to crystalline silica at 0.1 mg/m³ (approximately the current general industry PEL) range from 13 to 60 deaths per 1,000 workers. For exposure to the proposed PEL of 0.05 mg/m³, the lifetime risk estimates calculated by OSHA are in the range of 6 to 26 deaths per 1,000 workers. For a 45-year exposure at the proposed action level of 0.025 mg/m³, OSHA estimates the risk to range from 3 to 23 deaths per 1,000 workers. The results from these assessments are reasonably consistent despite the use of data from different cohorts and the reliance on different analytical techniques for evaluating dose-response relationships. Furthermore, OSHA notes that in this range of exposure, 0.025—0.1 mg/m³, there is statistical consistency between the risk estimates, as evidenced by the considerable overlap in the 95-percent confidence intervals of the risk estimates presented in Table VI-1.

OSHA also estimates the lung cancer risk associated with 45 years of exposure to the current construction/shipyard PEL (in the range of 0.25 to 0.5 mg/m³) to range from 37 to 653 deaths per 1,000 workers. Exposure to 0.25 or 0.5 mg/m³ over 45 years represents cumulative exposures of 11.25 and 22.5 mg-years/m³, respectively. This range of cumulative exposure is well above the median cumulative exposure for most of the cohorts used in the risk assessment, primarily because most of the individuals in these cohorts had not been exposed for as long as 45 years. Thus, estimating lung cancer excess risks over this higher range of cumulative exposures of interest to OSHA required some degree of extrapolation and adds uncertainty to the estimates.

C. Silicosis and Non-Malignant Respiratory Disease Mortality

There are two published quantitative risk assessment studies of silicosis and non-malignant respiratory disease (NMRD) mortality; a pooled analysis of silicosis mortality by Mannetje et al. (2002b) of data from six epidemiological studies, and an exposure-response analysis of NMRD mortality among diatomaceous earth workers (Park et al., 2002).

1. Mannetje et al. (2002b) Six Cohort Pooled Analysis

The Mannetje et al. (2002b) silicosis analysis was part of the IARC ten cohort pooled study included in the Steenland et al. (2001a) lung cancer mortality analysis above. These studies included 18,634 subjects and 170 silicosis deaths (n = 150 for silicosis, and n = 20 unspecified pneumoconiosis). The silicosis deaths had a median duration of exposure of 28 years, a median cumulative exposure of 7.2 mg/m³-years, and a median average exposure of 0.26 mg/m³, while the respective values of the whole cohort were 10 years, 0.62 mg/m³-years, and 0.07 mg/m³. Rates for silicosis adjusted for age, calendar time, and study were estimated by Poisson regression; rates increased nearly monotonically with deciles of cumulative exposure, from a mortality rate of 5/100,000 person-years in the lowest exposure category (0-0.99 mg/m³-years) to 299/100,000 person-years in the highest category (>28.10 mg/m³-years). Quantitative estimates of exposure to respirable silica (mg/m³) were available for all six cohorts (Mannetje et al. 2002a). Lifetime risk of silicosis mortality was estimated by accumulating mortality rates over time using the formula

Risk = 1 − exp(−∑time * rate).

To estimate the risk of silicosis mortality at the current and proposed PELs, OSHA used the model described by Mannetje et al. (2002b) to estimate risk to age 85 but used rate ratios that were estimated from a nested case-control design that was part of a sensitivity analysis conducted by Toxichemica, Inc. (2004), rather than the Poisson regression originally conducted by Mannetje et al. (2002b). The case-control design was selected because it was expected to better control for age; in addition, the rate ratios derived from the case-control study reflect exposure measurement uncertainty via conduct of a Monte Carlo analysis (Toxichemica, Inc., 2004).

2. Park et al. (2002) Study of Diatomaceous Earth Workers

Park et al. (2002) analyzed the California diatomaceous earth cohort data originally studied by Checkoway et al. (1997), consisting of 2,570 diatomaceous earth workers employed for 12 months or more from 1942 to 1994, to quantify the relationship between exposure to cristobalite and mortality from chronic lung disease other than cancer (LDOC). Diseases in this category included pneumoconiosis (which included silicosis), chronic bronchitis, and emphysema, but excluded pneumonia and other infectious diseases. Industrial hygiene data for the cohort were available from the employer for total dust, silica (mostly cristobalite), and asbestos. Park et al. (2002) used the exposure assessment previously reported by Seixas et al. (1997) and used by Rice et al. (2001) to estimate cumulative crystalline silica exposures for each worker in the cohort based on detailed work history files. The mean silica concentration for the cohort overall was 0.29 mg/m³ over the period of employment (Seixas et al., 1997). The mean cumulative exposure values for total respirable dust and respirable crystalline silica were 7.31 and 2.16 mg/m³-year, respectively. Similar cumulative exposure estimates were made for asbestos. Smoking information was available for about 50 percent of the cohort and for 22 of the 67 LDOC deaths available for analysis, permitting Park et al. (2002) to at least partially adjust for smoking. Estimates of LDOC mortality risks were derived via Poisson and Cox's proportional hazards models; a variety of relative rate model forms were fit to the data, with a linear relative rate model being selected for risk estimation.

3. Summary Risk Estimates for Silicosis and NMRD Mortality

Table VI-2 presents OSHA's risk estimates for silicosis and NMRD mortality derived from the Mannetje et al. (2002b) and Park et al. (2002) studies, respectively. For 45 years of exposure to the current general industry PEL (approximately 0.1 mg/m³ respirable crystalline silica), OSHA's estimates of excess lifetime risk are 11 deaths per 1,000 workers for the pooled analysis and 83 deaths per 1,000 workers based on Park et al.'s (2002) estimates. At the proposed PEL, estimates of silicosis and NMRD mortality are 7 and 43 deaths per 1,000, respectively. For exposures up to 0.25 mg/m³, the estimates based on Park et al. are about 5 to 11 times as great as those calculated for the pooled analysis of silicosis mortality (Mannetje et al., 2002b). However, these two sets of risk estimates are not directly comparable. First, the Park et al. analysis used untransformed cumulative exposure as the exposure metric, whereas the Mannertje et al. analysis used log cumulative exposure, which causes the exposure-response to flatten out in the higher exposure ranges. Second, the mortality endpoint for the Park et al. (2002) analysis is death from all non-cancer lung diseases, including pneumoconiosis, emphysema, and chronic bronchitis, whereas the pooled analysis by Mannetje et al. (2002b) included only deaths coded as silicosis or other pneumoconiosis. Less than 25 percent of the LDOC deaths in the Park et al. (2002) analysis were coded as silicosis or other pneumoconiosis (15 of 67). As noted by Park et al. (2002), it is likely that silicosis as a cause of death is often misclassified as emphysema or chronic bronchitis; thus, Mannetje et al.'s (2002b) selection of deaths may tend to underestimate the true risk of silicosis mortality, and Park et al.'s (2002) analysis would more fairly capture the total respiratory mortality risk from all non-malignant causes, including silicosis and chronic obstructive pulmonary disease.

D. Renal Disease Mortality

Steenland et al. (2002a) examined renal disease mortality in three cohorts and evaluated exposure-response relationships from the pooled cohort data. The three cohorts included U.S. gold miners (Steenland and Brown, 1995a), U.S. industrial sand workers (Steenland et al., 2001b), and Vermont granite workers (Costello and Graham, 1988), all three of which are included in both the lung cancer mortality and silicosis mortality pooled analyses reported above. Follow up for the U.S. gold miners study was extended six years from that in the other pooled analyses. Steenland et al. (2002a) reported that these cohorts were chosen because data were available for both underlying cause mortality and multiple cause mortality; this was believed important because renal disease is often listed on death certificates without being identified as an underlying cause of death. In the three cohorts, there were 51 total renal disease deaths using underlying cause, and 204 total renal deaths using multiple cause mortality.

The combined cohort for the pooled analysis (Steenland et al., 2002a) consisted of 13,382 workers with exposure information available for 12,783 (95 percent). Exposure matrices for the three cohorts had been used in previous studies (Steenland and Brown, 1995a; Attfield and Costello, 2001; Steenland et al., 2001b). The mean duration of exposure, the mean cumulative exposure, and the mean concentration of respirable silica for the pooled cohort were 13.6 years, 1.2 mg/m³-years, and 0.07 mg/m³, respectively. SMRs (compared to the U.S. population) for renal disease (acute and chronic glomerulonephritis, nephrotic syndrome, acute and chronic renal failure, renal sclerosis, and nephritis/nephropathy) were statistically significantly elevated using multiple cause data (SMR 1.29, 95% CI 1.10-1.47, 193 deaths) and underlying cause data (SMR 1.41, 95% CI 1.05-1.85, 51 observed deaths).

OSHA's estimates of renal disease mortality appear in Table VI-2. Based on the life table analysis, OSHA estimates that exposure to the current (0.10 mg/m³) and proposed general industry PEL (0.0.05 mg/m³) over a working life would result in a lifetime excess renal disease risk of 39 (95% CI 2-200) and 32 (95% CI 1.7-147) deaths per 1,000, respectively. For exposure to the current construction/shipyard PEL, OSHA estimates the excess lifetime risk to range from 52 (95% CI 2.2-289) to 63 (95% CI 2.5-368) deaths per 1,000 workers.

E. Silicosis Morbidity

OSHA's Preliminary QRA summarizes the principal cross-sectional and cohort studies that have quantitatively characterized relationships between exposure to crystalline silica and development of radiographic evidence of silicosis. Each of these studies relied on estimates of cumulative exposure to evaluate the relationship between exposure and silicosis prevalence in the worker populations examined. The health endpoint of interest in these studies is the appearance of opacities on chest roentgenograms indicative of pulmonary fibrosis.

The International Labour Organization's (ILO) 1980 International Classification of Radiographs of the Pneumoconioses is accepted as the standard against which chest radiographs are measured in epidemiologic studies, for medical surveillance and for clinical evaluation. According to this standard, if radiographic findings are or may be consistent with pneumoconiosis, then the size, shape, and extent of profusion of opacities are characterized by comparing the radiograph to standard films. Classification by shape (rounded vs. irregular) and size involves identifying primary and secondary types of small opacities on the radiograph and classifying them into one of six size/shape categories. The extent of profusion is judged from the concentrations of opacities as compared with that on the standard radiographs and is graded on a 12-point scale of four major categories (0-3, with Category 0 representing absence of opacities), each with three subcategories. Most of the studies reviewed by OSHA considered a finding consistent with an ILO classification of 1/1 to be a positive diagnosis of silicosis, although some also considered an x-ray classification of 1/0 or 0/1 to be positive.

Chest radiography is not the most sensitive tool used to diagnose or detect silicosis. In 1993, Hnizdo et al. reported the results of a study that compared autopsy and radiological findings of silicosis in a cohort of 557 white South African gold miners. The average period from last x-ray to autopsy was 2.7 years. Silicosis was not diagnosed radiographically for over 60 percent of the miners for whom pathological examination of lung tissue showed slight to marked silicosis. The likelihood of false negatives (negative by x-ray, but silicosis is actually present) increased with years of mining and average dust exposure of the miners. The low sensitivity seen for radiographic evaluation suggests that risk estimates derived from radiographic evidence likely understate the true risk of developing fibrotic lesions as a result of exposure to crystalline silica.

OSHA's Preliminary QRA examines multiple studies from which silicosis occupational morbidity risks can be estimated. The studies evaluated fall into three major types. Some are cross-sectional studies in which radiographs taken at a point in time were examined to ascertain cases (Kreiss and Zhen, 1996; Love et al., 1999; Ng and Chan, 1994; Rosenman et al., 1996; Churchyard et al., 2003, 2004); these radiographs may have been taken as part of a health survey conducted by the investigators or represent the most recent chest x-ray available for study subjects. Other studies were designed to examine radiographs over time in an effort to determine onset of disease. Some of these studies examined primarily active, or current, workers (Hughes et al., 1998; Muir et al., 1989a, 1989b; Park et al., 2002), while others included both active and retired workers (Chen et al., 2001, 2005; Hnizdo and Sluis-Cremer, 1993; Miller et al., 1998; Buchanan et al., 2003; Steenland and Brown, 1995b).

Even though OSHA has presented silicosis risk estimates for all of the studies identified, the Agency is relying primarily on those studies that examined radiographs over time and included both active and retired workers. It has been pointed out by others (Chen et al., 2001; Finkelstein, 2000; NIOSH, 2002) that lack of follow-up of retired workers consistently resulted in lower risk estimates compared to studies that included retired workers. OSHA believes that the most reliable estimates of silicosis morbidity, as detected by chest radiographs, come from the studies that evaluated radiographs over time, included radiographic evaluation of workers after they left employment, and derived cumulative or lifetime estimates of silicosis disease risk. Brief descriptions of these cumulative risk studies used to estimate silicosis morbidity risks are presented below.

1. Hnizdo and Sluis Cremer (1993) Study of South African White Gold Miners

Hnizdo and Sluis-Cremer (1993) described the results of a retrospective cohort study of 2,235 white gold miners in South Africa. These workers had received annual examinations and chest x-rays while employed; most returned for occasional examinations after employment. A case was defined as one with an x-ray classification of ILO 1/1 or greater. A total of 313 miners had developed silicosis and had been exposed for an average of 27 years at the time of diagnosis. Forty-three percent of the cases were diagnosed while employed and the remaining 57 percent were diagnosed an average of 7.4 years after leaving the mines. The average latency for the cohort was 35 years (range of 18-50 years) from start of exposure to diagnosis.

The average respirable dust exposure for the cohort overall was 0.29 mg/m³ (range 0.11-0.47), corresponding to an estimated average respirable silica concentration of 0.09 mg/m³ (range 0.033-0.14). The average cumulative dust exposure for the overall cohort was 6.6 mg/m³-years (range 1.2-18.7), or an average cumulative silica exposure of 1.98 mg/m³-years (range 0.36-5.61). OSHA believes that the exposure estimates for the cohort are uncertain given the need to rely on particle count data generated over a fairly narrow production period.

Silicosis risk increased exponentially with cumulative exposure to respirable dust and was modeled using log-logistic regression. Using the exposure-response relationship developed by Hnizdo and Sluis-Cremer (1993), and assuming a quartz content of 30 percent in respirable dust, Rice and Stayner (1995) and NIOSH (2002) estimated the risk of silicosis to be 70 percent and 13 percent for a 45-year exposure to 0.1 and 0.05 mg/m³ respirable crystalline silica, respectively.

2. Steenland and Brown (1995b) Study of South Dakota Gold Miners

Three thousand three hundred thirty South Dakota gold miners who had worked at least a year underground between 1940 and 1965 were studied by Steenland and Brown (1995b). Workers were followed though 1990 with 1,551 having died; loss to follow up was low (2 percent). Chest x-rays taken in cross-sectional surveys in 1960 and 1976 and death certificates were used to ascertain cases of silicosis. One hundred twenty eight cases were found via death certificate, 29 by x-ray (defined as ILO 1/1 or greater), and 13 by both. Nine percent of deaths had silicosis mentioned on the death certificate. Inclusion of death certificate diagnoses probably increases the risk estimates from this study compared to those that rely exclusively on radiographic findings to evaluate silicosis morbidity risk (see discussion of Hnizdo et al. (1993) above).

Exposure was estimated by conversion of impinger (particle count) data and was based on measurements indicating an average of 13 percent silica in the dust. Based on these data, the authors estimated the mean exposure concentration to be 0.05 mg/m³ for the overall cohort, with those hired before 1930 exposed to an average of 0.15 mg/m³. The average duration of exposure for cases was 20 years (s.d = 8.7) compared to 8.2 years (s.d = 7.9) for the rest of the cohort. This study found that cumulative exposure was the best disease predictor, followed by duration of exposure and average exposure. Lifetime risks were estimated from Poisson regression models using standard life table techniques. The authors estimated a risk of 47 percent associated with 45 years of exposure to 0.09 mg/m³ respirable crystalline silica, which reduced to 35 percent after adjustment for age and calendar time.

3. Miller et al. (1995, 1998) and Buchanan et al. (2003) Study of Scottish Coal Miners

Miller et al. (1995, 1998) and Buchanan et al. (2003) reported on a 1990/1991 follow-up study of 547 survivors of a 1,416 member cohort of Scottish coal workers from a single mine. These men had all worked in the mine during a period between early 1971 and mid 1976, during which they had experienced “unusually high concentrations of freshly cut quartz in mixed coalmine dust. The population's exposures to both coal and quartz dust had been measured in unique detail, for a substantial proportion of the men's working lives.” Thus, this cohort allowed for the study of the effects of both higher and lower silica concentrations, and exposure-rate effects on the development of silicosis. The 1,416 men had all had previous radiographs dating from before, during, or just after this high concentration period, and the 547 participating survivors received their follow-up chest x-rays between November 1990 and April 1991. Follow-up interviews consisted of questions on current and past smoking habits, and occupational history since leaving the coal mine, which closed in 1981.

Silicosis cases were identified as such if the median classification of the three readers indicated an ILO (1980) classification of 1/0 or greater, plus a progression from the earlier reading. Of the 547 men, 203 (38 percent) showed progression of at least one ILO category from the 1970's surveys to the 1990-91 survey; in 128 of these (24 percent) there was progression of two or more steps. In the 1970's survey 504 men had a profusion score of 0; of these, 120 (24 percent) progressed to an ILO classification of 1/0 or greater. Of the 36 men who had shown earlier profusions of 1/0 or greater, 27 (75 percent) showed further progression at the 1990/1991 follow-up. Only one subject showed a regression from any earlier reading, and that was slight, from ILO 1/0 to 0/1.

To study the effects of exposure to high concentrations of quartz dust, the Buchanan et al. (2003) analysis presented the results of logistic regression modeling that incorporated two independent terms for cumulative exposure, one arising from exposure to concentrations less than 2 mg/m³ respirable quartz and the other from exposure to concentrations greater than or equal to 2 mg/m³. Both of the cumulative quartz exposure concentration variables were “highly statistically significant in the presence of the other,” and independent of the presence of coal dust. Since these quartz variables were in the same units, g-hr/m³, the authors noted that coefficient for exposure concentrations equal to or above 2.0 mg/m³ was 3 times that of the coefficient for concentrations less than 2.0 mg/m³. From this, the authors concluded that their analysis showed that “the risk of silicosis over a working lifetime can rise dramatically with exposure to such high concentrations over a timescale of merely a few months.”

Buchanan et al., (2003) provided analysis and risk estimates only for silicosis cases defined as having an x-ray classified as ILO 2/1+, after adjusting for the disproportionately severe effect of exposure to high concentrations on silicosis risk. Estimating the risk of acquiring a chest x-ray classified as ILO 1/0+ from the Buchanan (2003) or the earlier Miller et al. (1995, 1998) publications can only be roughly approximated because of the limited summary information included; this information suggests that the risk of silicosis defined as an ILO classification of 1/0+ could be about three times higher than the risk of silicosis defined as an ILO 2/1+ x-ray. OSHA has a high degree of confidence in the estimates of progression to stages 2/1+ from this Scotland coal mine study, mainly because of the highly detailed and extensive exposure measurements, the radiographic records, and the detailed analyses of high exposure-rate effects.

4. Chen et al. (2001) Study of Tin Miners

Chen et al. (2001) reported the results of a retrospective study of a Chinese cohort of 3,010 underground miners who had worked in tin mines at least one year between 1960 and 1965. They were followed through 1994, by which time 2,426 (80.6%) workers had either retired or died, and only 400 (13.3%) remained employed at the mines.

The study incorporated occupational histories, dust measurements and medical examination records. Exposure data consisted of high-flow, short-term gravimetric total dust measurements made routinely since 1950; the authors used data from 1950 to represent earlier exposures since dust control measures were not implemented until 1958. Results from a 1998-1999 survey indicated that respirable silica measurements were 3.6 percent (s.d = 2.5 percent) of total dust measurements. Annual radiographs were taken since 1963 and all cohort members continued to have chest x-rays taken every 2 or 3 years after leaving work. Silicosis was diagnosed when at least 2 of 3 radiologists classified a radiograph as being a “suspected case” or at Stage I, II, or III under the 1986 Chinese pneumoconiosis roentgen diagnostic criteria. According to Chen et al. (2001), these four categories under the Chinese system were found to agree closely with ILO categories 0/1, Category 1, Category 2, and Category 3, respectively, based on studies comparing the Chinese and ILO classification systems. Silicosis was observed in 33.7 percent of the group; 67.4 percent of the cases developed after exposure ended.

5. Chen et al. (2005) Study of Chinese Pottery Workers, Tin Miners, and Tungsten Miners

In a later study, Chen et al. (2005) investigated silicosis morbidity risks among three cohorts to determine if the risk varied among workers exposed to silica dust having different characteristics. The cohorts consisted of 4,547 pottery workers, 4,028 tin miners, and 14,427 tungsten miners selected from a total of 20 workplaces. Cohort members included all males employed after January 1, 1950 and who worked for at least one year between 1960 and 1974. Radiological follow-up was through December 31, 1994 and x-rays were scored according to the Chinese classification system as described above by Chen et al. (2001) for the tin miner study. Exposure estimates of cohort members to respirable crystalline silica were based on the same data as described by Chen et al. (2001). In addition, the investigators measured the extent of surface occlusion of crystalline silica particles by alumino-silicate from 47 dust samples taken at 13 worksites using multiple-voltage scanning electron microscopy and energy dispersive X-ray spectroscopy (Harrison et al., 2005); this method yielded estimates of the percent of particle surface that is occluded.

Compared to tin and tungsten miners, pottery workers were exposed to significantly higher mean total dust concentrations (8.2 mg/m³, compared to 3.9 mg/m³ for tin miners and 4.0 mg/m³ for tungsten miners), worked more net years in dusty occupations (mean of 24.9 years compared to 16.4 years for tin miners and 16.5 years for tungsten miners), and had higher mean cumulative dust exposures (205.6 mg/m³-years compared to 62.3 mg/m³-years for tin miners and 64.9 mg/m³-years for tungsten miners) (Chen et al., 2005). Applying the authors' conversion factors to estimate respirable crystalline silica from Chinese total dust measurements, the approximate mean cumulative exposures to respirable silica for pottery, tin, and tungsten workers are 6.4 mg/m³-years, 2.4 mg/m³-years, and 3.2 mg/m³-years, respectively. Measurement of particle surface occlusion indicated that, on average, 45 percent of the surface area of respirable particles collected from pottery factory samples was occluded, compared to 18 percent of the particle surface area for tin mine samples and 13 percent of particle surface area for tungsten mines.

Based on Chen et al. (2005), OSHA estimated the cumulative silicosis risk associated with 45 years of exposure to 0.1 mg/m³ respirable crystalline silica (a cumulative exposure of 4.5 mg/m³-years) to be 6 percent for pottery workers, 12 percent for tungsten miners, and 40 percent for tin miners. For a cumulative exposure of 2.25 mg/m³-years (i.e., 45 years of exposure to 0.05 mg/m³), cumulative silicosis morbidity risks were estimated to be 2, 2, and 10 percent for pottery workers, tungsten miners, and tin miners, respectively. When cumulative silica exposure was adjusted to reflect exposure to surface-active quartz particles (i.e., not occluded), the estimated cumulative risk among pottery workers more closely approximated those of the tin and tungsten miners, suggesting to the authors that alumino-silicate occlusion of the crystalline particles in pottery factories at least partially explained the lower risk seen among workers, despite their having been more heavily exposed.

6. Summary of Silicosis Morbidity Risk Estimates.

Table VI-2 presents OSHA's risk estimates for silicosis morbidity that are derived from each of the studies described above. Estimates of silicosis morbidity derived from the seven cohorts in cumulative risk studies with post-employment follow-up range from 60 to 773 per 1,000 workers for 45-year exposures to the current general industry PEL of 0.10 mg/m³, and from 20 to 170 per 1,000 workers for a 45-year exposure to the proposed PEL of 0.05 mg/m³. The study results provide substantial evidence that the disease can progress for years after exposure ends. Results from an autopsy study (Hnizdo et al., 1993), which found pathological evidence of silicosis absent radiological signs, suggest that silicosis cases based on radiographic diagnosis alone tend to underestimate risk since pathological evidence of silicosis. Other results (Chen et al., 2005) suggest that surface properties among various types of silica dusts can have different silicosis potencies. Results from the Buchanan et al. (2003) study of Scottish coal miners suggest that short-term exposures to >2 mg/m³ silica can cause a disproportionately higher risk of silicosis than would be predicted by cumulative exposure alone, suggesting a dose-rate effect for exposures to concentrations above this level. OSHA believes that, given the consistent finding of a monotonic exposure-response relationship for silicosis morbidity with cumulative exposure in the studies reviewed, that cumulative exposure is a reasonable exposure metric upon which to base risk estimates in the exposure range of interest to OSHA (i.e., between 0.025 and 0.5 mg/m³ respirable crystalline silica).

F. Other Considerations in OSHA's Risk Analysis

Uncertainties are inherent to any risk modeling process and analysis; assessing risk and associated complexities of silica exposure among workers is no different. However, the Agency has a high level of confidence that the preliminary risk assessment results reasonably reflect the range of risks experienced by workers exposed to silica in all occupational settings. First, the preliminary assessment is based on an analysis of a wide range of studies, conducted in multiple industries across a wide range of exposure distributions, which included cumulative exposures equivalent to 45 years of exposure to and below the current PEL.

Second, risk models employed in this assessment are based on a cumulative exposure metric, which is the product of average daily silica concentration and duration of worker exposure for a specific job. Consequently, these models predict the same risk for a given cumulative exposure regardless of the pattern of exposure. For example, a manufacturing plant worker exposed to silica at 0.05 mg/m³ for eight hours per day will have the same cumulative exposure over a given period of time as a construction worker who is exposed each day to silica at 0.1 mg/m³ for one hour, at 0.075 mg/m³ for four hours and not exposed to silica for three hours. The cumulative exposure metric thus reflects a worker's long-term average exposure without regard to the pattern of exposure experienced by the worker, and is therefore generally applicable to all workers who are exposed to silica in the various industries. For example, at construction sites, conditions may change often since the nature of work can be intermittent and involve working with a variety of materials that contain different concentrations of quartz. Additionally, workers may perform construction operations for relatively short periods of time where they are exposed to concentrations of silica that may be significantly higher than many continuous operations in general industry. However, these differences are taken into account by the use of the cumulative exposure metric that relates exposure to disease risk. OSHA believes that use of cumulative exposure is the most appropriate dose-metric because each of the studies that provide the basis for the risk assessment demonstrated strong exposure-response relationships between cumulative exposure and disease risk. This metric is especially important in terms of progression of silica-related disease, as discussed in Section VII of the preamble, Significance of Risk, in section B.1.a.

OSHA's risk assessment relied upon many studies that utilized cumulative exposures of cohort members. Table VI-3 summarizes these lung cancer studies, including worker exposure quartile data across a number of industry sectors. The cumulative exposures exhibited in these studies are equivalent to the cumulative exposure that would result from 45 years of exposure to the current and proposed PELs (i.e., 4.5 and 2,25 mg/m³, respectively). For this reason, OSHA has a high degree of confidence in the risk estimates associated with exposure to the current and proposed PELs; additionally, the risk assessment does not require significant low-dose extrapolation of the model beyond the observed range of exposures. OSHA acknowledges there is greater uncertainty in the risk estimates for the proposed action level of 0.025 mg/m³, particularly given some evidence of a threshold for silicosis between the proposed PEL and action level. Given the Agency's findings that controlling exposures below the proposed PEL would not be technologically feasible for employers, OSHA believes that estimating risk for exposures below the proposed action level, which becomes increasingly more uncertain, is not necessary to further inform the Agency's regulatory action.

Although the Agency believes that the results of its risk assessment are broadly relevant to all occupational exposure situations involving crystalline silica, OSHA acknowledges that differences exist in the relative toxicity of crystalline silica particles present in different work settings due to factors such as the presence of mineral or metal impurities on quartz particle surfaces, whether the particles have been freshly fractured or are aged, and size distribution of particles. At this time, however, OSHA preliminarily concludes that it is not yet possible to use available information on factors that mediate the potency of silica to refine available quantitative estimates of the lung cancer and silicosis mortality risks, and that the estimates from the studies and analyses relied upon are fairly representative of a wide range of workplaces reflecting differences in silica polymorphism, surface properties, and impurities.

VII. Significance of Risk

A. Legal Requirements

To promulgate a standard that regulates workplace exposure to toxic materials or harmful physical agents, OSHA must first determine that the standard reduces a “significant risk” of “material impairment.” The first part of this requirement, “significant risk,” refers to the likelihood of harm, whereas the second part, “material impairment,” refers to the severity of the consequences of exposure.

The Agency's burden to establish significant risk derives from the OSH Act, 29 U.S.C. 651 et seq. Section 3(8) of the Act requires that workplace safety and health standards be “reasonably necessary and appropriate to provide safe or healthful employment.” 29 U.S.C. 652(8). The Supreme Court, in the “benzene” decision, stated that section 3(8) “implies that, before promulgating any standard, the Secretary must make a finding that the workplaces in question are not safe.” Indus. Union Dep't, AFL-CIO v. Am. Petroleum Inst., 448 U.S. 607, 642 (1980). Examining section 3(8) more closely, the Court described OSHA's obligation to demonstrate significant risk:

“[S]afe” is not the equivalent of “risk-free.” A workplace can hardly be considered “unsafe” unless it threatens the workers with a significant risk of harm. Therefore, before the Secretary can promulgate any permanent health or safety standard, he must make a threshold finding that the place of employment is unsafe in the sense that significant risks are present and can be eliminated or lessened by a change in practices.

Id. While clarifying OSHA's responsibilities, the Court emphasized the Agency's discretion in determining what constitutes significant risk, stating, “[the Agency's] determination that a particular level of risk is `significant' will be based largely on policy considerations.” Benzene, 448 U.S. at 655, n. 62. The Court explained that significant risk is not a “mathematical straitjacket,” and maintained that OSHA could meet its burden without “wait[ing] for deaths to occur before taking any action.” Benzene, 448 U.S. at 655.

Because section 6(b)(5) of the Act requires that the Agency base its findings on the “best available evidence,” a reviewing court must “give OSHA some leeway where its findings must be made on the frontiers of scientific knowledge.” Benzene, 448 U.S. at 656. Thus, while OSHA's significant risk determination must be supported by substantial evidence, the Agency “is not required to support the finding that a significant risk exists with anything approaching scientific certainty.” Id. Furthermore, “the Agency is free to use conservative assumptions in interpreting the data with respect to carcinogens, risking error on the side of over protection rather than under protection,” so long as such assumptions are based in “a body of reputable scientific thought.” Id.

The Act also requires that the Agency make a finding that the toxic material or harmful physical agent at issue causes material impairment to workers' health. Section 6(b)(5) of the Act directs the Secretary of Labor to “set the standard which most adequately assures, to the extent feasible, on the basis of the best available evidence, that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard . . . for the period of his working life.” 29 U.S.C. 655(b)(5). As with significant risk, what constitutes material impairment in any given case is a policy determination for which OSHA is given substantial leeway. “OSHA is not required to state with scientific certainty or precision the exact point at which each type of [harm] becomes a material impairment.” AFL-CIO v. OSHA, 965 F.2d 962, 975 (11th Cir. 1992). Courts have also noted that OSHA should consider all forms and degrees of material impairment—not just death or serious physical harm—and that OSHA may act with a “pronounced bias towards worker safety.” Id; Bldg & Constr. Trades Dep't v. Brock, 838 F.2d 1258, 1266 (D.C. Cir. 1988).

It is the Agency's practice to estimate risk to workers by using quantitative risk assessment and determining the significance of that risk based on judicial guidance, the language of the OSH Act, and Agency policy considerations. Thus, using the best available evidence, OSHA identifies material health impairments associated with potentially hazardous occupational exposures, and, when possible, provides a quantitative assessment of exposed workers' risk of these impairments. The Agency then evaluates whether these risks are severe enough to warrant regulatory action and determines whether a new or revised rule will substantially reduce these risks.

In this case, OSHA has reviewed extensive toxicological, epidemiological, and experimental research pertaining to adverse health effects of occupational exposure to respirable crystalline silica, including silicosis, other non-malignant respiratory disease, lung cancer, and autoimmune and renal diseases. As a result of this review, the Agency has developed preliminary quantitative estimates of the excess risk of mortality and morbidity that is attributable to currently allowable respirable crystalline silica exposure concentrations. The Agency is proposing a new PEL of 0.05 mg/m³ because exposures at and above this level present a significant risk to workers' health. Even though OSHA's preliminary risk assessment indicates that a significant risk exists at the proposed action level of 0.025 mg/m³, the Agency is not proposing a PEL below the proposed 0.05 mg/m³ limit because OSHA must also consider technological and economic feasibility in determining exposure limits. As explained in the Summary and Explanation for paragraph (c), Permissible Exposure Limit (PEL), OSHA has preliminary determined that the proposed PEL of 0.05 mg/m³ is technologically and economically feasible, but that a lower PEL of 0.025 mg/m³ is not technologically feasible. OSHA has preliminarily determined that long-term exposure at the current PEL presents a significant risk of material harm to workers' health, and that adoption of the proposed PEL will substantially reduce this risk to the extent feasible.

As discussed in Section V of this preamble (Health Effects Summary), inhalation exposure to respirable crystalline silica increases the risk of a variety of adverse health effects, including silicosis, chronic obstructive pulmonary disease (COPD), lung cancer, immunological effects, kidney disease, and infectious tuberculosis (TB). OSHA considers each of these conditions to be a material impairment of health. These diseases result in significant discomfort, permanent functional limitations including permanent disability or reduced ability to work, reduced quality of life, and decreased life expectancy. When these diseases coexist, as is common, the effects are particularly debilitating (Rice and Stayner, 1995; Rosenman et al., 1999). Based on these findings and on the scientific evidence that respirable crystalline silica substantially increases the risk of each of these conditions, OSHA preliminarily concludes that workers who are exposed to respirable crystalline silica at the current PEL are at significant risk of material impairment of health or functional capacity.

B. OSHA's Preliminary Findings

1. Material Impairments of Health

Section I of OSHA's Health Effects Literature Review and Preliminary Quantitative Risk Assessment (available in Docket OSHA-2010-0034) describes in detail the adverse health conditions that workers who are exposed to respirable crystalline silica are at risk of developing. The Agency's findings are summarized in Section V of this preamble (Health Effects Summary). The adverse health effects discussed include lung cancer, silicosis, other non-malignant respiratory disease (NMRD), and immunological and renal effects.

a. Silicosis

Silicosis refers to a spectrum of lung diseases attributable to the inhalation of respirable crystalline silica. As described in Section V (Health Effects Summary), the three types of silicosis are acute, accelerated, and chronic. Acute silicosis can occur within a few weeks to months after inhalation exposure to extremely high levels of respirable crystalline silica. Death from acute silicosis can occur within months to a few years of disease onset, with the exposed person drowning in their own lung fluid (NIOSH, 1996). Accelerated silicosis results from exposure to high levels of airborne respirable crystalline silica, and disease usually occurs within 5 to 10 years of initial exposure (NIOSH, 1996). Both acute and accelerated silicosis are associated with exposures that are substantially above the current general industry PEL, although precise information on the relationships between exposure and occurrence of disease are not available.

Chronic silicosis is the most common form of silicosis seen today, and is a progressive and irreversible condition characterized as a diffuse nodular pulmonary fibrosis (NIOSH, 1996). Chronic silicosis generally occurs after 10 years or more of inhalation exposure to respirable crystalline silica at levels below those associated with acute and accelerated silicosis. Affected workers may have a dry chronic cough, sputum production, shortness of breath, and reduced pulmonary function. These symptoms result from airway restriction caused by the development of fibrotic scarring in the alveolar sacs and the ends of the lung tissue. The scarring can be detected in chest x-ray films when the lesions become large enough to appear as visible opacities. The result is restriction of lung volumes and decreased pulmonary compliance with concomitant reduced gas transfer (Balaan and Banks, 1992). Chronic silicosis is characterized by small, rounded opacities that are symmetrically distributed in the upper lung zones on chest radiograph.

The diagnosis of silicosis is based on a history of exposure to respirable crystalline silica, chest radiograph findings, and the exclusion of other conditions, including tuberculosis (TB). Because workers affected by early stages of chronic silicosis are often asymptomatic, the finding of opacities in the lung is key to detecting silicosis and characterizing its severity. The International Labour Organization (ILO) International Classification of Radiographs of Pneumoconioses (ILO, 1980, 2002, 2011) is the currently accepted standard against which chest radiographs are evaluated in epidemiologic studies, for medical surveillance, and for clinical evaluation. The ILO system standardizes the description of chest x-rays, and is based on a 12-step scale of severity and extent of silicosis as evidenced by the size, shape, and density of opacities seen on the x-ray film. Profusion (frequency) of small opacities is classified on a 4-point major category scale (0-3), with each major category divided into three, giving a 12-point scale between 0/− and 3/+. Large opacities are defined as any opacity greater than 1 cm that is present in a film.

The small rounded opacities seen in early stage chronic silicosis (i.e., ILO major category 1 profusion) may progress (through ILO major categories 2 and/or 3) and develop into large fibrotic masses that destroy the lung architecture, resulting in progressive massive fibrosis (PMF). This stage of advanced silicosis is usually characterized by impaired pulmonary function, disability, and premature death. In cases involving PMF, death is commonly attributable to progressive respiratory insufficiency (Balaan and Banks, 1992).

The appearance of ILO category 2 or 3 background profusion of small opacities has been shown to increase the risk of developing large opacities characteristic of PMF. In one study of silicosis patients in Hong Kong, Ng and Chan (1991) found the risk of PMF increased by 42 and 64 percent among patients whose chest x-ray films were classified as ILO major category 2 or 3, respectively. Research has shown that people with silicosis advanced beyond ILO major category 1 have reduced median survival times compared to the general population (Infante-Rivard et al., 1991; Ng et al., 1992a; Westerholm, 1980).

Silicosis is the oldest known occupational lung disease and is still today the cause of significant premature mortality. In 2005, there were 161 deaths in the U.S. where silicosis was recorded as an underlying or contributing cause of death on a death certificate (NIOSH, 2008c). Between 1996 and 2005, deaths attributed to silicosis resulted in an average of 11.6 years of life lost by affected workers (NIOSH, 2007). In addition, exposure to respirable crystalline silica remains an important cause of morbidity and hospitalizations. State-based hospital discharge data show that in the year 2000, 1,128 silicosis-related hospitalizations occurred, indicating that silicosis continues to be a significant health issue in the U.S. (CSTE, 2005). Although there is no national silicosis disease surveillance system in the U.S., a published analysis of state-based surveillance data from the time period 1987-1996 estimated that between 3,600-7,000 new cases of silicosis occurred in the U.S. each year (Rosenman et al., 2003). It has been widely reported that available statistics on silicosis-related mortality and morbidity are likely to be understated due to misclassification of causes of death (for example, as tuberculosis, chronic bronchitis, emphysema, or cor pulmonale), errors in recording occupation on death certificates, or misdiagnosis of disease by health care providers (Goodwin, 2003; Windau et al., 1991; Rosenman et al., 2003). Furthermore, reliance on chest x-ray findings may miss cases of silicosis because fibrotic changes in the lung may not be visible on chest radiograph; thus, silicosis may be present absent x-ray signs or may be more severe than indicated by x-ray (Hnizdo et al., 1993; Craighead and Vallyathan, 1980; Rosenman et al., 1997).

Although most workers with early-stage silicosis (ILO categories 0/1 or 1/0) typically do not experience respiratory symptoms, the primary risk to the affected worker is progression of disease with progressive decline of lung function. Several studies of workers exposed to crystalline silica have shown that, once silicosis is detected by x-ray, a substantial proportion of affected workers can progress beyond ILO category 1 silicosis, even after exposure has ceased (for example, Hughes et al., 1982; Hessel et al., 1988; Miller et al., 1998; Ng et al., 1987a; Yang et al., 2006). In a population of coal miners whose last chest x-ray while employed was classified as major category 0, and who were examined again 10 years after the mine had closed, 20 percent had developed opacities consistent with a classification of at least 1/0, and 4 percent progressed further to at least 2/1 (Miller et al., 1998). Although there were periods of extremely high exposure to respirable quartz in the mine (greater than 2 mg/m³ in some jobs between 1972 and 1976, and more than 10 percent of exposures between 1969 and 1977 were greater than 1 mg/m³), the mean cumulative exposure for the cohort over the period 1964-1978 was 1.8 mg/m³-years, corresponding to an average silica concentration of 0.12 mg/m³. In a population of granite quarry workers exposed to an average respirable silica concentration of 0.48 mg/m³ (mean length of employment was 23.4 years), 45 percent of those diagnosed with simple silicosis showed radiological progression of disease after 2 to 10 years of follow up (Ng et al., 1987a). Among a population of gold miners, 92 percent progressed in 14 years; exposures of high-, medium-, and low-exposure groups were 0.97, 0.45, and 0.24 mg/m³, respectively (Hessel et al., 1988). Chinese mine and factory workers categorized under the Chinese system of x-ray classification as “suspected” silicosis cases (analogous to ILO 0/1) had a progression rate to stage I (analogous to ILO major category 1) of 48.7 percent and the average interval was about 5.1 years (Yang et al., 2006). These and other studies discussed in the Health Effects section are of populations of workers exposed to average concentrations of respirable crystalline silica above those permitted by OSHA's current general industry PEL. The studies, however, are of interest to OSHA because the Agency's current enforcement data indicate that exposures in this range are still common in some industry sectors. Furthermore, the Agency's preliminary risk assessment is based on use of an exposure metric that is less influenced by exposure pattern and, instead, characterizes the accumulated exposure of workers over time. Further, the use of a cumulative exposure metric reflects the progression of silica-related diseases: While it is not known that silicosis is a precursor to lung cancer, continued exposure to respirable crystalline silica among workers with silicosis has been shown to be associated with malignant respiratory disease (Chen et al., 1992). The Chinese pottery workers study offers an example of silicosis-associated lung cancer among workers in the clay industry, reflecting the variety of health outcomes associated with diverse silica exposures across industrial settings.

The risk of silicosis, and particularly its progression, carries with it an increased risk of reduced lung function. There is strong evidence in the literature for the finding that lung function deteriorates more rapidly in workers exposed to silica, especially those with silicosis, than what is expected from a normal aging process (Cowie 1998; Hughes et al., 1982; Malmberg et al., 1993; Ng and Chan, 1992). The rates of decline in lung function are greater in those whose disease showed evidence of radiologic progression (Bégin et al., 1987a; Cowie 1998; Ng and Chan, 1992; Ng et al., 1987a). Additionally, the average deterioration of lung function exceeds that in smokers (Hughes et al., 1982).

Several studies have reported no decrease in pulmonary function with an ILO category 1 level of profusion of small opacities but found declines in pulmonary function with categories 2 and 3 (Ng et al., 1987a; Begin et al., 1988; Moore et al., 1988). A study by Cowie (1998), however, found a statistically significantly greater annual loss in FVC and FEV₁ among those with category 1 profusion compared to category 0. In another study, Cowie and Mabena (1991) found that the degree of profusion of opacities was associated with reductions in several pulmonary function metrics. Still, other studies have reported no associations between radiographic silicosis and decreases in pulmonary function (Ng et al., 1987a; Wiles et al., 1992; Hnizdo, 1992), with some studies (Ng et al., 1987a; Wang et al., 1997) finding that measurable changes in pulmonary function are evident well before the changes seen on chest x-ray. This may reflect the general insensitivity of chest radiography in detecting lung fibrosis, and/or may reflect that exposure to respirable silica has also been shown to increase the risk of chronic obstructive pulmonary disease (COPD) (see Section V, Health Effects Summary).

Finally, silicosis, and exposure to respirable crystalline silica in and of itself, increases the risk that latent tuberculosis infection can convert to active disease. Early descriptions of dust diseases of the lung did not distinguish between TB and silicosis, and most fatal cases described in the first half of this century were a combination of silicosis and TB (Castranova et al., 1996). More recent findings demonstrate that exposure to silica, even without silicosis, increases the risk of infectious (i.e., active) pulmonary TB (Sherson et al., 1990; Cowie, 1994; Hnizdo and Murray, 1998; WaterNaude et al., 2006). Both conditions together can hasten the development of respiratory impairment and increase mortality risk even beyond that experienced by unexposed persons with active TB (Banks, 2005).

Based on the information presented above and in its review of the health literature, OSHA preliminarily concludes that silicosis remains a significant cause of early mortality and of serious morbidity, despite the existence of an enforceable exposure limit over the past 40 years. Silicosis in its later stages of progression (i.e., with chest x-ray findings of ILO category 2 or 3 profusion of small opacities, or the presence of large opacities) is characterized by the likely appearance of respiratory symptoms and decreased pulmonary function, as well as increased risk of progression to PMF, disability, and early mortality. Early-stage silicosis, although without symptoms among many who are affected, nevertheless reflects the formation of fibrotic lesions in the lung and increases the risk of progression to later stages, even after exposure to respirable crystalline silica ceases. In addition, the presence of silicosis increases the risk of pulmonary infections, including conversion of latent TB infection to active TB. Silicosis is not a reversible condition and there is no specific treatment for the disease, other than administration of drugs to alleviate inflammation and maintain open airways, or administration of oxygen therapy in severe cases. Based on these considerations, OSHA preliminarily finds that silicosis of any form, and at any stage, is a material impairment of health and that fibrotic scarring of the lungs represents loss of functional respiratory capacity.

b. Lung Cancer

OSHA considers lung cancer, an irreversible and usually fatal disease, to be a clear material impairment of health. According to the National Cancer Institute (Horner et al., 2009), the five-year survival rate for all forms of lung cancer is only 15.6 percent, a rate that has not improved in nearly two decades. OSHA's preliminary finding that respirable crystalline silica exposure substantially increases the risk of lung cancer mortality is based on the best available toxicological and epidemiological data, reflects substantial supportive evidence from animal and mechanistic research, and is consistent with the conclusions of other government and public health organizations, including the International Agency for Research on Cancer (IARC, 1997), the National Toxicology Program (NTP, 2000), the National Institute for Occupational Safety and Health (NIOSH, 2002), the American Thoracic Society (1997), and the American Conference of Governmental Industrial Hygienists (ACGIH, 2001). The Agency's primary evidence comes from evaluation of more than 50 studies of occupational cohorts from many different industry sectors in which exposure to respirable crystalline silica occurs, including granite and stone quarrying; the refractory brick industry; gold, tin, and tungsten mining; the diatomaceous earth industry; the industrial sand industry; and construction. Studies key to OSHA's risk assessment are outlined in Table VII-1, which summarizes exposure characterization and related lung cancer risk across several different industries. In addition, the association between exposure to respirable crystalline silica and lung cancer risk was reported in a national mortality surveillance study (Calvert et al., 2003) and in two community-based studies (Pukkala et al., 2005; Cassidy et al., 2007), as well as in a pooled analysis of 10 occupational cohort studies (Steenland et al., 2001a).

Toxicity studies provide additional evidence of the carcinogenic potential of crystalline silica (Health Effects Summary, Section V). Acellular studies using DNA exposed directly to freshly fractured crystalline silica demonstrate the direct effect silica has on DNA breakage. Cell culture research has investigated the processes by which crystalline silica disrupts normal gene expression and replication (Section V). Studies demonstrate that chronic inflammatory and fibrotic processes resulting in oxidative and cellular damage set up another possible mechanism that leads to neoplastic changes in the lung (Goldsmith, 1997; see also Health Effects discussion in Section V). In addition, the biologically damaging physical characteristics of crystalline silica, and the direct and indirect genotoxicity of crystalline silica (Schins, 2002; Borm and Driscoll, 1996), support the Agency's preliminary position that respirable crystalline silica should be considered as an occupational carcinogen that causes lung cancer, a clear material impairment of health.

c. Non-Malignant Respiratory Disease (Other Than Silicosis)

Exposure to respirable crystalline silica increases the risk of developing chronic obstructive pulmonary disease (COPD), in particular chronic bronchitis and emphysema. COPD results in loss of pulmonary function that restricts normal activity in individuals afflicted with these conditions (ATS, 2003). Both chronic bronchitis and emphysema can occur in conjunction with development of silicosis. Several studies have documented increased prevalence of chronic bronchitis and emphysema among silica-exposed workers even absent evidence of silicosis (see Section I of the Health Effects Literature Review and Preliminary Quantitative Risk Assessment; NIOSH, 2002; ATS, 1997). There is evidence that smoking may have an additive or synergistic effect on silica-related COPD morbidity or mortality (Hnizdo, 1990; Hnizdo et al., 1990; Wyndham et al., 1986; NIOSH, 2002). In a study of diatomaceous earth workers, Park et al. (2002) found a positive exposure-response relationship between exposure to respirable cristobalite and increased mortality from non-malignant respiratory disease.

Decrements in pulmonary function have often been found among workers exposed to respirable crystalline silica absent radiologic evidence of silicosis. Several cross-sectional studies have reported such findings among granite workers (Theriault, 1974a, 1974b; Ng et al., 1992b; Montes et al., 2004b), South African gold miners (Irwig and Rocks, 1978; Hnizdo et al., 1990; Cowie and Mabena, 1991), gemstone cutters (Ng et al., 1987b), concrete workers (Meijer et al., 2001), refractory brick workers (Wang et al., 1997), hard rock miners (Manfreda et al., 1982; Kreiss et al., 1989), pottery workers (Neukirch et al., 1994), slate workers (Suhr et al., 2003), and potato sorters (Jorna et al., 1994).

OSHA also evaluated several longitudinal studies where exposed workers were examined over a period of time to track changes in pulmonary function. Among both active and retired Vermont granite workers exposed to an average of 60 μg/m³, Graham did not find exposure-related decrements in pulmonary function (Graham et al., 1981, 1994). However, Eisen et al. (1995) did find significant pulmonary decrements among a subset of granite workers (termed “dropouts”) who left work and consequently did not voluntarily participate in the last of a series of annual pulmonary function tests. This group of workers experienced steeper declines in FEV₁ compared to the subset of workers who remained at work and participated in all tests (termed “survivors”), and these declines were significantly related to dust exposure. Thus, in this study, workers who had left work had exposure-related declines in pulmonary function to a greater extent than did workers who remained on the job, clearly demonstrating a survivor effect among the active workers. Exposure-related changes in lung function were also reported in a 12-year study of granite workers (Malmberg et al., 1993), in two 5-year studies of South African miners (Hnizdo, 1992; Cowie, 1998), and in a study of foundry workers whose lung function was assessed between 1978 and 1992 (Hertzberg et al., 2002).

Each of these studies reported their findings in terms of rates of decline in any of several pulmonary function measures, such as FVC, FEV₁, and FEV₁/FVC. To put these declines in perspective, Eisen et al. (1995), reported that the rate of decline in FEV₁ seen among the dropout subgroup of Vermont granite workers was 4 ml per mg/m³-year of exposure to respirable granite dust; by comparison, FEV₁ declines at a rate of 10 ml/year from smoking one pack of cigarettes daily. From their study of foundry workers, Hertzberg et al., (2002) reported finding a 1.1 ml/year decline in FEV₁ and a 1.6 ml/year decline in FVC for each mg/m³-year of respirable silica exposure after controlling for ethnicity and smoking. From these rates of decline, they estimated that exposure to the current OSHA quartz standard of 0.1 mg/m³ for 40 years would result in a total loss of FEV₁ and FVC that is less than but still comparable to smoking a pack of cigarettes daily for 40 years. Hertzberg et al. (2002) also estimated that exposure to the current standard for 40 years would increase the risk of developing abnormal FEV₁ or FVC by factors of 1.68 and 1.42, respectively. OSHA believes that this magnitude of reduced pulmonary function, as well as the increased morbidity and mortality from non-malignant respiratory disease that has been documented in the studies summarized above, constitute material impairments of health and loss of functional respiratory capacity.

d. Renal and Autoimmune Effects

OSHA's review of the literature summarized in Section V, Health Effects Summary, reflects substantial evidence that exposure to crystalline silica increases the risk of renal and autoimmune diseases. Epidemiologic studies have found statistically significant associations between occupational exposure to silica dust and chronic renal disease (e.g., Calvert et al., 1997), subclinical renal changes including proteinurea and elevated serum creatinine (e.g., Ng et al., 1992c; Rosenman et al., 2000; Hotz et al., 1995), end-stage renal disease morbidity (e.g., Steenland et al., 1990), chronic renal disease mortality (Steenland et al., 2001b, 2002a), and Wegener's granulomatosis (Nuyts et al., 1995), the latter of which represents severe injury to the glomeruli that, if untreated, rapidly leads to renal failure. Possible mechanisms suggested for silica-induced renal disease include a direct toxic effect on the kidney, deposition in the kidney of immune complexes (IgA) following silica-related pulmonary inflammation, or an autoimmune mechanism (Calvert et al., 1997; Gregorini et al., 1993). Steenland et al. (2002a) demonstrated a positive exposure-response relationship between exposure to respirable crystalline silica and end-stage renal disease mortality.

In addition, there are a number of studies that show exposure to be related to increased risks of autoimmune disease, including scleroderma (e.g., Sluis-Cremer et al., 1985), rheumatoid arthritis (e.g. Klockars et al., 1987; Rosenman and Zhu, 1995), and systemic lupus erythematosus (e.g., Brown et al., 1997). Scleroderma is a degenerative disorder that leads to over-production of collagen in connective tissue that can cause a wide variety of symptoms including skin discoloration and ulceration, joint pain, swelling and discomfort in the extremities, breathing problems, and digestive problems. Rheumatoid arthritis is characterized by joint pain and tenderness, fatigue, fever, and weight loss. Systemic lupus erythematosus is a chronic disease of connective tissue that can present a wide range of symptoms including skin rash, fever, malaise, joint pain, and, in many cases, anemia and iron deficiency. OSHA believes that chronic renal disease, end-stage renal disease mortality, Wegener's granulomatosis, scleroderma, rheumatoid arthritis, and systemic lupus erythematosus clearly represent material impairments of health.

2. Significance of Risk

To evaluate the significance of the health risks that result from exposure to hazardous chemical agents, OSHA relies on toxicological, epidemiological, and experimental data, as well as statistical methods. The Agency uses these data and methods to characterize the risk of disease resulting from workers' exposure to a given hazard over a working lifetime at levels of exposure reflecting both compliance with current standards and compliance with the new standard being proposed. In the case of crystalline silica, the current general industry, construction, and shipyard PELs are formulas that limit 8-hour TWA exposures to respirable dust; the limit on exposure decreases with increasing crystalline silica content of the dust. OSHA's current general industry PEL for respirable quartz is expressed both in terms of a particle count as well as a gravimetric concentration, while the current construction and shipyard employment PELs for respirable quartz are only expressed in terms of a particle count formula. For general industry, the gravimetric formula PEL for quartz approaches 0.1 mg/m³ (100 μg/m³) of respirable crystalline silica when the quartz content of the dust is about 10 percent or greater. For the construction and shipyard industries, the current PEL is a formula that is based on concentration of respirable particles in the air; on a mass concentration basis, it is believed by OSHA to lie within a range of between about 0.25 mg/m³ (250 μg/m³) to 0.5 mg/m³ (500 μg/m³) expressed as respirable quartz (see Section VI). In general industry, the current PELs for cristobalite and tridymite are one-half the PEL for quartz.

OSHA is proposing to revise the current PELs for general industry, construction, and shipyards to 0.05 mg/m³ (50 μg/m³) of respirable crystalline silica. OSHA is also proposing an action level of 0.025 mg/m³ (25 μg/m³). In the Summary of the Preliminary Quantitative Risk Assessment (Section VI of the preamble), OSHA presents estimates of health risks associated with 45 years of exposure to 0.025, 0.05, and 0.1 mg/m³ respirable crystalline silica to represent the risks associated with exposure over a working lifetime to the proposed action level, proposed PEL, and current general industry PEL, respectively. OSHA also presents estimates associated with exposure to 0.25 and 0.5 mg/m³ to represent a range of risks likely to be associated with exposure to the current construction and shipyard PELs. Risk estimates are presented for mortality due to lung cancer, silicosis and other non-malignant lung disease, and end-stage renal disease, as well as silicosis morbidity. The preliminary findings from this assessment are summarized below.

a. Summary of Excess Risk Estimates for Excess Lung Cancer Mortality

For preliminary estimates of lung cancer risk from crystalline silica exposure, OSHA has relied upon studies of exposure-response relationships presented in a pooled analysis of 10 cohort studies (Steenland, et al. 2001a; Toxichemica, Inc., 2004) as well as on individual studies of granite (Attfield and Costello, 2004), diatomaceous earth (Rice et al., 2001), and industrial sand (Hughes et al., 2001) worker cohorts, and a study of coal miners exposed to respirable quartz (Miller et al., 2007; Miller and MacCalman, 2009). OSHA believes these studies are suitable for use to quantitatively characterize health risks to exposed workers because (1) study populations were of sufficient size to provide adequate power to detect low levels of risk, (2) sufficient quantitative exposure data were available to characterize cumulative exposures of cohort members to respirable crystalline silica, (3) the studies either adjusted for or otherwise adequately addressed confounding factors such as smoking and exposure to other carcinogens, and (4) investigators developed quantitative assessments of exposure-response relationships using appropriate statistical models or otherwise provided sufficient information that permits OSHA to do so. Where investigators estimated excess lung cancer risks associated with exposure to the current PEL or NIOSH recommended exposure limit, OSHA provided these estimates in its Preliminary Quantitative Risk Assessment. However, OSHA implemented all risk models in its own life table analysis so that the use of background lung cancer rates and assumptions regarding length of exposure and lifetime were constant across each of the models, and so OSHA could estimate lung cancer risks associated with exposure to specific levels of silica of interest to the Agency.

The Steenland et al. (2001a) study consisted of a pooled exposure-response analysis and risk assessment based on raw data obtained for ten cohorts of silica-exposed workers (65,980 workers, 1,072 lung cancer deaths). The cohorts in this pooled analysis include U.S. gold miners (Steenland and Brown, 1995a), U.S. diatomaceous earth workers (Checkoway et al., 1997), Australian gold miners (deKlerk and Musk, 1998), Finnish granite workers (Koskela et al., 1994), South African gold miners (Hnizdo et al., 1997), U.S. industrial sand employees (Steenland et al., 2001b), Vermont granite workers (Costello and Graham, 1988), and Chinese pottery workers, tin miners, and tungsten miners (Chen et al., 1992). The investigators used a nested case-control design with cases and controls matched for race, sex, age (within five years) and study; 100 controls were matched for each case. An extensive exposure assessment for this pooled analysis was developed and published by Mannetje et al. (2002a). Exposure measurement data were available for all 10 cohorts and included measurements of particle counts, total dust mass, respirable dust mass, and, for one cohort, respirable quartz. Cohort-specific conversion factors were used to estimate cumulative exposures to respirable crystalline silica. A case-control analysis of silicosis mortality (Mannetje et al., 2002b) showed a strong positive exposure-response trend, indicating that cumulative exposure estimates for the cohorts were not subject to random misclassification errors of such a magnitude so as to obscure observing an exposure-response relationship between silica and silicosis despite the variety of dust measurement metrics relied upon and the need to make assumptions to convert the data to a single exposure metric (i.e., mass concentration of respirable crystalline silica). In effect, the known relationship between exposure to respirable silica and silicosis served as a positive control to assess the validity of exposure estimates. Quantitative assessment of lung cancer risks were based on use of a log-linear model (log RR = βx, where x represents the exposure variable and β the coefficient to be estimated) with a 15-year exposure lag providing the best fit. Models based on untransformed or log-transformed cumulative dose metrics provided an acceptable fit to the pooled data, with the model using untransformed cumulative dose providing a slightly better fit. However, there was substantial heterogeneity among the exposure-response coefficients derived from the individual cohorts when untransformed cumulative dose was used, which could result in one or a few of the cohorts unduly influencing the pooled exposure-response coefficient. For this reason, the authors preferred the use of log-transformed cumulative exposure in the model to derive the pooled coefficient since heterogeneity was substantially reduced.

OSHA's implementation of this model is based on a re-analysis conducted by Steenland and Bartow (Toxichemica, 2004), which corrected small errors in the assignment of exposure estimates in the original analysis. In addition, subsequent to the Toxichemica report, and in response to suggestions made by external peer reviewers, Steenland and Bartow conducted additional analyses based on use of a linear relative risk model having the general form RR = 1 + βx, as well as a categorical analysis (personal communication, Steenland 2010). The linear model was implemented with both untransformed and log-transformed cumulative exposure metrics, and was also implemented as a 2-piece spline model.

The categorical analysis indicates that, for the pooled data set, lung cancer relative risks increase steeply at low exposures, after which the rate of increase in relative risk declines and the exposure-response curve becomes flat (see Figure II-2 of the Preliminary Quantitative Risk Assessment). Use of either the linear relative risk or log-linear relative risk model with untransformed cumulative exposure (with or without a 15-year lag) failed to capture this initial steep slope, resulting in an underestimate of the relative risk compared to that suggested by the categorical analysis. In contrast, use of log-transformed cumulative exposure with the linear or log-linear model, and use of the 2-piece linear spline model with untransformed exposure, better reflected the initial rise and subsequent leveling out of the exposure-response curve, with the spline model fitting somewhat better than either the linear or log-linear models (all models incorporated a 15-year exposure lag). Of the three models that best reflect the shape of the underlying exposure-response curve suggested by the categorical analysis, there is no clear rationale to prefer one over the other. Use of log-transformed cumulative exposure in either the linear or log-linear models has the advantage of reducing heterogeneity among the 10 pooled studies, lessening the likelihood that the pooled coefficient would be overtly influenced by outliers; however, use of a log-transformed exposure metric complicates comparing results with those from other risk analyses considered by OSHA that are based on untransformed exposure metrics. Since all three of these models yield comparable estimates of risk the choice of model is not critical for the purpose of assessing significance of the risk, and therefore OSHA believes that the risk estimates derived from the pooled study are best represented as a range of estimates based on all three of these models.

From these models, the estimated lung cancer risk associated with 45 years of exposure to 0.1 mg/m³ (about equal to the current general industry PEL) is between 22 and 29 deaths per 1,000 workers. The estimated risk associated with exposure to silica concentrations in the range of 0.25 and 0.5 mg/m³ (about equal to the current construction and shipyard PELs) is between 27 and 38 deaths per 1,000. At the proposed PEL of 0.05 mg/m³, the estimated excess risk ranges from 18 to 26 deaths per 1,000, and, at the proposed action level of 0.025 mg/m³, from 9 to 23 deaths per 1,000.

As previously discussed, the exposure-response coefficients derived from each of the 10 cohorts exhibited significant heterogeneity; risk estimates based on the coefficients derived from the individual studies for untransformed cumulative exposure varied by almost two orders of magnitude, with estimated risks associated with exposure over a working lifetime to the current general industry PEL ranging from a low of 0.8 deaths per 1,000 (from the Chinese pottery worker study) to a high of 69 deaths per 1,000 (from the South African miner study). It is possible that the differences seen in the slopes of the exposure-response relationships reflect physical differences in the nature of crystalline silica particles generated in these workplaces and/or the presence of different substances on the crystal surfaces that could mitigate or enhance their toxicity (see Section V, Health Effects Summary). It may also be that exposure estimates for some cohorts were subject to systematic misclassification errors resulting in under- or over-estimation of exposures due to the use of assumptions and conversion factors that were necessary to estimate mass respirable crystalline silica concentrations from exposure samples analyzed as particle counts or total and respirable dust mass. OSHA believes that, given the wide range of risk estimates derived from these 10 studies, use of log-transformed cumulative exposure or the 2-piece spline model is a reasonable approach for deriving a single summary statistic that represents the lung cancer risk across the range of workplaces and exposure conditions represented by the studies. However, use of these approaches results in a non-linear exposure-response and suggests that the relative risk of silica-related lung cancer begins to attenuate at cumulative exposures in the range of those represented by the current PELs. Although such exposure-response relationships have been described for some carcinogens (for example, from metabolic saturation or a healthy worker survivor effect, see Staynor et al., 2003), OSHA is not aware of any specific evidence that would suggest that such a result is biologically plausible for silica, except perhaps the possibility that lung cancer risks increase more slowly with increasing exposure because of competing risks from other silica-related diseases. Attenuation of the exposure-response can also result from misclassification of exposure estimates for the more highly-exposed cohort members (Staynor et al., 2003). OSHA's evaluation of individual cohort studies discussed below indicates that, with the exception of the Vermont granite cohort, attenuation of exposure-related lung cancer response has not been directly observed.

In addition to the pooled cohort study, OSHA's Preliminary Quantitative Risk Assessment presents risk estimates derived from four individual studies where investigators presented either lung cancer risk estimates or exposure-response coefficients. Two of these studies, one on diatomaceous earth workers (Rice et al., 2001) and one on Vermont granite workers (Attfield and Costello, 2004), were included in the 10-cohort pooled study (Steenland et al., 2001a; Toxichemica, 2004). The other two were of British coal miners (Miller et al., 2007; Miller and MacCalman, 2010) and North American industrial sand workers (Hughes et al., 2001).

Rice et al. (2001) presents an exposure-response analysis of the diatomaceous worker cohort studied by Checkoway et al. (1993, 1996, 1997), who found a significant relationship between exposure to respirable cristobalite and increased lung cancer mortality. The cohort consisted of 2,342 white males employed for at least one year between 1942 and 1987 in a California diatomaceous earth mining and processing plant. The cohort was followed until 1994, and included 77 lung cancer deaths. The risk analysis relied on an extensive job-specific exposure assessment developed by Sexias et al. (1997), which included use of over 6,000 samples taken during the period 1948 through 1988. The mean cumulative exposure for the cohort was 2.16 mg/m³-years for respirable crystalline silica dust. Rice et al. (2001) evaluated several model forms for the exposure-response analysis and found exposure to respirable cristobalite to be a significant predictor of lung cancer mortality with the best-fitting model being a linear relative risk model (with a 15-year exposure lag). From this model, the estimates of the excess risk of lung cancer mortality are 34, 17, and 9 deaths per 1,000 workers for 45-years of exposure to 0.1, 0.05, and 0.025 mg/m³, respectively. For exposures in the range of the current construction and shipyard PELs over 45 years, estimated risks lie in a range between 81 and 152 deaths per 1,000 workers.

Somewhat higher risk estimates are derived from the analysis presented by Attfield and Costello (2004) of Vermont granite workers. This study involved a cohort of 5,414 male granite workers who were employed in the Vermont granite industry between 1950 and 1982 and who were followed through 1994. Workers' cumulative exposures were estimated by Davis et al. (1983) based on historical exposure data collected in six environmental surveys conducted between 1924 and 1977. A categorical analysis showed an increasing trend of lung cancer risk ratios with increasing exposure, and Poisson regression was used to evaluate several exposure-response models with varying exposure lags and use of either untransformed or log-transformed exposure metrics. The best-fitting model was based on use of a 15-year lag, use of untransformed cumulative exposure, and omission of the highest exposure group. The investigators believed that the omission of the highest exposure group was appropriate since: (1) The underlying exposure data for the high-exposure group was weaker than for the others; (2) there was a greater likelihood that competing causes of death and misdiagnoses of causes of death attenuated the lung cancer death rate in the highest exposure group; (3) all of the remaining groups comprised 85 percent of the deaths in the cohort and showed a strong linear increase in lung cancer mortality with increasing exposure; and (4) the exposure-response relationship seen in the lower exposure groups was more relevant given that the exposures of these groups were within the range of current occupational standards. OSHA's use of the exposure coefficient from this analysis in a log-linear relative risk model yielded a risk estimate of 60 deaths per 1,000 workers for 45 years of exposure to the current general industry PEL of 0.1 mg/m³, 25 deaths per 1,000 for 45 years of exposure to the proposed PEL of 0.05 mg/m³, and 11 deaths per 1,000 for 45 years of exposure at the proposed action level of 0.025 mg/m³. Estimated risks associated with 45 years of exposure at the current construction PEL range from 250 to 653 deaths per 1,000.

Hughes et al. (2001) conducted a nested case-control study of 95 lung cancer deaths from a cohort of 2,670 industrial sand workers in the U.S. and Canada studied by McDonald et al. (2001). (This cohort overlaps with the cohort studied by Steenland and Sanderson (2001), which was included in the 10-cohort pooled study by Steenland et al., 2001a). Both categorical analyses and conditional logistic regression were used to examine relationships with cumulative exposure, log of cumulative exposure, and average exposure. Exposure levels over time were estimated via a job-exposure matrix developed for this study (Rando et al., 2001). The 50th percentile (median) exposure level of cases and controls for lung cancer were 0.149 and 0.110 mg/m³ respirable crystalline silica, respectively, slightly above the current OSHA general industry standard. There did not appear to be substantial misclassification of exposures, as evidenced by silicosis mortality showing a positive exposure-response trend with cumulative exposure and average exposure concentration. Statistically significant positive exposure-response trends for lung cancer were found for both cumulative exposure (lagged 15 years) and average exposure concentration, but not for duration of employment, after controlling for smoking. There was no indication of an interaction effect of smoking and cumulative silica exposure. Hughes et al. (2001) reported the exposure coefficients for both lagged and unlagged cumulative exposure; there was no significant difference between the two (0.13 per mg/m³-year for lagged vs. 0.14 per mg/m³-year for unlagged). Use of the coefficient from Hughes et al. (2001) that incorporated a 15-year lag generates estimated cancer risks of 34, 15, and 7 deaths per 1,000 for 45 years exposure to the current general industry PEL of 0.1, the proposed PEL of 0.05 mg/m³, and the proposed action level of 0.025 mg/m³ respirable silica, respectively. For 45 years of exposure to the construction PEL, estimated risks range from 120 to 387 deaths per 1,000 workers.

Miller and MacCalman (2010, also reported in Miller et al., 2007) extended the follow-up of a previously published cohort mortality study (Miller and Buchanan, 1997). The follow-up study included 17,800 miners from 10 coal mines in the U.K. who were followed through the end of 2005; observation in the original study began in 1970. By 2005, there were 516,431 person years of observation, an average of 29 years per miner, with 10,698 deaths from all causes. Exposure estimates of cohort members were not updated from the earlier study since the mines closed in the 1980s; however, some of these men might have had additional exposure at other mines or facilities. An analysis of cause-specific mortality was performed using external controls; it demonstrated that lung cancer mortality was statistically significantly elevated for coal miners exposed to silica. An analysis using internal controls was performed via Cox proportional hazards regression methods, which allowed for each individual miner's measurements of age and smoking status, as well as the individual's detailed dust and quartz time-dependent exposure measurements. From the Cox regression, Miller and MacCalman (2009) estimated that cumulative exposure of 5 g-h/m³ respirable quartz (incorporating a 15-year lag) was associated with a relative risk of 1.14 for lung cancer. This cumulative exposure is about equivalent to 45 years of exposure to 0.055 mg/m³ respirable quartz, or a cumulative exposure of 2.25 mg/m³-yr, assuming 2,000 hours of exposure per year. OSHA applied this slope factor in a log-relative risk model and estimated the lifetime lung cancer mortality risk to be 13 per 1,000 for 45 years of exposure to 0.1 mg/m³ respirable crystalline silica. For the proposed PEL of 0.05 mg/m³ and proposed action level of 0.025 mg/m³, the lifetime risks are estimated to be 6 and 3 deaths per 1,000, respectively. The range of risks estimated to result from 45 years of exposure to the current construction and shipyard PELs is from 37 to 95 deaths per 1,000 workers.

The analysis from the Miller and MacCalman (2009) study yields risk estimates that are lower than those obtained from the other cohort studies described above. Possible explanations for this include: (1) Unlike the studies on diatomaceous earth workers and granite workers, the mortality analysis of the coal miners was adjusted for smoking; (2) lung cancer risks might have been lower among the coal miners due to high competing mortality risks observed in the cohort (mortality was significantly increased for several diseases, including tuberculosis, chronic bronchitis, and non-malignant respiratory disease); and (3) the lower risk estimates derived from the coal miner study could reflect an actual difference in the cancer potency of the quartz dust in the coal mines compared to that present in the work environments studied elsewhere. OSHA believes that the risk estimates derived from this study are credible. In terms of design, the cohort was based on union rolls with very good participation rates and good reporting. The study group was the largest of any of the individual cohort studies reviewed here (over 17,000 workers) and there was an average of nearly 30 years of follow-up, with about 60 percent of the cohort having died by the end of follow-up. Just as important were the high quality and detail of the exposure measurements, both of total dust and quartz.

b. Summary of Risk Estimates for Silicosis and Other Chronic Lung Disease Mortality

OSHA based its quantitative assessment of silicosis mortality risks on a pooled analysis conducted by Mannetje et al. (2002b) of data from six of the ten epidemiological studies in the Steenland et al. (2001a) pooled analysis of lung cancer mortality. Cohorts included in the silicosis study were U.S. diatomaceous earth workers (Checkoway et al., 1997); Finnish granite workers (Koskela et al., 1994); U.S. granite workers (Costello and Graham, 1988); U.S. industrial sand workers (Steenland and Sanderson, 2001); U.S. gold miners (Steenland and Brown, 1995b); and Australian gold miners (deKlerk and Musk, 1998). These six cohorts contained 18,634 subjects and 170 silicosis deaths, where silicosis mortality was defined as death from silicosis (ICD-9 502, n=150) or from unspecified pneumoconiosis (ICD-9 505, n = 20). Analysis of exposure-response was performed in a categorical analysis where the cohort was divided into cumulative exposure deciles and Poisson regression was used to estimate silicosis rate ratios for each category, adjusted for age, calendar period, and study. Exposure-response was examined in more detail using a nested case-control design and logistic regression. Although Mannetje et al. (2002b) estimated silicosis risks at the current OSHA PEL from the Poisson regression, a subsequent analysis based on the case-control design was conducted by Steenland and Bartow (Toxichemica, 2004), which resulted in slightly lower estimates of risk. Based on the Toxichemica analysis, OSHA estimates that the lifetime risk (over 85 years) of silicosis mortality associated with 45 years of exposure to the current general industry PEL of 0.1 mg/m³ is 11 deaths per 1,000 workers. Exposure for 45 years to the proposed PEL of 0.05 mg/m³ and action level of 0.025 mg/m³ results in an estimated 7 and 4 silicosis deaths per 1,000, respectively. Lifetime risks associated with exposure at the current construction and shipyard PELs range from 17 to 22 deaths per 1,000 workers.

To study non-malignant respiratory diseases, of which silicosis is one, Park et al. (2002) analyzed the California diatomaceous earth cohort data originally studied by Checkoway et al. (1997), consisting of 2,570 diatomaceous earth workers employed for 12 months or more from 1942 to 1994. The authors quantified the relationship between exposure to cristobalite and mortality from chronic lung disease other than cancer (LDOC). Diseases in this category included pneumoconiosis (which included silicosis), chronic bronchitis, and emphysema, but excluded pneumonia and other infectious diseases. Less than 25 percent of the LDOC deaths in the analysis were coded as silicosis or other pneumoconiosis (15 of 67). As noted by Park et al. (2002), it is likely that silicosis as a cause of death is often misclassified as emphysema or chronic bronchitis. Exposure-response relationships were explored using both Poisson regression models and Cox's proportional hazards models fit to the same series of relative rate exposure-response models that were evaluated by Rice et al. (2001) for lung cancer (i.e., log-linear, log-square root, log-quadratic, linear relative rate, a power function, and a shape function). Relative or excess rates were modeled using internal controls and adjusting for age, calendar time, ethnicity (Hispanic versus white), and time since first entry into the cohort, or using age- and calendar time-adjusted external standardization to U.S. population mortality rates. There were no LDOC deaths recorded among workers having cumulative exposures above 32 mg/m³-years, causing the response to level off or decline in the highest exposure range; possible explanations considered included survivor selection, depletion of susceptible populations in high dust areas, and/or a higher degree of misclassification of exposures in the earlier years where exposure data were lacking and when exposures were presumably the highest. Therefore, Park et al. (2002) performed exposure-response analyses that restricted the dataset to observations where cumulative exposures were below 10 mg/m³-years, a level more than four times higher than that resulting from 45 years of exposure to the current general industry PEL for cristobalite (which is about 0.05 mg/m³), as well as analyses using the full dataset. Among the models based on the restricted dataset, the best-fitting model with a single exposure term was the linear relative rate model using external adjustment.

OSHA's estimates of the lifetime chronic lung disease mortality risk based on this model are substantially higher than those that OSHA derived from the Mannetje et al. (2002b) silicosis analysis. For the current general industry PEL of 0.1 mg/m³, exposure for 45 years is estimated to result in 83 deaths per 1,000 workers. At the proposed PEL of 0.05 mg/m³ and action level of 0.025 mg/m³, OSHA estimates the lifetime risk from 45 years of exposure to be 43 and 22 deaths per 1,000, respectively. The range of risks associated with exposure at the construction and shipyard PELs over a working lifetime is from 188 to 321 deaths per 1,000 workers. It should be noted that the Mannetje study (2002b) was not adjusted for smoking while the Park study (2002) had data on smoking habits for about one-third of the workers who died from LDOC and about half of the entire cohort. The Poisson regression on which the risk model is based was partially stratified on smoking. Furthermore, analyses without adjustment for smoking suggested to the authors that smoking was acting as a negative confounder.

c. Summary of Risk Estimates for Renal Disease Mortality

OSHA's analysis of the health effects literature included several studies that have demonstrated that exposure to crystalline silica increases the risk of renal and autoimmune disease (see Section V, Health Effects Summary). Studies have found statistically significant associations between occupational exposure to silica dust and chronic renal disease, sub-clinical renal changes, end-stage renal disease morbidity, chronic renal disease mortality, and Wegener's granulomatosis. A strong exposure-response association for renal disease mortality and silica exposure has also been demonstrated.

OSHA's assessment of the renal disease risks that result from exposure to respirable crystalline silica are based on an analysis of pooled data from three cohort studies (Steenland et al., 2002a). The combined cohort for the pooled analysis (Steenland et al., 2002a) consisted of 13,382 workers and included industrial sand workers (Steenland et al., 2001b), U.S. gold miners (Steenland and Brown, 1995a), and Vermont granite workers (Costello and Graham, 1998). Exposure data were available for 12,783 workers and analyses conducted by the original investigators demonstrated monotonically increasing exposure-response trends for silicosis, indicating that exposure estimates were not likely subject to significant random misclassification. The mean duration of exposure, cumulative exposure, and concentration of respirable silica for the combined cohort were 13.6 years, 1.2 mg/m³-years, and 0.07 mg/m³, respectively. There were highly statistically significant trends for increasing renal disease mortality with increasing cumulative exposure for both multiple cause analysis of mortality (p<0.000001) and underlying cause analysis (p = 0.0007). Exposure-response analysis was also conducted as part of a nested case-control study, which showed statistically significant monotonic trends of increasing risk with increasing exposure again for both multiple cause (p = 0.004 linear trend, 0.0002 log trend) and underlying cause (p = 0.21 linear trend, 0.03 log trend) analysis. The authors found that use of log-cumulative dose in a log relative risk model fit the pooled data better than cumulative exposure, average exposure, or lagged exposure. OSHA's estimates of renal disease mortality risk, which are based on the log relative risk model with log cumulative exposure, are 39 deaths per 1,000 for 45 years of exposure at the current general industry PEL of 0.1 mg/m³, 32 deaths per 1,000 for exposure at the proposed PEL of 0.05 mg/m³, and 25 deaths per 1,000 at the proposed action level of 0.025 mg/m³. OSHA also estimates that 45 years of exposure at the current construction and shipyard PELs would result in a renal disease mortality risk ranging from 52 to 63 deaths per 1,000 workers.

d. Summary of Risk Estimates for Silicosis Morbidity

OSHA's Preliminary Quantitative Risk Assessment reviewed several cross-sectional studies designed to characterize relationships between exposure to respirable crystalline silica and development of silicosis as determined by chest radiography. Several of these studies could not provide information on exposure or length of employment prior to disease onset. Others did have access to sufficient historical medical data to retrospectively determine time of disease onset but included medical examination at follow up of primarily active workers with little or no post-employment follow-up. Although OSHA presents silicosis risk estimates that were reported by the investigators of these studies, OSHA believes that such estimates are likely to understate lifetime risk of developing radiological silicosis; in fact, the risk estimates reported in these studies are generally lower than those derived from studies that included retired workers in follow up medical examinations.

Therefore, OSHA believes that the most useful studies for characterizing lifetime risk of silicosis morbidity are retrospective cohort studies that included a large proportion of retired workers in the cohort and that were able to evaluate disease status over time, including post-retirement. OSHA identified studies of six cohorts for which the inclusion of retirees was deemed sufficient to adequately characterize silicosis morbidity risks well past employment (Hnizdo and Sluis-Cremer, 1993; Steenland and Brown, 1995b; Miller et al., 1998; Buchanan et al., 2003; Chen et al., 2001; Chen et al., 2005). Study populations included five mining cohorts and a Chinese pottery worker cohort. Except for the Chinese studies (Chen et al., 2001; Chen et al., 2005), chest radiographs were interpreted in accordance with the ILO system described earlier in this section, and x-ray films were read by panels of B-readers. In the Chinese studies, films were evaluated using a Chinese system of classification that is analogous to the ILO system. In addition, the Steenland and Brown (1995b) study of U.S. gold miners included silicosis mortality as well as morbidity in its analysis. OSHA's estimates of silicosis morbidity risks are based on implementing the various exposure-response models reported by the investigators; these are considered to be cumulative risk models in the sense that they represent the risk observed in the cohort at the time of the last medical evaluation and do not reflect all of the risk that may become manifest over a lifetime. With the exception of a coal miner study (Buchanan et al., 2003), risk estimates reflect the risk that a worker will acquire an abnormal chest x-ray classified as ILO major category 1 or greater; the coal miner study evaluated the risk of acquiring an abnormal chest x-ray classified as major category 2 or higher.

For miners exposed to freshly cut crystalline silica, the estimated risk of developing lesions consistent with an ILO classification of category 1 or greater is estimated to range from 120 to 773 cases per 1,000 workers exposed at the current general industry PEL of 0.1 mg/m³ for 45 years. For 45 years of exposure to the proposed PEL of 0.05 mg/m³, the range in estimated risk is from 20 to 170 cases per 1,000 workers. The risk predicted from exposure to the proposed action level of 0.025 mg/m³ ranges from 5 to 40 cases per 1,000. From the coal miner study of Buchanan et al. (2003), the estimated risks of acquiring an abnormal chest x-ray classified as ILO category 2 or higher are 301, 55, and 21 cases per 1,000 workers exposed for 45 years to 0.1, 0.05, and 0.025 mg/m³, respectively. These estimates are within the range of risks obtained from the other mining studies. At exposures at or above 0.25 mg/m³ for 45 years (equivalent to the current construction and shipyard PELs), the risk of acquiring an abnormal chest x-ray approaches unity. Risk estimates based on the pottery cohort are 60, 20, and 5 cases per 1,000 workers exposed for 45 years to 0.1, 0.05, and 0.025 mg/m³, respectively, which is generally below the range of risks estimated from the other studies and may reflect a lower toxicity of quartz particles in that work environment due to the presence of alumino-silicates on the particle surfaces. According to Chen et al. (2005), adjustment of the exposure metric to reflect the unoccluded surface area of silica particles resulted in an exposure-response of pottery workers that was similar to the mining cohorts. The finding of a reduced silicosis risk among pottery workers is consistent with other studies of clay and brick industries that have reported finding a lower prevalence of silicosis compared to that experienced in other industry sectors (Love et al., 1999; Hessel, 2006; Miller and Soutar, 2007) as well as a lower silicosis risk per unit of cumulative exposure (Love et al., 1999; Miller and Soutar, 2007).

3. Significance of Risk and Risk Reduction

The Supreme Court's benzene decision of 1980, discussed above in this section, states that “before he can promulgate any permanent health or safety standard, the Secretary [of Labor] is required to make a threshold finding that a place of employment is unsafe—in the sense that significant risks are present and can be eliminated or lessened by a change in practices.” Benzene, 448 U.S. at 642. While making it clear that it is up to the Agency to determine what constitutes a significant risk, the Court offered general guidance on the level of risk OSHA might determine to be significant.

It is the Agency's responsibility to determine in the first instance what it considers to be a “significant” risk. Some risks are plainly acceptable and others are plainly unacceptable. If, for example, the odds are one in a billion that a person will die from cancer by taking a drink of chlorinated water, the risk clearly could not be considered significant. On the other hand, if the odds are one in a thousand that regular inhalation of gasoline vapors that are 2% benzene will be fatal, a reasonable person might well consider the risk significant and take appropriate steps to decrease or eliminate it.

Benzene, 448 U.S. at 655. The Court further stated that the determination of significant risk is not a mathematical straitjacket and that “the Agency has no duty to calculate the exact probability of harm.” Id.

In this section, OSHA presents its preliminary findings with respect to the significance of the risks summarized above, and the potential of the proposed standard to reduce those risks. Findings related to mortality risk will be presented first, followed by silicosis morbidity risks.

a. Mortality Risks

OSHA's Preliminary Quantitative Risk Assessment (and the Summary of the Preliminary Quantitative Risk Assessment in section VI) presents risk estimates for four causes of excess mortality: Lung cancer, silicosis, non-malignant respiratory disease (including silicosis and COPD), and renal disease. Table VII-2 presents the estimated excess lifetime risks (i.e., to age 85) of these fatal diseases associated with various levels of crystalline silica exposure allowed under the current rule, based on OSHA's risk assessment and assuming 45 years of occupational exposure to crystalline silica.

The purpose of the OSH Act, as stated in Section 6(b), is to ensure “that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard . . . for the period of his working life.” 29 U.S.C. 655(b)(5). Assuming a 45-year working life, as OSHA has done in significant risk determinations for previous standards, the Agency preliminarily finds that the excess risk of disease mortality related to exposure to respirable crystalline silica at levels permitted by current OSHA standards is clearly significant. The Agency's estimate of such risk falls well above the level of risk the Supreme Court indicated a reasonable person might consider unacceptable. Benzene, 448 U.S. at 655. For lung cancer, OSHA estimates the range of risk at the current general industry PEL to be between 13 and 60 deaths per 1,000 workers. The estimated risk for silicosis mortality is lower, at 11 deaths per 1,000 workers; however, the estimated lifetime risk for non-malignant respiratory disease mortality, including silicosis, is about 8-fold higher than that for silicosis alone, at 83 deaths per 1,000. OSHA believes that the estimate for non-malignant respiratory disease mortality is better than the estimate for silicosis mortality at capturing the total respiratory disease burden associated with exposure to crystalline silica dust. The former captures deaths related to COPD, for which there is strong evidence of a causal relationship with exposure to silica, and is also more likely to capture those deaths where silicosis was a contributing factor but where the cause of death was misclassified. Finally, there is an estimated lifetime risk of renal disease mortality of 39 deaths per 1,000. Exposure for 45 years at levels of respirable crystalline silica in the range of the current limits for construction and shipyards result in even higher risk estimates, as presented in Table VII-2.

To further demonstrate significant risk, OSHA compares the risk from currently permissible crystalline silica exposures to risks found across a broad variety of occupations. The Agency has used similar occupational risk comparisons in the significant risk determination for substance-specific standards promulgated since the benzene decision. This approach is supported by evidence in the legislative record, with regard to Section 6(b)(5) of the Act (29 U.S.C. 655(b)(5)), that Congress intended the Agency to regulate unacceptably severe occupational hazards, and not “to establish a utopia free from any hazards” or to address risks comparable to those that exist in virtually any occupation or workplace. 116 Cong. Rec. 37614 (1970), Leg. Hist. 480-82. It is also consistent with Section 6(g) of the OSH Act, which states: “In determining the priority for establishing standards under this section, the Secretary shall give due regard to the urgency of the need for mandatory safety and health standards for particular industries, trades, crafts, occupations, businesses, workplaces or work environments.” 29 U.S.C. 655(g).

Fatal injury rates for most U.S. industries and occupations may be obtained from data collected by the Bureau of Labor Statistics. Table VII-3 shows annual fatality rates per 1,000 employees for several industries for 2007, as well as projected fatalities per 1,000 employees assuming exposure to workplace hazards for 45 years based on these annual rates (BLS, 2010). While it is difficult to meaningfully compare aggregate industry fatality rates to the risks estimated in the quantitative risk assessment for crystalline silica, which address one specific hazard (inhalation exposure to respirable crystalline silica) and several health outcomes (lung cancer, silicosis, NMRD, renal disease mortality), these rates provide a useful frame of reference for considering risk from inhalation exposure to crystalline silica. For example, OSHA's estimated range of 6-60 excess lung cancer deaths per 1,000 workers from regular occupational exposure to respirable crystalline silica in the range of 0.05—0.1 mg/m³ is roughly comparable to, or higher than, the expected risk of fatal injuries over a working life in high-risk occupations such as mining and construction (see Table VII-3). Regular exposures at higher levels, including the current construction and shipyard PELs for respirable crystalline silica, are expected to cause substantially more deaths per 1,000 workers from lung cancer (ranging from 37 to 653 per 1,000) than result from occupational injuries in most private industry. At the proposed PEL of 0.05 mg/m³ respirable crystalline silica, the Agency's estimate of excess lung cancer mortality, from 6 to 26 deaths per 1,000 workers, is still 3- to10-fold or more higher than private industry's average fatal injury rate, given the same employment time, and substantially exceeds those rates found in lower-risk industries such as finance and educational and health services.

Because there is little available information on the incidence of occupational cancer across all industries, risk from crystalline silica exposure cannot be compared with overall risk from other workplace carcinogens. However, OSHA's previous risk assessments provide estimates of risk from exposure to certain carcinogens. These risk assessments, as with the current assessment for crystalline silica, were based on animal or human data of reasonable or high quality and used the best information then available. Table VII-4 shows the Agency's best estimates of cancer risk from 45 years of occupational exposure to several carcinogens, as published in the preambles to final rules promulgated since the benzene decision in 1980. These risks were judged by the Agency to be significant.

The estimated excess lung cancer risks associated with respirable crystalline silica at the current general industry PEL, 13-60 deaths per 1,000 workers, are comparable to, and in some cases higher than, the estimated excess cancer risks for many other workplace carcinogens for which OSHA made a determination of significant risk (see Table VII-4, “Selected OSHA Risk Estimates for Prior and Current PELs”). The estimated excess lung cancer risks associated with exposure to the current construction and shipyard PELs are even higher. The estimated risk from lifetime occupational exposure to respirable crystalline silica at the proposed PEL is 6-26 excess lung cancer deaths per 1,000 workers, a range still higher than the risks from exposure to many other carcinogens regulated by OSHA (see Table VII-4, “Selected OSHA Risk Estimates for Prior and Current PELs”).

OSHA's preliminary risk assessment also shows that reduction of the current PELs to the proposed level of 0.05 mg/m³ will result in substantial reduction in risk, although quantification of that reduction is subject to model uncertainty. Risk models that reflect attenuation of the risk with increasing exposure, such as those relating risk to a log transformation of cumulative exposure, will result in lower estimates of risk reduction compared to linear risk models. Thus, for lung cancer risks, the assessment based on the 10-cohort pooled analysis by Steenland et al. (2001; also Toxichemica, 2004; Steenland 2010) suggests risk will be reduced by about 14 percent from the current general industry PEL and by 28-41 percent from the current construction/shipyard PEL (based on the midpoint of the ranges of estimated risk derived from the three models used for the pooled cohort data). These risk reduction estimates, however, are much lower than those derived from the single cohort studies (Rice et al., 2001; Attfield and Costello, 2004; Hughes et al., 2001; Miller and MacCalman, 2009), which used linear or log-linear relative risk models with untransformed cumulative exposure as the dose metric. These single cohort studies suggest that reducing the current PELs to the proposed PEL will reduce lung cancer risk by more than 50 percent in general industry and by more than 80 percent in construction and shipyards.

For silicosis mortality, OSHA's assessment indicates that risk will be reduced by 36 percent and by 58-68 percent as a result of reducing the current general industry and construction/shipyard PELs, respectively. Non-malignant respiratory disease mortality risks will be reduced by 48 percent and by 77-87 percent from reducing the general industry and construction/shipyard PELs, respectively, to the proposed PEL. There is also a substantial reduction in renal disease mortality risks; an 18-percent reduction associated with reducing the general industry PEL and a 38- to 49-percent reduction associated with reducing the construction/shipyard PEL.

Thus, OSHA believes that the proposed PEL of 0.05 mg/m³ respirable crystalline silica will substantially reduce the risk of material health impairments associated with exposure to silica. However, even at the proposed PEL, as well as the action level of 0.025 mg/m³, the risk posed to workers with 45 years of regular exposure to respirable crystalline silica is greater than 1 per 1,000 workers and is still clearly significant.

b. Silicosis Morbidity Risks

OSHA's Preliminary Risk Assessment characterizes the risk of developing lung fibrosis as detected by chest x-ray. For 45 years of exposure at the current general industry PEL, OSHA estimates that the risk of developing lung fibrosis consistent with an ILO category 1+ degree of small opacity profusion ranges from 60 to 773 cases per 1,000. For exposure at the construction and shipyard PELs, the risk approaches unity. The wide range of risk estimates derived from the underlying studies relied on for the risk assessment may reflect differences in the relative toxicity of quartz particles in different workplaces; nevertheless, OSHA believes that each of these risk estimates clearly represent a significant risk of developing fibrotic lesions in the lung. Exposure to the proposed PEL of 0.05 mg/m³ respirable crystalline silica for 45 years yields an estimated risk of between 20 and 170 cases per 1,000 for developing fibrotic lesions consistent with an ILO category of 1+. These risk estimates indicate that promulgation of the proposed PEL would result in a reduction in risk by about two-thirds or more, which the Agency believes is a substantial reduction of the risk of developing abnormal chest x-ray findings consistent with silicosis.

One study of coal miners also permitted the agency to evaluate the risk of developing lung fibrosis consistent with an ILO category 2+ degree of profusion of small opacities (Buchanan et al., 2003). This level of profusion has been shown to be associated with a higher prevalence of lung function decrement and an increased rate of early mortality (Ng et al., 1987a; Begin et al., 1998; Moore et al., 1988; Ng et al., 1992a; Infante-Rivard et al., 1991). From this study, OSHA estimates that the risk associated with 45 years of exposure to the current general industry PEL is 301 cases per 1,000 workers, again a clearly significant risk. Exposure to the proposed PEL of 0.05 mg/m³ respirable crystalline silica for 45 years yields an estimated risk of 55 cases per 1,000 for developing lesions consistent with an ILO category 2+ degree of small opacity profusion. This represents a reduction in risk of over 80 percent, again a clearly substantial reduction of the risk of developing radiologic silicosis consistent with ILO category 2+ degree of small opacity profusion.

As is the case for other health effects addressed in the preliminary risk assessment (i.e., lung cancer, silicosis morbidity defined as ILO 1+ level of profusion), there is some evidence that this risk will vary according to the nature of quartz particles present in different workplaces. In particular, risk may vary depending on whether quartz is freshly fractured during work operations and the co-existence of other minerals and substances that could alter the biological activity of quartz. Using medical and exposure data taken from a cohort of heavy clay workers first studied by Love et al. (1999), Miller and Soutar (2007) compared the silicosis prevalence within the cohort to that predicted by the exposure-response model derived by Buchanan et al. (2003) and used by OSHA to estimate the risk of radiologic silicosis with a classification of ILO 2+. They found that the model predicted about a 4-fold higher prevalence of workers having an abnormal x-ray than was actually seen in the clay cohort (31 cases predicted vs. 8 observed). Unlike the coal miner study, the clay worker cohort included only active workers and not retirees (Love et al., 1999); however, Miller and Soutar believed this could not explain the magnitude of the difference between the model prediction and observed silicosis prevalence in the clay worker cohort. OSHA believes that the result obtained by Miller and Soutar (2007) likely does reflect differences in the toxic potency of quartz particles in different work settings. Nevertheless, even if the risk estimates predicted by the model derived from the coal worker study were reduced substantially, even by more than a factor of 10, the resulting risk estimate would still reflect the presence of a significant risk.

The Preliminary Quantitative Risk Assessment also discusses the question of a threshold exposure level for silicosis. There is little quantitative data available with which to estimate a threshold exposure level for silicosis or any of the other silica-related diseases addressed in the risk assessment. The risk assessment discussed one study that perhaps provides the best information. This is an analysis by Kuempel et al. (2001) who used a rat-based toxicokinetic/toxicodynamic model along with a human lung deposition/clearance model to estimate a minimum lung burden necessary to cause the initial inflammatory events that can lead to lung fibrosis and an indirect genotoxic cause of lung cancer. They estimated that the threshold effect level of lung burden associated with this inflammation (M_crit) is the equivalent of exposure to 0.036 mg/m³ for 45 years; thus, exposures below this level would presumably not lead to an excess lung cancer risk (based on an indirect genotoxic mechanism) nor to silicosis, at least in the “average individual.” This might suggest that exposures to a concentration of silica at the proposed action level would not be associated with a risk of silicosis, and possibly not of lung cancer. However, OSHA does not believe that the analysis by Kuemple et al. is definitive with respect to a threshold for silica-related disease. First, since the critical quartz burden is a mean value derived from the model, the authors estimated that a 45-year exposure to a concentration as low as 0.005 mg/m³, or 5 times below the proposed action level, would result in a lung quartz burden that was equal to the 95-percent lower confidence limit on M_crit. Due to the statistical uncertainty in Kuemple et al.'s estimate of critical lung burden, OSHA cannot rule out the existence of a threshold lung burden that is below that resulting from exposure to the proposed action level. In addition, with respect to silica-related lung cancer, if at least some of the risk is from a direct genotoxic mechanism (see section II.F of the Health Effects Literature Review), then this threshold value is not relevant to the risk of lung cancer. Supporting evidence comes from Steenland and Deddens (2002), who found that, for the 10-cohort pooled data set, a risk model that incorporated a threshold did fit better than a no-threshold model, but the estimated threshold was very low, 0.010 mg/m³ (10 μg/m³). OSHA acknowledges that a threshold exposure level might lie within the range of the proposed action level, as suggested by the work of Kuempel et al. (2001) and that this possibility adds uncertainty to the estimated risks associated with exposure to the action level. However, OSHA believes that available information cannot firmly establish a threshold exposure level for silica-related effects, and there is no empirical evidence that a threshold exists at or above the proposed PEL of 0.05 mg/m³ for respirable crystalline silica.

VIII. Summary of the Preliminary Economic Analysis and Initial Regulatory Flexibility Analysis

A. Introduction and Summary

OSHA's Preliminary Economic Analysis and Initial Regulatory Flexibility Analysis (PEA) addresses issues related to the costs, benefits, technological and economic feasibility, and the economic impacts (including impacts on small entities) of this proposed respirable crystalline silica rule and evaluates regulatory alternatives to the proposed rule. Executive Orders 13563 and 12866 direct agencies to assess all costs and benefits of available regulatory alternatives and, if regulation is necessary, to select regulatory approaches that maximize net benefits (including potential economic, environmental, and public health and safety effects; distributive impacts; and equity). Executive Order 13563 emphasized the importance of quantifying both costs and benefits, of reducing costs, of harmonizing rules, and of promoting flexibility. The full PEA has been placed in OSHA rulemaking docket OSHA-2010-0034. This rule is an economically significant regulatory action under Sec. 3(f)(1) of Executive Order 12866 and has been reviewed by the Office of Information and Regulatory Affairs in the Office of Management and Budget, as required by executive order.

The purpose of the PEA is to:

• Identify the establishments and industries potentially affected by the proposed rule;
• Estimate current exposures and the technologically feasible methods of controlling these exposures;
• Estimate the benefits resulting from employers coming into compliance with the proposed rule in terms of reductions in cases of silicosis, lung cancer, other forms of chronic obstructive pulmonary disease, and renal failure;
• Evaluate the costs and economic impacts that establishments in the regulated community will incur to achieve compliance with the proposed rule;
• Assess the economic feasibility of the proposed rule for affected industries; and
• Assess the impact of the proposed rule on small entities through an Initial Regulatory Flexibility Analysis (IRFA), to include an evaluation of significant regulatory alternatives to the proposed rule that OSHA has considered.

The Preliminary Economic Analysis contains the following chapters:

Chapter I. Introduction

Chapter II. Assessing the Need for Regulation

Chapter III. Profile of Affected Industries

Chapter IV. Technological Feasibility

Chapter V. Costs of Compliance

Chapter VI. Economic Impacts

Chapter VII. Benefits and Net Benefits

Chapter VIII. Regulatory Alternatives

Chapter IX. Initial Regulatory Flexibility Analysis

Chapter X. Environmental Impacts

Key findings of these chapters are summarized below and in sections VIII.B through VIII.I of this PEA summary.

Profile of Affected Industries

The proposed rule would affect employers and employees in many different industries across the economy. As described in Section VIII.C and reported in Table VIII-3 of this preamble, OSHA estimates that a total of 2.1 million employees in 550,000 establishments and 533,000 firms (entities) are potentially at risk from exposure to respirable crystalline silica. This total includes 1.8 million employees in 477,000 establishments and 486,000 firms in the construction industry and 295,000 employees in 56,000 establishments and 47,000 firms in general industry and maritime.

Technological Feasibility

As described in more detail in Section VIII.D of this preamble and in Chapter IV of the PEA, OSHA assessed, for all affected sectors, the current exposures and the technological feasibility of the proposed PEL of 50 µ g/m³ and, for analytic purposes, an alternative PEL of 25 µ g/m³.

Tables VIII-6 and VIII-7 in section VIII.D of this preamble summarize all the industry sectors and construction activities studied in the technological feasibility analysis and show how many operations within each can achieve levels of 50 μg/m³ through the implementation of engineering and work practice controls. The table also summarizes the overall feasibility finding for each industry sector or construction activity based on the number of feasible versus infeasible operations. For the general industry sector, OSHA has preliminarily concluded that the proposed PEL of 50 μg/m³ is technologically feasible for all affected industries. For the construction activities, OSHA has determined that the proposed PEL of 50 μg/m³ is feasible in 10 out of 12 of the affected activities. Thus, OSHA preliminarily concludes that engineering and work practices will be sufficient to reduce and maintain silica exposures to the proposed PEL of 50 μg/m³ or below in most operations most of the time in the affected industries. For those few operations within an industry or activity where the proposed PEL is not technologically feasible even when workers use recommended engineering and work practice controls (seven out of 108 operations, see Tables VIII-6 and VIII-7), employers can supplement controls with respirators to achieve exposure levels at or below the proposed PEL.

Based on the information presented in the technological feasibility analysis, the Agency believes that 50 μg/m³ is the lowest feasible PEL. An alternative PEL of 25 μg/m³ would not be feasible because the engineering and work practice controls identified to date will not be sufficient to consistently reduce exposures to levels below 25 μg/m³ in most operations most of the time. OSHA believes that an alternative PEL of 25 μg/m³ would not be feasible for many industries, and that the use of respiratory protection would be necessary in most operations most of the time to achieve compliance. Additionally, the current methods of sampling analysis create higher errors and lower precision in measurement as concentrations of silica lower than the proposed PEL are analyzed. However, the Agency preliminarily concludes that these sampling and analytical methods are adequate to permit employers to comply with all applicable requirements triggered by the proposed action level and PEL.

Costs of Compliance

As described in more detail in Section VIII.E and reported by industry in Table VIII-8 of this preamble, the total annualized cost of compliance with the proposed standard is estimated to be about $658 million. The major cost elements associated with the revisions to the standard are costs for engineering controls, including controls for abrasive blasting ($344 million); medical surveillance ($79 million); exposure monitoring ($74 million); respiratory protection ($91 million); training ($50 million) and regulated areas or access control ($19 million). Of the total cost, $511 million would be borne by firms in the construction industry and $147 million would be borne by firms in general industry and maritime.

The compliance costs are expressed as annualized costs in order to evaluate economic impacts against annual revenue and annual profits, to be able to compare the economic impact of the rulemaking with other OSHA regulatory actions, and to be able to add and track Federal regulatory compliance costs and economic impacts in a consistent manner. Annualized costs also represent a better measure for assessing the longer-term potential impacts of the rulemaking. The annualized costs were calculated by annualizing the one-time costs over a period of 10 years and applying discount rates of 7 and 3 percent as appropriate.

The estimated costs for the proposed silica standard rule include the additional costs necessary for employers to achieve full compliance. They do not include costs associated with current compliance that has already been achieved with regard to the new requirements or costs necessary to achieve compliance with existing silica requirements, to the extent that some employers may currently not be fully complying with applicable regulatory requirements.

OSHA's exposure profile represents the Agency's best estimate of current exposures (i.e., baseline exposures). OSHA did not attempt to determine the extent to which current exposures in compliance with the current silica PELs are the result of baseline engineering controls or the result of circumstances leading to low exposures. This information is not needed to estimate the costs of (additional) engineering controls needed to comply with the proposed standard.

Because of the severe health hazards involved, the Agency expects that the estimated 15,446 abrasive blasters in the construction sector and the estimated 4,550 abrasive blasters in the maritime sector are currently wearing respirators in compliance with OSHA's abrasive blasting provisions. Furthermore, for the construction baseline, an estimated 241,269 workers, including abrasive blasters, will need to use respirators to achieve compliance with the proposed rule, and, based on the NIOSH/BLS respirator use survey (NIOSH/BLS, 2003), an estimated 56 percent of construction employers currently require such respiratory use and have respirator programs that meet OSHA's respirator standard. OSHA has not taken any costs for employers and their workers currently in compliance with the respiratory provisions in the proposed rule.

In addition, under both the general industry and construction baselines, an estimated 50 percent of employers have pre-existing training programs that address silica-related risks (as required under OSHA's hazard communication standard) and partially satisfy the proposed rule's training requirements (for costing purposes, estimated to satisfy 50 percent of the training requirements in the proposed rule). These employers will need fewer resources to achieve full compliance with the proposed rule than those employers without pre-existing training programs that address silica-related risks.

Other than respiratory protection and worker training concerning silica-related risks, OSHA did not assume baseline compliance with any ancillary provisions, even though some employers have reported that they do currently monitor silica exposure and some employers have reported conducting medical surveillance.

Economic Impacts

To assess the nature and magnitude of the economic impacts associated with compliance with the proposed rule, OSHA developed quantitative estimates of the potential economic impact of the new requirements on entities in each of the affected industry sectors. The estimated compliance costs were compared with industry revenues and profits to provide an assessment of the economic feasibility of complying with the revised standard and an evaluation of the potential economic impacts.

As described in greater detail in Section VIII.F of this preamble, the costs of compliance with the proposed rulemaking are not large in relation to the corresponding annual financial flows associated with each of the affected industry sectors. The estimated annualized costs of compliance represent about 0.02 percent of annual revenues and about 0.5 percent of annual profits, on average, across all firms in general industry and maritime, and about 0.05 percent of annual revenues and about 1.0 percent of annual profits, on average, across all firms in construction. Compliance costs do not represent more than 0.39 percent of revenues or more than 8.8 percent of profits in any affected industry in general industry or maritime, or more than 0.13 percent of revenues or more than 3 percent of profits in any affected industry in construction.

Based on its analysis of international trade effects, OSHA concluded that most or all costs arising from this proposed silica rule would be passed on in higher prices rather than absorbed in lost profits and that any price increases would result in minimal loss of business to foreign competition.

Given the minimal potential impact on prices or profits in the affected industries, OSHA has preliminarily concluded that compliance with the requirements of the proposed rulemaking would be economically feasible in every affected industry sector.

In addition, OSHA directed Inforum—a not-for-profit corporation with over 40 years of experience in the design and application of macroeconomic models—to run its LIFT (Long-term Interindustry Forecasting Tool) model of the U.S. economy to estimate the industry and aggregate employment effects of the proposed silica rule. Inforum developed estimates of the employment impacts over the ten-year period from 2014-2023 by feeding OSHA's year-by-year and industry-by-industry estimates of the compliance costs of the proposed rule into its LIFT model. The most important Inforum result is that the proposed silica rule would have a negligible—albeit slightly positive—net effect on aggregate U.S. employment.

Based on its analysis of the costs and economic impacts associated with this rulemaking and on Inforum's estimates of associated employment and other macroeconomic impacts, OSHA preliminarily concludes that the effect of the proposed standard on employment, wages, and economic growth for the United States would be negligible.

Benefits, Net Benefits, and Cost-Effectiveness

As described in more detail in Section VIII.G of this preamble, OSHA estimated the benefits, net benefits, and incremental benefits of the proposed silica rule. That section also contains a sensitivity analysis to show how robust the estimates of net benefits are to changes in various cost and benefit parameters. A full explanation of the derivation of the estimates presented there is provided in Chapter VII of the PEA for the proposed rule. OSHA invites comments on any aspect of its estimation of the benefits and net benefits of the proposed rule.

OSHA estimated the benefits associated with the proposed PEL of 50 μg/m³ and, for analytical purposes to comply with OMB Circular A-4, with an alternative PEL of 100 μg/m³ for respirable crystalline silica by applying the dose-response relationship developed in the Agency's quantitative risk assessment—summarized in Section VI of this preamble—to current exposure levels. OSHA determined current exposure levels by first developing an exposure profile (presented in Chapter IV of the PEA) for industries with workers exposed to respirable crystalline silica, using OSHA inspection and site-visit data, and then applying this exposure profile to the total current worker population. The industry-by-industry exposure profile is summarized in Table VIII-5 in Section VIII.C of this preamble.

By applying the dose-response relationship to estimates of current exposure levels across industries, it is possible to project the number of cases of the following diseases expected to occur in the worker population given current exposure levels (the “baseline”):

• Fatal cases of lung cancer,
• fatal cases of non-malignant respiratory disease (including silicosis),
• fatal cases of end-stage renal disease, and
• cases of silicosis morbidity.

Table VIII-1 provides a summary of OSHA's best estimate of the costs and benefits of the proposed rule using a discount rate of 3 percent. As shown, the proposed rule is estimated to prevent 688 fatalities and 1,585 silica-related illnesses annually once it is fully effective, and the estimated cost of the rule is $637 million annually. Also as shown in Table VIII-1, the discounted monetized benefits of the proposed rule are estimated to be $5.3 billion annually, and the proposed rule is estimated to generate net benefits of $4.6 billion annually. Table VIII-1 also presents the estimated costs and benefits of the proposed rule using a discount rate of 7 percent. The estimated costs and benefits of the proposed rule, disaggregated by industry sector, were previously presented in Table SI-3 in this preamble.

Initial Regulatory Flexibility Analysis

OSHA has prepared an Initial Regulatory Flexibility Analysis (IRFA) in accordance with the requirements of the Regulatory Flexibility Act, as amended in 1996. Among the contents of the IRFA are an analysis of the potential impact of the proposed rule on small entities and a description and discussion of significant alternatives to the proposed rule that OSHA has considered. The IRFA is presented in its entirety both in Chapter IX of the PEA and in Section VIII.I of this preamble.

The remainder of this section (Section VIII) of the preamble is organized as follows:

B. The Need for Regulation

C. Profile of Affected Industry

D. Technological Feasibility

E. Costs of Compliance

F. Economic Feasibility Analysis and Regulatory Flexibility Determination

G. Benefits and Net Benefits

H. Regulatory Alternatives

I. Initial Regulatory Flexibility Analysis.

B. Need for Regulation

Employees in work environments addressed by the proposed silica rule are exposed to a variety of significant hazards that can and do cause serious injury and death. As described in Chapter II of the PEA in support of the proposed rule, the risks to employees are excessively large due to the existence of various types of market failure, and existing and alternative methods of overcoming these negative consequences—such as workers' compensation systems, tort liability options, and information dissemination programs—have been shown to provide insufficient worker protection.

After carefully weighing the various potential advantages and disadvantages of using a regulatory approach to improve upon the current situation, OSHA concludes that, in the case of silica exposure, the proposed mandatory standards represent the best choice for reducing the risks to employees. In addition, rulemaking is necessary in this case in order to replace older existing standards with updated, clear, and consistent health standards.

C. Profile of Affected Industries

1. Introduction

Chapter III of the PEA presents profile data for industries potentially affected by the proposed silica rule. The discussion below summarizes the findings in that chapter. As a first step, OSHA identifies the North American Industrial Classification System (NAICS) industries, both in general industry and maritime and in the construction sector, with potential worker exposure to silica. Next, OSHA provides summary statistics for the affected industries, including the number of affected entities and establishments, the number of at-risk workers, and the average revenue for affected entities and establishments. Finally, OSHA presents silica exposure profiles for at-risk workers. These data are presented by sector and job category. Summary data are also provided for the number of workers in each affected industry who are currently exposed above the proposed silica PEL of 50 μg/m³, as well as above an alternative PEL of 100 μg/m³ for economic analysis purposes.

The methodological basis for the industry and at-risk worker data presented here comes from ERG (2007a, 2007b, 2008a, and 2008b). The actual data presented here comes from the technological feasibility analyses presented in Chapter IV of the PEA and from ERG (2013), which updated ERG's earlier spreadsheets to reflect the most recent industry data available. The technological feasibility analyses identified the job categories with potential worker exposure to silica. ERG (2007a, 2007b) matched the BLS Occupational Employment Survey (OES) occupational titles in NAICS industries with the at-risk job categories and then calculated the percentages of production employment represented by each at-risk job title. These percentages were then used to project the number of employees in the at-risk job categories by NAICS industry. OSHA welcomes additional information and data that might help improve the accuracy and usefulness of the industry profile presented here and in Chapter III of the PEA.

2. Selection of NAICS Industries for Analysis

The technological feasibility analyses presented in Chapter IV of the PEA identify the general industry and maritime sectors and the construction activities potentially affected by the proposed silica standard.

a. General Industry and Maritime

Employees engaged in various activities in general industry and maritime routinely encounter crystalline silica as a molding material, as an inert mineral additive, as a refractory material, as a sandblasting abrasive, or as a natural component of the base materials with which they work. Some industries use various forms of silica for multiple purposes. As a result, employers are challenged to limit worker exposure to silica in dozens of job categories throughout the general industry and maritime sectors.

Job categories in general industry and maritime were selected for analysis based on data from the technical industrial hygiene literature, evidence from OSHA Special Emphasis Program (SEP) results, and, in several cases, information from ERG site visit reports. These data sources provided evidence of silica exposures in numerous sectors. While the available data are not entirely comprehensive, OSHA believes that silica exposures in other sectors are quite limited.

The 25 industry subsectors in the overall general industry and maritime sectors that OSHA identified as being potentially affected by the proposed silica standard are as follows:

• Asphalt Paving Products
• Asphalt Roofing Materials
• Industries with Captive Foundries
• Concrete Products
• Cut Stone
• Dental Equipment and Supplies
• Dental Laboratories
• Flat Glass
• Iron Foundries
• Jewelry
• Mineral Processing
• Mineral Wool
• Nonferrous Sand Casting Foundries
• Non-Sand Casting Foundries
• Other Ferrous Sand Casting Foundries
• Other Glass Products
• Paint and Coatings
• Porcelain Enameling
• Pottery
• Railroads
• Ready-Mix Concrete
• Refractories
• Refractory Repair
• Shipyards
• Structural Clay

In some cases, affected industries presented in the technological feasibility analysis have been disaggregated to facilitate the cost and economic impact analysis. In particular, flat glass, mineral wool, and other glass products are subsectors of the glass industry described in Chapter IV of the PEA, and captive foundries, iron foundries, nonferrous sand casting foundries, non-sand cast foundries, and other ferrous sand casting foundries are subsectors of the overall foundries industry presented in Chapter IV of the PEA.

As described in ERG (2008b), OSHA identified the six-digit NAICS codes for these subsectors to develop a list of industries potentially affected by the proposed silica standard. Table VIII-2 presents the sectors listed above with their corresponding six-digit NAICS industries.

b. Construction

The construction sector is an integral part of the nation's economy, accounting for almost 6 percent of total employment. Establishments in this industry are involved in a wide variety of activities, including land development and subdivision, homebuilding, construction of nonresidential buildings and other structures, heavy construction work (including roadways and bridges), and a myriad of special trades such as plumbing, roofing, electrical, excavation, and demolition work.

Construction activities were selected for analysis based on historical data of recorded samples of construction worker exposures from the OSHA Integrated Management Information System (IMIS) and the National Institute for Occupational Safety and Health (NIOSH). In addition, OSHA reviewed the industrial hygiene literature across the full range of construction activities, and focused on dusty operations where silica sand was most likely to be fractured or abraded by work operations. These physical processes have been found to cause the silica exposures that pose the greatest risk of silicosis for workers.

The 12 construction activities, by job category, that OSHA identified as being potentially affected by the proposed silica standard are as follows:

• Abrasive Blasters
• Drywall Finishers
• Heavy Equipment Operators
• Hole Drillers Using Hand-Held Drills
• Jackhammer and Impact Drillers
• Masonry Cutters Using Portable Saws
• Masonry Cutters Using Stationary Saws
• Millers Using Portable or Mobile Machines
• Rock and Concrete Drillers
• Rock-Crushing Machine Operators and Tenders
• Tuckpointers and Grinders
• Underground Construction Workers

As shown in ERG (2008a) and in Chapter IV of the PEA, these construction activities occur in the following construction industries, accompanied by their four-digit NAICS codes:

• 2361 Residential Building Construction
• 2362 Nonresidential Building Construction
• 2371 Utility System Construction
• 2372 Land Subdivision
• 2373 Highway, Street, and Bridge Construction
2379 Other Heavy and Civil Engineering Construction
• 2381 Foundation, Structure, and Building Exterior Contractors
• 2382 Building Equipment Contractors
• 2383 Building Finishing Contractors
• 2389 Other Specialty Trade Contractors

Characteristics of Affected Industries

Table VIII-3 provides an overview of the industries and estimated number of workers affected by the proposed rule. Included in Table VIII-3 are summary statistics for each of the affected industries, subtotals for construction and for general industry and maritime, and grand totals for all affected industries combined.

The first five columns in Table VIII-3 identify each industry in which workers are routinely exposed to respirable crystalline silica (preceded by the industry's NAICS code) and the total number of entities, establishments, and employees for that industry. Note that not all entities, establishments, and employees in these affected industries necessarily engage in activities involving silica exposure.

The next three columns in Table VIII-3 show, for each affected industry, OSHA's estimate of the number of affected entities, establishments, and workers—that is, the number of entities and establishments in which workers are actually exposed to silica and the total number of workers exposed to silica. Based on ERG (2007a, 2007b), OSHA's methodology focused on estimation of the number of affected workers. The number of affected establishments was set equal to the total number of establishments in an industry (based on Census data) unless the number of affected establishments would exceed the number of affected employees in the industry. In that case, the number of affected establishments in the industry was set equal to the number of affected employees, and the number of affected entities in the industry was reduced so as to maintain the same ratio of entities to establishments in the industry.

As shown in Table VIII-3, OSHA estimates that a total of 533,000 entities (486,000 in construction; 47,000 in general industry and maritime), 534,000 establishments (477,500 in construction; 56,100 in general industry and maritime), and 2.1 million workers (1.8 million in construction; 0.3 million in general industry and maritime) would be affected by the proposed silica rule. Note that only slightly more than 50 percent of the entities and establishments, and about 12 percent of the workers in affected industries, actually engage in activities involving silica exposure.

The ninth column in Table VIII-3, with data only for construction, shows for each affected NAICS construction industry the number of full-time-equivalent (FTE) affected workers that corresponds to the total number of affected construction workers in the previous column. This distinction is necessary because affected construction workers may spend large amounts of time working on tasks with no risk of silica exposure. As shown in Table VIII-3, the 1.8 million affected workers in construction converts to approximately 652,000 FTE affected workers. In contrast, OSHA based its analysis of the affected workers in general industry and maritime on the assumption that they were engaged full time in activities with some silica exposure.

The last three columns in Table VIII-3 show combined total revenues for all entities (not just affected entities) in each affected industry, and the average revenue per entity and per establishment in each affected industry. Because OSHA did not have data to distinguish revenues for affected entities and establishments in any industry, average revenue per entity and average revenue per affected entity (as well as average revenue per establishment and average revenue per affected establishment) are estimated to be equal in value.

Silica Exposure Profile of At-Risk Workers

The technological feasibility analyses presented in Chapter IV of the PEA contain data and discussion of worker exposures to silica throughout industry. Exposure profiles, by job category, were developed from individual exposure measurements that were judged to be substantive and to contain sufficient accompanying description to allow interpretation of the circumstance of each measurement. The resulting exposure profiles show the job categories with current overexposures to silica and, thus, the workers for whom silica controls would be implemented under the proposed rule.

Chapter IV of the PEA includes a section with a detailed description of the methods used to develop the exposure profile and to assess the technological feasibility of the proposed standard. That section documents how OSHA selected and used the data to establish the exposure profiles for each operation in the affected industry sectors, and discusses sources of uncertainly including the following:

• Data Selection—OSHA discusses how exposure samples with sample durations of less than 480 minutes (an 8-hour shift) are used in the analysis.
• Use of IMIS data—OSHA discusses the limitations of data from its Integrated Management Information System.
• Use of analogous information—OSHA discusses how information from one industry or operation is used to describe exposures in other industries or operations with similar characteristics.
• Non-Detects—OSHA discusses how exposure data that is identified as “less than the LOD (limit of detection)” is used in the analysis.

OSHA seeks comment on the assumptions and data selection criteria the Agency used to develop the exposure profiles shown in Chapter IV of the PEA.

Table VIII-4 summarizes, from the exposure profiles, the total number of workers at risk from silica exposure at any level, and the distribution of 8-hour TWA respirable crystalline silica exposures by job category for general industry and maritime sectors and for construction activities. Exposures are grouped into the following ranges: less than 25 μg/m³; ≥ 25 μg/m³ and ≤ 50 μg/m³; > 50 μg/m³ and ≤ 100 μg/m³; > 100 μg/m³ and ≤ 250 μg/m³; and greater than 250 μg/m³. These frequencies represent the percentages of production employees in each job category and sector currently exposed at levels within the indicated range.

Table VIII-5 presents data by NAICS code—for each affected general, maritime, and construction industry—on the estimated number of workers currently at risk from silica exposure, as well as the estimated number of workers at risk of silica exposure at or above 25 μg/m³, above 50 μg/m³, and above 100 μg/m³. As shown, an estimated 1,026,000 workers (851,000 in construction; 176,000 in general industry and maritime) currently have silica exposures at or above the proposed action level of 25 μg/m³; an estimated 770,000 workers (648,000 in construction; 122,000 in general industry and maritime) currently have silica exposures above the proposed PEL of 50 μg/m³; and an estimated 501,000 workers (420,000 in construction; 81,000 in general industry and maritime) currently have silica exposures above 100 μg/m³—an alternative PEL investigated by OSHA for economic analysis purposes.

D. Technological Feasibility Analysis of the Proposed Permissible Exposure Limit to Crystalline Silica Exposures

Chapter IV of the Preliminary Economic Analysis (PEA) provides the technological feasibility analysis that guided OSHA's selection of the proposed PEL, consistent with the requirements of the Occupational Safety and Health Act (“OSH Act”), 29 U.S.C. 651 et seq. Section 6(b)(5) of the OSH Act requires that OSHA “set the standard which most adequately assures, to the extent feasible, on the basis of the best available evidence, that no employee will suffer material impairment of health or functional capacity.” 29 U.S.C. 655(b)(5) (emphasis added). The Court of Appeals for the D.C. Circuit has clarified the Agency's obligation to demonstrate the technological feasibility of reducing occupational exposure to a hazardous substance:

OSHA must prove a reasonable possibility that the typical firm will be able to develop and install engineering and work practice controls that can meet the PEL in most of its operations . . . The effect of such proof is to establish a presumption that industry can meet the PEL without relying on respirators . . . Insufficient proof of technological feasibility for a few isolated operations within an industry, or even OSHA's concession that respirators will be necessary in a few such operations, will not undermine this general presumption in favor of feasibility. Rather, in such operations firms will remain responsible for installing engineering and work practice controls to the extent feasible, and for using them to reduce . . . exposure as far as these controls can do so.

United Steelworkers of America, AFL-CIO-CIC v. Marshall, 647 F.2d 1189, 1272 (D.C. Cir. 1980).

Additionally, the D.C. Circuit has explained that “[f]easibility of compliance turns on whether exposure levels at or below [the PEL] can be met in most operations most of the time. . . .” American Iron & Steel Inst. v. OSHA, 939 F.2d 975, 990 (D.C. Cir. 1991).

To demonstrate the limits of feasibility, OSHA's analysis examines the technological feasibility of the proposed PEL of 50 μg/m³, as well as the technological feasibility of an alternative PEL of 25 μg/m³. In total, OSHA analyzed technological feasibility in 108 operations in general industry, maritime, and construction industries. This analysis addresses two different aspects of technological feasibility: (1) The extent to which engineering controls can reduce and maintain exposures; and (2) the capability of existing sampling and analytical methods to measure silica exposures. The discussion below summarizes the findings in Chapter IV of the PEA (see Docket No. OSHA-2010-0034).

Methodology

The technological feasibility analysis relies on information from a wide variety of sources. These sources include published literature, OSHA inspection reports, NIOSH reports and engineering control feasibility studies, and information from other federal agencies, state agencies, labor organizations, industry associations, and other groups. OSHA has limited the analysis to job categories that are associated with substantial direct silica exposure. The technological feasibility analyses group the general industry and maritime workplaces into 23 industry sectors. The Agency has divided each industry sector into specific job categories on the basis of common materials, work processes, equipment, and available exposure control methods. OSHA notes that these job categories are intended to represent job functions; actual job titles and responsibilities might differ depending on the facility.

OSHA has organized the construction industry by grouping workers into 12 general construction activities. The Agency organized construction workers into general activities that create silica exposures rather than organizing them by job titles because construction workers often perform multiple activities and job titles do not always coincide with the sources of exposure. In organizing construction worker activity this way, OSHA was able to create a more accurate exposure profile and apply control methods to workers who perform these activities in any segment of the construction industry.

The exposure profiles include silica exposure data only for workers in the United States. Information on international exposure levels is occasionally referenced for perspective or in discussions of control options. It is important to note that the vast majority of crystalline silica encountered by workers in the United States is in the quartz form, and the terms crystalline silica and quartz are often used interchangeably. Unless specifically indicated otherwise, all silica exposure data, samples, and results discussed in the technological feasibility analysis refer to measurements of personal breathing zone (PBZ) respirable crystalline silica.

In general and maritime industries, the exposure profiles in the technological feasibility analysis consist mainly of full-shift samples, collected over periods of 360 minutes or more. By using full-shift sampling results, OSHA minimizes the number of results that are less than the limit of detection (LOD) and eliminates the ambiguity associated with the LOD for low air volume samples. Thus, results that are reported in the original data source as below the LOD are included without contributing substantial uncertainty regarding their relationship to the proposed PEL. This is particularly important for general industry samples, which on average have lower silica levels than typical results for many tasks in the construction industry.

In general and maritime industries, the exposure level for the period sampled is assumed to have continued over any unsampled portion of the worker's shift. OSHA has preliminarily determined that this sample criterion is valid because workers in these industries are likely to work at the same general task or same repeating set of tasks over most of their shift; thus, unsampled periods generally are likely to be similar to the sampled periods.

In the construction industry, much of the data analyzed for the defined activities consisted of full-shift samples collected over periods of 360 minutes or more. Construction workers are likely to spend a shift working at multiple discrete tasks, independent of occupational titles, and do not normally engage in those discrete tasks for the entire duration of a shift. Therefore, the Agency occasionally included partial-shift samples (periods of less than 360 minutes), but has limited the use of partial-shift samples with results below the LOD, giving preference to data covering a greater part of the workers' shifts.

OSHA believes that the partial-shift samples were collected for the entire duration of the task and that the exposure to silica ended when the task was completed. Therefore, OSHA assumes that the exposure to silica was zero for the remaining unsampled time. OSHA understands that this may not always be the case, and that there may be activities other than the sampled tasks that affect overall worker exposures, but the documentation regarding these factors is insufficient to use in calculating a time-weighted average. It is important to note, however, that the Agency has identified to the best of its ability the construction activities that create significant exposures to respirable crystalline silica.

In cases where exposure information from a specific job category is not available, OSHA has based that portion of the exposure profile on surrogate data from one or more similar job categories in related industries. The surrogate data is selected based on strong similarities of raw materials, equipment, worker activities, and exposure duration between the job categories. When used, OSHA has clearly identified the surrogate data and the relationship between the industries or job categories.

1. Feasibility Determination of Sampling and Analytical Methods

As part of its technological feasibility analysis, OSHA examined the capability of currently available sampling methods and sensitivity and precision of currently available analytical methods to measure respirable crystalline silica (please refer to the “Feasibility of Measuring Respirable Crystalline Silica Exposures at The Proposed PEL” section in Chapter IV of the PEA). The Agency understands that several commercially available personal sampling cyclones exist that can be operated at flow rates that conform to the ISO/CEN particle size selection criteria with an acceptable level of bias. Some of these sampling devices are the Dorr-Oliver, Higgens-Dowel, BGI GK 2.69, and the SKC G-3 cyclones. Bias against the ISO/CEN criteria will fall within ±20 percent, and often is within ±10 percent.

Additionally, the Agency preliminarily concludes that all of the mentioned cyclones are capable of allowing a sufficient quantity of quartz to be collected from atmospheric concentrations as low as 25 μg/m³ to exceed the limit of quantification for the OSHA ID-142 analytical method, provided that a sample duration is at least 4 hours. Furthermore, OSHA believes that these devices are also capable of collecting more than the minimum amount of cristobalite at the proposed PEL and action level necessary for quantification with OSHA's method ID-142 for a full shift. One of these cyclones (GK 2.69) can also collect an amount of cristobalite exceeding OSHA's limit of quantification (LOQ) with a 4-hour sample at the proposed PEL and action level.

Regarding analytical methods to measure silica, OSHA investigated the sensitivity and precision of available methods. The Agency preliminarily concludes that the X-Ray Diffraction (XRD) and Infrared Spectroscopy (IR) methods of analysis are both sufficiently sensitive to quantify levels of quartz and cristobalite that would be collected on air samples taken from concentrations at the proposed PEL and action level. Available information shows that poor inter-laboratory agreement and lack of specificity render colorimetric spectrophotometry (another analytical method) inferior to XRD or IR techniques. As such, OSHA is proposing not to permit employers to rely on exposure monitoring results based on analytical methods that use colorimetric methods.

For the OSHA XRD Method ID-142 (revised December 1996), precision is ±23 percent at a working range of 50 to 160 µg crystalline silica, and the SAE (sampling and analytical error) is ±19 percent. The NIOSH and MSHA XRD and IR methods report a similar degree of precision. OSHA's Salt Lake Technical Center (SLTC) evaluated the precision of ID-142 at lower filter loadings and has shown an acceptable level of precision is achieved at filter loadings of approximately 40 µg and 20 µg corresponding to the amounts collected from full-shift sampling at the proposed PEL and action level, respectively. This analysis showed that at filter loadings corresponding to the proposed PEL, the precision and SAE for quartz are ±17 and ±14 percent, respectively. For cristobalite, the precision and SAE are ±19 and ±16 percent, respectively. These results indicate that employers can have confidence in sampling results for the purpose of assessing compliance with the PEL and identifying when additional engineering and work practice controls and/or respiratory protection are needed.

For example, given an SAE for quartz of 0.14 at a filter load of 40 µg, employers can be virtually certain that the PEL is not exceeded where exposures are less than 43 µg/m³, which represents the lower 95-percent confidence limit (i.e., 50 µg/m³ minus 50*0.14). At 43 µg/m³, a full-shift sample that collects 816 L of air will result in a filter load of 35 µg of quartz, or more than twice the LOQ for Method ID-142. Thus, OSHA believes that the method is sufficiently sensitive and precise to allow employers to distinguish between operations that have sufficient dust control to comply with the PEL from those that do not. Finally, OSHA's analysis of PAT data indicates that most laboratories achieve good agreement in results for samples having filter loads just above 40 µg quartz (49-70 µg).

At the proposed action level, the study by SLTC found the precision and SAE of the method for quartz at 20 µg to be ±19 and ±16 percent, respectively. For cristobalite, the precision and SAE at 20 µg were also ±19 and ±16 percent, respectively. OSHA believes that these results show that Method ID-142 can achieve a sufficient degree of precision for the purpose of identifying those operations where routine exposure monitoring should be conducted.

However, OSHA also believes that limitations in the characterization of the precision of the analytical method in this range of filter load preclude the Agency from proposing a PEL of 25 µg/m³ at this time. First, the measurement error increases by about 4 to 5 percent for a full-shift sample taken at 25 µg/m³ compared to one taken at 50 µg/m³, and the error would be expected to increase further as filter loads approach the limit of detection. Second, for an employer to be virtually certain that an exposure to quartz did not exceed 25 µg/m³ as an exposure limit, the exposure would have to be below 21 µg/m³ given the SAE of ±16 percent calculated from the SLTC study. For a full-shift sample of 0.816 L of air, only about 17 µg of quartz would be collected at 21 µg/m³, which is near the LOQ for Method ID-142 and at the maximum acceptable LOD that would be required by the proposed rule. Thus, given a sample result that is below a laboratory's reported LOD, employers might not be able to rule out whether a PEL of 25 µg/m³ was exceeded.

Finally, there are no available data that describe the total variability seen between laboratories at filter loadings in the range of 20 µg crystalline silica since the lowest filter loading used in PAT samples is about 50 µg. Given these considerations, OSHA believes that a PEL of 50 µg/m³ is more appropriate in that employers will have more confidence that sampling results are properly informing them where additional dust controls and respiratory protection is needed.

Based on the evaluation of the nationally recognized sampling and analytical methods for measuring respirable crystalline silica presented in the section titled “Feasibility of Measuring Respirable Crystalline Silica Exposures at The Proposed PEL” in Chapter IV of the PEA, OSHA preliminarily concludes that it is technologically feasible to reliably measure exposures of workers at the proposed PEL of 50 µg/m³ and action level of 25 µg/m³. OSHA notes that the sampling and analytical error is larger at the proposed action level than that for the proposed PEL. In the “Issues” section of this preamble (see Provisions of the Standards—Exposure Assessment), OSHA solicits comments on whether measurements of exposures at the proposed action level and PEL are sufficiently precise to permit employers to adequately determine when additional exposure monitoring is necessary under the standard, when to provide workers with the required medical surveillance, and when to comply with all other requirements of the proposed standard. OSHA also solicits comments on the appropriateness of specific requirements in the proposed standard for laboratories that perform analyses of respirable crystalline silica samples to reduce the variability between laboratories.

2. Feasibility Determination of Control Technologies

The Agency has conducted a feasibility analysis for each of the identified 23 general industry sectors and 12 construction industry activities that are potentially affected by the proposed silica standard. Additionally, the Agency identified 108 operations within those sectors/activities and developed exposure profiles for each operation, except for two industries, engineered stone products and landscape contracting industries. For these two industries, data satisfying OSHA's criteria for inclusion in the exposure profile were unavailable (refer to the Methodology section in Chapter 4 of the PEA for criteria). However, the Agency obtained sufficient information in both of these industries to make feasibility determinations (see Chapter IV Sections C.7 and C.11 of the PEA). Each feasibility analysis contains a description of the applicable operations, the baseline conditions for each operation (including the respirable silica samples collected), additional controls necessary to reduce exposures, and final feasibility determinations for each operation.

3. Feasibility Findings for the Proposed Permissible Exposure Limit of 50 μg/m³

Tables VIII-6 and VIII-7 summarize all the industry sectors and construction activities studied in the technological feasibility analysis and show how many operations within each can achieve levels of 50 μg/m³ through the implementation of engineering and work practice controls. The tables also summarize the overall feasibility finding for each industry sector or construction activity based on the number of feasible versus not feasible operations. For the general industry sector, OSHA has preliminarily concluded that the proposed PEL of 50 μg/m³ is technologically feasible for all affected industries. For the construction activities, OSHA has determined that the proposed PEL of 50 μg/m³ is feasible in 10 out of 12 of the affected activities. Thus, OSHA preliminarily concludes that engineering and work practices will be sufficient to reduce and maintain silica exposures to the proposed PEL of 50 μg/m³ or below in most operations most of the time in the affected industries. For those few operations within an industry or activity where the proposed PEL is not technologically feasible even when workers use recommended engineering and work practice controls (seven out of 108 operations, see Tables VIII-6 and VIII-7), employers can supplement controls with respirators to achieve exposure levels at or below the proposed PEL.

4. Feasibility Findings for an Alternative Permissible Exposure Limit of 25 μg/m³

Based on the information presented in the technological feasibility analysis, OSHA believes that engineering and work practice controls identified to date will not be sufficient to consistently reduce exposures to PELs lower than 50 μg/m³. The Agency believes that a proposed PEL of 25 μg/m³, for example, would not be feasible for many industries, and to use respiratory protection would have to be required in most operations and most of the time to achieve compliance.

However, OSHA has data indicating that an alternative PEL of 25 μg/m³ has already been achieved in several industries (e.g. asphalt paving products, dental laboratories, mineral processing, and paint and coatings manufacturing in general industry, and drywall finishers and heavy equipment operators in construction). In these industries, airborne respirable silica concentrations are inherently low because either small amounts of silica containing materials are handled or these materials are not subjected to high energy processes that generate large amounts of respirable dust.

For many of the other industries, OSHA believes that engineering and work practice controls will not be able to reduce and maintain exposures to an alternative PEL of 25 μg/m³ in most operations and most of the time. This is especially the case in industries that use silica containing material in substantial quantities and industries with high energy operations. For example, in general industry, the ferrous foundry industry would not be able to comply with an alternative PEL of 25 μg/m³ without widespread respirator use. In this industry, silica containing sand is transported, used, and recycled in significant quantities to create castings, and as a result, workers can be exposed to high levels of silica in all steps of the production line. Additionally, some high energy operations in foundries create airborne dust that causes high worker exposures to silica. One of these operations is the shakeout process, where operators monitor equipment that separates castings from mold materials by mechanically vibrating or tumbling the casting. The dust generated from this process causes elevated silica exposures for shakeout operators and often contributes to exposures for other workers in a foundry. For small, medium, and large castings, exposure information with engineering controls in place show that exposures below 50 μg/m³ can be consistently achieved, but exposures above an alternative PEL of 25 μg/m³ still occur. With engineering controls in place, exposure data for these operations range from 13 μg/m³ to 53 μg/m³, with many of the reported exposures above 25 μg/m³.

In the construction industry, OSHA estimates that an alternative PEL of 25 μg/m³ would be infeasible in most operations because most of them are high energy operations that produce significant levels of dust, causing workers to have elevated exposures, and available engineering controls would not be able to maintain exposures at or below the alternative PEL most of the time. For example, jackhammering is a high energy operation that creates a large volume of silica containing dust, which disburses rapidly in highly disturbed air. OSHA estimates that the exposure levels of most workers operating jackhammers outdoors will be reduced to less that 100 μg/m³ as an 8-hour TWA, by using either wet methods or LEV paired with a suitable vacuum.

OSHA believes that typically, the majority of jackhammering is performed for less than four hours of a worker's shift, and in these circumstances the Agency estimates that most workers will experience levels below 50 μg/m³. Jackhammer operators who work indoors or with multiple jackhammers will achieve similar results granted that the same engineering controls are used and that fresh air circulation is provided to prevent accumulation of respirable dust in a worker's vicinity. OSHA does not have any data indicating that these control strategies would reduce exposures of most workers to levels of 25 μg/m³ or less.

5. Overall Feasibility Determination

Based on the information presented in the technological feasibility analysis, the Agency believes that 50 μg/m³ is the lowest feasible PEL. An alternative PEL of 25 μg/m³ would not be feasible because the engineering and work practice controls identified to date will not be sufficient to consistently reduce exposures to levels below 25 μg/m³ in most operations most of the time. OSHA believes that an alternative PEL of 25 μg/m³ would not be feasible for many industries, and that the use of respiratory protection would be necessary in most operations most of the time to achieve compliance. Additionally, the current methods of sampling analysis create higher errors and lower precision in measurement as concentrations of silica lower than the proposed PEL are analyzed. However, the Agency preliminarily concludes that these sampling and analytical methods are adequate to permit employers to comply with all applicable requirements triggered by the proposed action level and PEL.

E. Costs of Compliance

Chapter V of the PEA in support of the proposed silica rule provides a detailed assessment of the costs to establishments in all affected industry sectors of reducing worker exposures to silica to an eight-hour time-weighted average (TWA) permissible exposure limit (PEL) of 50 μg/m³ and of complying with the proposed standard's ancillary requirements. The discussion below summarizes the findings in the PEA cost chapter. OSHA's preliminary cost assessment is based on the Agency's technological feasibility analysis presented in Chapter IV of the PEA (2013); analyses of the costs of the proposed standard conducted by OSHA's contractor, Eastern Research Group (ERG, 2007a, 2007b, and 2013); and the comments submitted to the docket as part of the SBREFA panel process.

OSHA estimates that the proposed rule will cost $657.9 million per year in 2009 dollars. Costs originally estimated for earlier years were adjusted to 2009 dollars using the appropriate price indices. All costs are annualized using a discount rate of 7 percent. (A sensitivity analysis using discount rates of 3 percent and 0 percent is presented in the discussion of net benefits.) One-time costs are annualized over 10-year annualization period, and capital goods are annualized over the life of the equipment. OSHA has historically annualized one-time costs over at least a 10-year period, which approximately reflects the average life of a business in the United States. (The Agency has chosen a longer annualization period under special circumstances, such as when a rule involves longer and more complex phase-in periods. In general, a longer annualization period, in such cases, will tend to reduce annualized costs slightly.)

The estimated costs for the proposed silica standard rule include the additional costs necessary for employers to achieve full compliance. They do not include costs associated with current compliance that has already been achieved with regard to the new requirements or costs necessary to achieve compliance with existing silica requirements, to the extent that some employers may currently not be fully complying with applicable regulatory requirements.

Table VIII-8 provides the annualized costs of the proposed rule by cost category for general industry, maritime, and construction. As shown in Table VIII-8, of the total annualized costs of the proposed rule, $132.5 million would be incurred by general industry, $14.2 million by maritime, and $511.2 million by construction.

Table VIII-9 shows the annualized costs of the proposed rule by cost category and by industry for general industry and maritime, and Table VIII-10 shows the annualized costs similarly disaggregated for construction. These tables show that engineering control costs represent 69 percent of the costs of the proposed standard for general industry and maritime and 47 percent of the costs of the proposed standard for construction. Considering other leading cost categories, costs for exposure assessment and respirators represent, respectively, 20 percent and 5 percent of the costs of the proposed standard for general industry and maritime; costs for respirators and medical surveillance represent, respectively, 16 percent and 15 percent of the costs of the proposed standard for construction.

While the costs presented here represent the Agency's best estimate of the costs to industry of complying with the proposed rule under static conditions (that is, using existing technology and the current deployment of workers), OSHA recognizes that the actual costs could be somewhat higher or lower, depending on the Agency's possible overestimation or underestimation of various cost factors. In Chapter VII of the PEA, OSHA provides a sensitivity analysis of its cost estimates by modifying certain critical unit cost factors. Beyond the sensitivity analysis, however, OSHA believes its cost estimates may significantly overstate the actual costs of the proposed rule because, in response to the rule, industry may be able to take two types of actions to reduce compliance costs.

First, in construction, 53 percent of the estimated costs of the proposed rule (all costs except engineering controls) vary directly with the number of workers exposed to silica. However, as shown in Table VIII-3 of this preamble, almost three times as many construction workers would be affected by the proposed rule as would the number of full-time-equivalent construction workers necessary to do the work. This is because most construction workers currently do work involving silica exposure for only a portion of their workday. In response to the proposed rule, many employers are likely to assign work so that fewer construction workers perform tasks involving silica exposure; correspondingly, construction work involving silica exposure will tend to become a full-time job for some construction workers. Were this approach fully implemented in construction, the actual cost of the proposed rule would decline by over 25 percent, or by $180 million annually, to under $480 million annually.

Second, the costs presented here do not take into account the likely development and dissemination of cost-reducing compliance technology in response to the proposed rule. One possible example is the development of safe substitutes for silica sand in abrasive blasting operations, repair and replacement of refractory materials, foundry operations, and the railroad transportation industry. Another is expanded uses of automated processes, which would allow workers to be isolated from the points of operation that involve silica exposure (such as tasks between the furnace and the pouring machine in foundries and at sand transfer stations in structural clay production facilities). Yet another example is the further development and use of bags with valves that seal effectively when filled, thereby preventing product leakage and worker exposure (for example, in mineral processing and concrete products industries). Probably the most pervasive and significant technological advances, however, will likely come from the integration of compliant control technology into production equipment as standard equipment. Such advances would both increase the effectiveness and reduce the costs of silica controls retrofitted to production equipment. Possible examples include local exhaust ventilation (LEV) systems attached to portable tools used by grinders and tuckpointers; enclosed operator cabs equipped with air filtration and air conditioning in industries that mechanically transfer silica or silica-containing materials; and machine-integrated wet dust suppression systems used, for example, in road milling operations. Of course, all the possible technological advances in response to the proposed rule and their effects on costs are difficult to predict.

OSHA has decided at this time not to create a more dynamic and predictive analysis of possible cost-reducing technological advances or worker specialization because the technological and economic feasibility of the proposed rule can easily be demonstrated using existing technology and employment patterns. However, OSHA believes that actual costs, if future developments of this type were fully accounted for, would be lower than those estimated here.

OSHA invites comment on this discussion concerning the costs of the proposed rule.

1. Unit Costs, Other Cost Parameters, and Methodological Assumptions by Major Provision

Below, OSHA summarizes its methodology for estimating unit and total costs for the major provisions required under the proposed silica standard. For a full presentation of the cost analysis, see Chapter V of the PEA and ERG (2007a, 2007b, 2011, 2013). OSHA invites comment on all aspects of its preliminary cost analysis.

a. Engineering Controls

Engineering controls include such measures as local exhaust ventilation, equipment hoods and enclosures, dust suppressants, spray booths and other forms of wet methods, high efficient particulate air (HEPA) vacuums, and control rooms.

Following ERG's (2011) methodology, OSHA estimated silica control costs on a per-worker basis, allowing the costs to be related directly to the estimates of the number of overexposed workers. OSHA then multiplied the estimated control cost per worker by the numbers of overexposed workers for both the proposed PEL of 50 μg/m³ and the alternative PEL of 100 μg/m³, introduced for economic analysis purposes. The numbers of workers needing controls (i.e., workers overexposed) are based on the exposure profiles for at-risk occupations developed in the technological feasibility analysis in Chapter IV of the PEA and estimates of the number of workers employed in these occupations developed in the industry profile in Chapter III of the PEA. This worker-based method is necessary because, even though the Agency has data on the number of firms in each affected industry, on the occupations and industrial activities with worker exposure to silica, on exposure profiles of at-risk occupations, and on the costs of controlling silica exposure for specific industrial activities, OSHA does not have a way to match up these data at the firm level. Nor does OSHA have facility-specific data on worker exposure to silica or even facility-specific data on the level of activity involving worker exposure to silica. Thus, OSHA could not directly estimate per-affected-facility costs, but instead, first had to estimate aggregate compliance costs and then calculate the average per-affected-facility costs by dividing aggregate costs by the number of affected facilities.

In general, OSHA viewed the extent to which exposure controls are already in place to be reflected in the distribution of overexposures among the affected workers. Thus, for example, if 50 percent of workers in a given occupation are found to be overexposed relative to the proposed silica PEL, OSHA judged this equivalent to 50 percent of facilities lacking the relevant exposure controls. The remaining 50 percent of facilities are expected either to have installed the relevant controls or to engage in activities that do not require that the exposure controls be in place. OSHA recognizes that some facilities might have the relevant controls in place but are still unable, for whatever reason, to achieve the PEL under consideration. ERG's review of the industrial hygiene literature and other source materials (as noted in ERG, 2007b), however, suggest that the large majority of overexposed workers lack relevant controls. Thus, OSHA has generally assumed that overexposures occur due to the absence of suitable controls. This assumption results in an overestimate of costs since, in some cases, employers may merely need to upgrade or better maintain existing controls or to improve work practices rather than to install and maintain new controls.

There are two situations in which the proportionality assumption may oversimplify the estimation of the costs of the needed controls. First, some facilities may have the relevant controls in place but are still unable, for whatever reason, to achieve the PEL under consideration for all employees. ERG's review of the industrial hygiene literature and other source materials (as noted in ERG, 2007b, pg. 3-4), however, suggest that the large majority of overexposed workers lack relevant controls. Thus, OSHA has generally assumed that overexposures occur due to the absence of suitable controls. This assumption could, in some cases, result in an overestimate of costs where employers merely need to upgrade or better maintain existing controls or to improve work practices rather than to install and maintain new controls. Second, there may be situations where facilities do not have the relevant controls in place but nevertheless have only a fraction of all affected employees above the PEL. If, in such situations, an employer would have to install all the controls necessary to meet the PEL, OSHA may have underestimated the control costs. However, OSHA believes that, in general, employers could come into compliance by such methods as checking the work practices of the employee who is above the PEL or installing smaller amounts of LEV at costs that would be more or less proportional to the costs for all employees. Nevertheless there may be situations in which a complete set of controls would be necessary if even one employee in a work area is above the PEL. OSHA welcomes comment on the extent to which this approach may yield underestimates or overestimates of costs.

At many workstations, employers must improve ventilation to reduce silica exposures. Ventilation improvements will take a variety of forms at different workstations and in different facilities and industries. The cost of ventilation enhancements generally reflects the expense of ductwork and other equipment for the immediate workstation or individual location and, potentially, the cost of incremental capacity system-wide enhancements and increased operation costs for the heating, ventilation, and air conditioning (HVAC) system for the facility.

For a number of occupations, the technological feasibility analysis indicates that, in addition to ventilation, the use of wet methods, improved housekeeping practices, and enclosure of process equipment are needed to reduce silica exposures. The degree of incremental housekeeping depends upon how dusty the operations are and the applicability of HEPA vacuums or other equipment to the dust problem. The incremental costs for most such occupations arise due to the labor required for these additional housekeeping efforts. Because additional labor for housekeeping will be required on virtually every work shift by most of the affected occupations, the costs of housekeeping are substantial. Employers also need to purchase HEPA vacuums and must incur the ongoing costs of HEPA vacuum filters. To reduce silica exposures by enclosure of process equipment, such as in the use of conveyors near production workers in mineral processing, covers can be particularly effective where silica-containing materials are transferred (and notable quantities of dust become airborne), or, as another example, where dust is generated, such as in sawing or grinding operations.

For construction, ERG (2007a) defined silica dust control measures for each representative job as specified in Table 1 of the proposed rule. Generally, these controls involve either a dust collection system or a water-spray approach (wet method) to capture and suppress the release of respirable silica dust. Wet-method controls require a water source (e.g., tank) and hoses. The size of the tank varies with the nature of the job and ranges from a small hand-pressurized tank to a large tank for earth drilling operations. Depending on the tool, dust collection methods entail vacuum equipment, including a vacuum unit and hoses, and either a dust shroud or an extractor. For example, concrete grinding operations using hand-held tools require dust shroud adapters for each tool and a vacuum. The capacity of the vacuum depends on the type and size of tool being used. Some equipment, such as concrete floor grinders, comes with a dust collection system and a port for a vacuum hose. The estimates of control costs for those jobs using dust collection methods assume that an HEPA filter will be required.

For each job, ERG estimated the annual cost of the appropriate controls and translated this cost to a daily charge. The unit costs for control equipment were based on price information collected from manufacturers and vendors. In some cases, control equipment costs were based on data on equipment rental charges.

As noted above, included among the engineering controls in OSHA's cost model are housekeeping and dust-suppression controls in general industry. For the maritime industry and for construction, abrasive blasting operations are expected to require the use of wet methods to control silica dust.

Tables V-3, V-4, V-21, V-22, and V-31 in Chapter V of the PEA and Tables V-A-1 and V-A-2 in Appendix V-A provide details on the unit costs, other unit parameters, and methodological assumptions applied by OSHA to estimate engineering control costs.

b. Respiratory Protection

OSHA's cost estimates assume that implementation of the recommended silica controls prevents workers in general industry and maritime from being exposed over the PEL in most cases. Specifically, based on its technological feasibility analysis, OSHA expects that the technical controls are adequate to keep silica exposures at or below the PEL for an alternative PEL of 100 μ g/m (introduced for economic analysis purposes). For the proposed 50 μ g/m³ PEL, OSHA's feasibility analysis suggests that the controls that employers use, either because of technical limitations or imperfect implementation, might not be adequate in all cases to ensure that worker exposures in all affected job categories are at or below 50 μ g/m³. For this preliminary cost analysis, OSHA estimates that ten percent of the at-risk workers in general industry would require respirators, at least occasionally, after the implementation of engineering controls to achieve compliance with the proposed PEL of 50 μ g/m³. For workers in maritime, the only activity with silica exposures above the proposed PEL of 50 μ g/m³ is abrasive blasting, and maritime workers engaged in abrasive blasting are already required to use respirators under the existing OSHA ventilation standard (29 CFR 1910.94(a)). Therefore, OSHA has estimated no additional costs for maritime workers to use respirators as a result of the proposed silica rule.

For construction, employers whose workers receive exposures above the PEL are assumed to adopt the appropriate task-specific engineering controls and, where required, respirators prescribed in Table 1 and under paragraph (g)(1) in the proposed standard. Respirator costs in the construction industry have been adjusted to take into account OSHA's estimate (consistent with the findings from the NIOSH Respiratory Survey, 2003) that 56 percent of establishments in the construction industry are already using respirators that would be in compliance with the proposed silica rule.

ERG (2013) used respirator cost information from a 2003 OSHA respirator study to estimate the annual cost of $570 (in 2009 dollars) for a half-mask, non-powered, air-purifying respirator and $638 per year (in 2009 dollars) for a full-face non-powered air-purifying respirator (ERG, 2003). These unit costs reflect the annualized cost of respirator use, including accessories (e.g., filters), training, fit testing, and cleaning.

In addition to bearing the costs associated with the provision of respirators, employers will incur a cost burden to establish respirator programs. OSHA projects that this expense will involve an initial 8 hours for establishments with 500 or more employees and 4 hours for all other firms. After the first year, OSHA estimates that 20 percent of establishments would revise their respirator program every year, with the largest establishments (500 or more employees) expending 4 hours for program revision, and all other employers expending two hours for program revision. Consistent with the findings from the NIOSH Respiratory Survey (2003), OSHA estimates that 56 percent of establishments in the construction industry that would require respirators to achieve compliance with the proposed PEL already have a respirator program. OSHA further estimates that 50 percent of firms in general industry and all maritime firms that would require respirators to achieve compliance already have a respirator program.

c. Exposure Assessment

Most establishments wishing to perform exposure monitoring will require the assistance of an outside consulting industrial hygienist (IH) to obtain accurate results. While some firms might already employ or train qualified staff, ERG (2007b) judged that the testing protocols are fairly challenging and that few firms have sufficiently skilled staff to eliminate the need for outside consultants.

Table V-8 in the PEA shows the unit costs and associated assumptions used to estimate exposure assessment costs. Unit costs for exposure sampling include direct sampling costs, the costs of productivity losses, and recordkeeping costs, and, depending on establishment size, range from $225 to $412 per sample in general industry and maritime and from $228 to $415 per sample in construction.

For costing purposes, based on ERG (2007b), OSHA estimated that there are four workers per work area. OSHA interpreted the initial exposure assessment as requiring first-year testing of at least one worker in each distinct job classification and work area who is, or may reasonably be expected to be, exposed to airborne concentrations of respirable crystalline silica at or above the action level. This may result in overestimated exposure assessment costs in construction because OSHA anticipates that many employers, aware that their operations currently expose their workers to silica levels above the PEL, will simply choose to comply with Table 1 and avoid the costs of conducting exposure assessments.

For periodic monitoring, the proposed standard provides employers an option of assessing employee exposures either under a fixed schedule (paragraph (d)(3)(i)) or a performance-based schedule (paragraph (d)(3)(ii)). Under the fixed schedule, the proposed standard requires semi-annual sampling for exposures at or above the action level and quarterly sampling for exposures above the 50 μ g/m³ PEL. Monitoring must be continued until the employer can demonstrate that exposures are no longer at or above the action level. OSHA used the fixed schedule option under the frequency-of-monitoring requirements to estimate, for costing purposes, that exposure monitoring will be conducted (a) twice a year where initial or subsequent exposure monitoring reveals that employee exposures are at or above the action level but at or below the PEL, and (b) four times a year where initial or subsequent exposure monitoring reveals that employee exposures are above the PEL.

As required under paragraph (d)(4) of the proposed rule, whenever there is a change in the production, process, control equipment, personnel, or work practices that may result in new or additional exposures at or above the action level or when the employer has any reason to suspect that a change may result in new or additional exposures at or above the action level, the employer must conduct additional monitoring. Based on ERG (2007a, 2007b), OSHA estimated that approximately 15 percent of workers whose initial exposure or subsequent monitoring was at or above the action level would undertake additional monitoring.

A more detailed description of unit costs, other unit parameters, and methodological assumptions for exposure assessments is presented in Chapter V of the PEA.

d. Medical Surveillance

Paragraph (h) of the proposed standard requires an initial health screening and then triennial periodic screenings for workers exposed above the proposed PEL of 50 μ g/m³ for 30 days or more per year. ERG (2013) assembled information on representative unit costs for initial and periodic medical surveillance. Separate costs were estimated for current employees and for new hires as a function of the employment size (i.e., 1-19, 20-499, or 500+ employees) of affected establishments. Table V-10 in the PEA presents ERG's unit cost data and modeling assumptions used by OSHA to estimate medical surveillance costs.

In accordance with the paragraph (h)(2) of the proposed rule, the initial (baseline) medical examination would consist of (1) a medical and work history, (2) a physical examination with special emphasis on the respiratory system, (3) a chest X-ray that is interpreted according to guidelines of the International Labour Organization, (4) a pulmonary function test that meets certain criteria and is administered by spirometry technician with current certification from a NIOSH-approved spirometry course, (5) testing for latent tuberculosis (TB) infection, and (6) any other tests deemed appropriate by the physician or licensed health care professional (PLHCP).

As shown in Table V-10 in the PEA, the estimated unit cost of the initial health screening for current employees in general industry and maritime ranges from approximately $378 to $397 and includes direct medical costs, the opportunity cost of worker time (i.e., lost work time, evaluated at the worker's 2009 hourly wage, including fringe benefits) for offsite travel and for the initial health screening itself, and recordkeeping costs. The variation in the unit cost of the initial health screening is due entirely to differences in the percentage of workers expected to travel offsite for the health screening. In OSHA's experience, the larger the establishment the more likely it is that the selected PLHCP would provide the health screening services at the establishment's worksite. OSHA estimates that 20 percent of establishments with fewer than 20 employees, 75 percent of establishments with 20-499 employees, and 100 percent of establishments with 500 or more employees would have the initial health screening for current employees conducted onsite.

The unit cost components of the initial health screening for new hires in general industry and maritime are identical to those for existing employees with the exception that the percentage of workers expected to travel offsite for the health screening would be somewhat larger (due to fewer workers being screened annually, in the case of new hires, and therefore yielding fewer economies of onsite screening). OSHA estimates that 10 percent of establishments with fewer than 20 employees, 50 percent of establishments with 20-499 employees, and 90 percent of establishments with 500 or more employees would have the initial health screening for new hires conducted onsite. As shown in Chapter V in the PEA, the estimated unit cost of the initial health screening for new hires in general industry and maritime ranges from approximately $380 to $399.

The unit costs of medical surveillance in construction were derived using identical methods. As shown in Table V-39 of the PEA, the estimated unit costs of the initial health screening for current employees in construction range from approximately $389 to $425; the estimated unit costs of the initial health screening for new hires in construction range from approximately $394 to $429.

In accordance with paragraph (h)(3) of the proposed rule, the periodic medical examination (every third year after the initial health screening) would consist of (1) a medical and work history review and update, (2) a physical examination with special emphasis on the respiratory system, (3) a chest X-ray that meets certain standards of the International Labour Organization, (4) a pulmonary function test that meets certain criteria and is administered by a spirometry technician with current certification from a NIOSH-approved spirometry course, (5) testing for latent TB infection, if recommended by the PLHCP, and (6) any other tests deemed appropriate by the PLHCP.

The estimated unit cost of periodic health screening also includes direct medical costs, the opportunity cost of worker time, and recordkeeping costs. As shown in Table V-10 in the PEA, these triennial unit costs in general industry and maritime vary from $378 to $397. For construction, as shown in Table V-39 in the PEA, the triennial unit costs for periodic health screening vary from roughly $389 to $425. The variation in the unit cost (with or without the chest X-ray and pulmonary function test) is due entirely to differences in the percentage of workers expected to travel offsite for the periodic health screening. OSHA estimated that the share of workers traveling offsite, as a function of establishment size, would be the same for the periodic health screening as for the initial health screening for existing employees.

ERG (2013) estimated a turnover rate of 27.2 percent in general industry and maritime and 64.0 percent in construction, based on estimates of the separations rate (layoffs, quits, and retirements) provided by the Bureau of Labor Statistics (BLS, 2007). However, not all new hires would require initial medical testing. As specified in paragraph (h)(2) of the proposed rule, employees who had received a qualifying medical examination within the previous twelve months would be exempt from the initial medical examination. OSHA estimates that 25 percent of new hires in general industry and maritime and 60 percent of new hires in construction would be exempt from the initial medical examination.

Although OSHA believes that some affected establishments in general industry, maritime, and construction currently provide some medical testing to their silica-exposed employees, the Agency doubts that many provide the comprehensive health screening required under the proposed rule. Therefore for costing purposes for the proposed rule, OSHA has assumed no current compliance with the proposed health screening requirements. OSHA requests information from interested parties on the current levels and the comprehensiveness of health screening in general industry, maritime, and construction.

Finally, OSHA estimated the unit cost of a medical examination by a pulmonary specialist for those employees found to have signs or symptoms of silica-related disease or are otherwise referred by the PLHCP. OSHA estimates that a medical examination by a pulmonary specialist costs approximately $307 for workers in general industry and maritime and $333 for workers in construction. This cost includes direct medical costs, the opportunity cost of worker time, and recordkeeping costs. In all cases, OSHA anticipates that the worker will travel offsite to receive the medical examination by a pulmonary specialist.

See Chapter V in the PEA for a full discussion of OSHA's analysis of medical surveillance costs under the proposed standard.

e. Information and Training

As specified in paragraph (i) of the proposed rule and 29 CFR 1910.1200, training is required for all employees in jobs where there is potential exposure to respirable crystalline silica. In addition, new hires would require training before starting work. As previously noted, ERG (2013) provided an estimate of the new-hire rate in general industry and maritime, based on the BLS-estimated separations rate of 27.2 percent in manufacturing, and an estimate of the new-hire rate in construction, based on the BLS-estimated separations rate in construction of 64.0 percent.

OSHA estimated separate costs for initial training of current employees and for training new hires. Given that new-hire training might need to be performed frequently during the year, OSHA estimated a smaller class size for new hires. OSHA anticipates that training, in accordance with the requirements of the proposed rule, will be conducted by in-house safety or supervisory staff with the use of training modules or videos and will last, on average, one hour. ERG (2007b) judged that establishments could purchase sufficient training materials at an average cost of $2 per worker, encompassing the cost of handouts, video presentations, and training manuals and exercises. ERG (2013) included in the cost estimates for training the value of worker and trainer time as measured by 2009 hourly wage rates (to include fringe benefits). ERG also developed estimates of average class sizes as a function of establishment size. For initial training, ERG estimated an average class size of 5 workers for establishments with fewer than 20 employees, 10 workers for establishments with 20 to 499 employees, and 20 workers for establishments with 500 or more employees. For new hire training, ERG estimated an average class size of 2 workers for establishments with fewer than 20 employees, 5 workers for establishments with 20 to 499 employees, and 10 workers for establishments with 500 or more employees.

The unit costs of training are presented in Tables V-14 (for general industry/maritime) and V-43 (for construction) in the PEA. Based on ERG's work, OSHA estimated the annualized cost (annualized over 10 years) of initial training per current employee at between $3.02 and $3.57 and the annual cost of new-hire training at between $22.50 and $32.72 per employee in general industry and maritime, depending on establishment size. For construction, OSHA estimated the annualized cost of initial training per employee at between $3.68 and $4.37 and the annual cost of new hire training at between $27.46 and $40.39 per employee, depending on establishment size.

OSHA recognizes that many affected establishments currently provide training on the hazards of respirable crystalline silica in the workplace. Consistent with some estimates developed by ERG (2007a and 2007b), OSHA estimates that 50 percent of affected establishments already provide such training. However, some of the training specified in the proposed rule requires that workers be familiar with the training and medical surveillance provisions in the rule. OSHA expects that these training requirements in the proposed rule are not currently being provided. Therefore, for costing purposes for the proposed rule, OSHA has estimated that 50 percent of affected establishments currently provide their workers, and would provide new hires, with training that would comply with approximately 50 percent of the training requirements. In other words, OSHA estimates that those 50 percent of establishments currently providing training on workplace silica hazards would provide an additional 30 minutes of training to comply with the proposed rule; the remaining 50 percent of establishments would provide 60 minutes of training to comply with the proposed rule. OSHA also recognizes that many new hires may have been previously employed in the same industry, and in some cases by the same establishment, so that they might have already received (partial) silica training. However, for purposes of cost estimation, OSHA estimates that all new hires will receive the full silica training from the new employer. OSHA requests comments from interested parties on the reasonableness of these assumptions.

f. Regulated Areas and Access Control

Paragraph (e)(1) of the proposed standard requires that wherever an employee's exposure to airborne concentrations of respirable crystalline silica is, or can reasonably be expected to be, in excess of the PEL, each employer shall establish and implement either a regulated area in accordance with paragraph (e)(2) or an access control plan in accordance with paragraph (e)(3). For costing purposes, OSHA estimated that employers in general industry and maritime would typically prefer and choose option (e)(2) and would therefore establish regulated areas when an employee's exposure to airborne concentrations of silica exceeds, or can reasonably be expected to exceed, the PEL. OSHA believes that general industry and maritime employers will prefer this option as it is expected to be the most practical alternative in fixed worksites. Requirements in the proposed rule for a regulated area include demarcating the boundaries of the regulated area (as separate from the rest of the workplace), limiting access to the regulated area, providing an appropriate respirator to each employee entering the regulated area, and providing protective clothing as needed in the regulated area.

Based on ERG (2007b), OSHA derived unit cost estimates for establishing and maintaining regulated areas to comply with these requirements and estimated that one area would be necessary for every eight workers in general industry and maritime exposed above the PEL. Unit costs include planning time (estimated at eight hours of supervisor time annually); material costs for signs and boundary markers (annualized at $63.64 in 2009 dollars); and costs of $500 annually for two disposable respirators per day to be used by authorized persons (other than those who regularly work in the regulated area) who might need to enter the area in the course of their job duties. In addition, for costing purposes, OSHA estimates that, in response to the protective work clothing requirements in regulated areas, ten percent of employees in regulated areas would wear disposable protective clothing daily, estimated at $5.50 per suit, for an annual clothing cost of $1,100 per regulated area. Tables V-16 in the PEA shows the cost assumptions and unit costs applied in OSHA's cost model for regulated areas in general industry and maritime. Overall, OSHA estimates that each regulated area would, on average, cost employers $1,732 annually in general industry and maritime.

For construction, OSHA estimated that some employers would select the (e)(2) option concerning regulated areas while other employers would prefer the (e)(3) option concerning written access control plans whenever an employee's exposure to airborne concentrations of respirable crystalline silica exceeds, or can reasonably be expected to exceed, the PEL.

Based on the respirator specifications developed by ERG (2007a) and shown in Table V-34 in the PEA, ERG derived the full-time-equivalent number of workers engaged in construction tasks where respirators are required and estimated the costs of establishing a regulated area for these workers.

Under the second option for written access control plans, the employer must include the following elements in the plan: competent person provisions; notification and demarcation procedures; multi-employer workplace procedures; provisions for limiting access; provisions for supplying respirators; and protective clothing procedures. OSHA anticipates that employers will incur costs for labor, materials, respiratory protection, and protective clothing to comply with the proposed access control plan requirements.

Table V-45 in the PEA shows the unit costs and assumptions for developing costs for regulated areas and for access control plans in construction. ERG estimated separate development and implementation costs. ERG judged that developing either a regulated area or an access control plan would take approximately 4 hours of a supervisor's time. The time allowed to set up a regulated area or an access control plan is intended to allow for the communication of access restrictions and locations at multi-employer worksites. ERG estimated a cost of $116 per job based on job frequency and the costs for hazard tape and warning signs (which are reusable). ERG estimated a labor cost of $27 per job for implementing a written access control plan (covering the time expended for revision of the access control plan for individual jobs and communication of the plan). In addition, OSHA estimated that there would be annual disposable clothing costs of $333 per crew for employers who implement either regulated areas or the access control plan option. In addition, OSHA estimated that there would be annual respirator costs of $60 per crew for employers who implement either option.

ERG aggregated costs by estimating an average crew size of four in construction and an average job length of ten days. ERG judged that employers would choose to establish regulated areas in 75 percent of the instances where either regulated areas or an access control plan is required, and that written access control plans would be established for the remaining 25 percent.

See Chapter V in the PEA for a full discussion of OSHA's analysis of costs for regulated areas and written access control plans under the proposed standard.

F. Economic Feasibility Analysis and Regulatory Flexibility Determination

Chapter VI of the PEA presents OSHA's analysis of the economic impacts of its proposed silica rule on affected employers in general industry, maritime, and construction. The discussion below summarizes the findings in that chapter.

As a first step, the Agency explains its approach for achieving the two major objectives of its economic impact analysis: (1) To establish whether the proposed rule is economically feasible for all affected industries, and (2) to determine if the Agency can certify that the proposed rule will not have a significant economic impact on a substantial number of small entities. Next, this approach is applied to industries with affected employers in general industry and maritime and then to industries with affected employers in construction. Finally, OSHA directed Inforum—a not-for-profit corporation (based at the University of Maryland) specializing in the design and application of macroeconomic models of the United States (and other countries)—to estimate the industry and aggregate employment effects of the proposed silica rule. The Agency invites comment on any aspect of the methods and data presented here or in Chapter VI of the PEA.

1. Analytic Approach

a. Economic Feasibility

The Court of Appeals for the D.C. Circuit has long held that OSHA standards are economically feasible so long as their costs do not threaten the existence of, or cause massive economic dislocations within, a particular industry or alter the competitive structure of that industry. American Iron and Steel Institute. v. OSHA, 939 F.2d 975, 980 (D.C. Cir. 1991); United Steelworkers of America, AFL-CIO-CLC v. Marshall, 647 F.2d 1189, 1265 (D.C. Cir. 1980); Industrial Union Department v. Hodgson, 499 F.2d 467, 478 (D.C. Cir. 1974).

In practice, the economic burden of an OSHA standard on an industry—and whether the standard is economically feasible for that industry—depends on the magnitude of compliance costs incurred by establishments in that industry and the extent to which they are able to pass those costs on to their customers. That, in turn, depends, to a significant degree, on the price elasticity of demand for the products sold by establishments in that industry.

The price elasticity of demand refers to the relationship between the price charged for a product and the demand for that product: the more elastic the relationship, the less an establishment's compliance costs can be passed through to customers in the form of a price increase and the more it has to absorb compliance costs in the form of reduced profits. When demand is inelastic, establishments can recover most of the costs of compliance by raising the prices they charge; under this scenario, profit rates are largely unchanged and the industry remains largely unaffected. Any impacts are primarily on those customers using the relevant product. On the other hand, when demand is elastic, establishments cannot recover all compliance costs simply by passing the cost increase through in the form of a price increase; instead, they must absorb some of the increase from their profits. Commonly, this will mean reductions both in the quantity of goods and services produced and in total profits, though the profit rate may remain unchanged. In general, “[w]hen an industry is subjected to a higher cost, it does not simply swallow it; it raises its price and reduces its output, and in this way shifts a part of the cost to its consumers and a part to its suppliers,” in the words of the court in American Dental Association v. Secretary of Labor (984 F.2d 823, 829 (7th Cir. 1993)).

The court's summary is in accord with microeconomic theory. In the long run, firms can remain in business only if their profits are adequate to provide a return on investment that ensures that investment in the industry will continue. Over time, because of rising real incomes and productivity increases, firms in most industries are able to ensure an adequate profit. As technology and costs change, however, the long-run demand for some products naturally increases and the long-run demand for other products naturally decreases. In the face of additional compliance costs (or other external costs), firms that otherwise have a profitable line of business may have to increase prices to stay viable. Increases in prices typically result in reduced quantity demanded, but rarely eliminate all demand for the product. Whether this decrease in the total production of goods and services results in smaller output for each establishment within the industry or the closure of some plants within the industry, or a combination of the two, is dependent on the cost and profit structure of individual firms within the industry.

If demand is perfectly inelastic (i.e., the price elasticity of demand is zero), then the impact of compliance costs that are 1 percent of revenues for each firm in the industry would result in a 1 percent increase in the price of the product, with no decline in quantity demanded. Such a situation represents an extreme case, but might be observed in situations in which there were few if any substitutes for the product in question, or if the products of the affected sector account for only a very small portion of the revenue or income of its customers.

If the demand is perfectly elastic (i.e., the price elasticity of demand is infinitely large), then no increase in price is possible and before-tax profits would be reduced by an amount equal to the costs of compliance (net of any cost savings—such as reduced workers' compensation insurance premiums—resulting from the proposed standard) if the industry attempted to maintain production at the same level as previously. Under this scenario, if the costs of compliance are such a large percentage of profits that some or all plants in the industry could no longer operate in the industry with hope of an adequate return on investment, then some or all of the firms in the industry would close. This scenario is highly unlikely to occur, however, because it can only arise when there are other products—unaffected by the proposed rule—that are, in the eyes of their customers, perfect substitutes for the products the affected establishments make.

A common intermediate case would be a price elasticity of demand of one (in absolute terms). In this situation, if the costs of compliance amount to 1 percent of revenues, then production would decline by 1 percent and prices would rise by 1 percent. As a result, industry revenues would remain the same, with somewhat lower production, but with similar profit rates (in most situations where the marginal costs of production net of regulatory costs would fall as well). Customers would, however, receive less of the product for their (same) expenditures, and firms would have lower total profits; this, as the court described in American Dental Association v. Secretary of Labor, is the more typical case.

A decline in output as a result of an increase in price may occur in a variety of ways: individual establishments could each reduce their levels of production; some marginal plants could close; or, in the case of an expanding industry, new entry may be delayed until demand equals supply. In many cases it will be a combination of all three kinds of reductions in output. Which possibility is most likely depends on the form that the costs of the regulation take. If the costs are variable costs (i.e., costs that vary with the level of production at a facility), then economic theory suggests that any reductions in output will take the form of reductions in output at each affected facility, with few if any plant closures. If, on the other hand, the costs of a regulation primarily take the form of fixed costs (i.e., costs that do not vary with the level of production at a facility), then reductions in output are more likely to take the form of plant closures or delays in new entry.

Most of the costs of this regulation, as estimated in Chapter V of the PEA, are variable costs. Almost all of the major costs of program elements, such as medical surveillance and training, will vary in proportion to the number of employees (which is a rough proxy for the amount of production). Exposure monitoring costs will vary with the number of employees, but do have some economies of scale to the extent that a larger firm need only conduct representative sampling rather than sample every employee. The costs of engineering controls in construction also vary by level of production because almost all necessary equipment can readily be rented and the productivity costs of using some of these controls vary proportionally to the level of production. Finally, the costs of operating engineering controls in general industry (the majority of the annualized costs of engineering controls in general industry) vary by the number of hours the establishment works, and thus vary by the level of production and are not fixed costs in the strictest sense.

This leaves two kinds of costs that are, in some sense, fixed costs—capital costs of engineering controls in general industry and certain initial costs that new entries to the industry will not have to bear.

Capital costs of engineering controls in general industry due to this standard are relatively small as compared to the total costs, representing less than 8 percent of total annualized costs and approximately $362 per year per affected establishment in general industry.

Some initial costs are fixed in the sense that they will only be borne by firms in the industry today—these include initial costs for general training not currently required and initial costs of medical surveillance. Both of these costs will disappear after the initial year of the standard and thus would be difficult to pass on. These costs, however, represent less than 4 percent of total costs and less than $55 per affected establishment.

As a result of these considerations, OSHA expects that it is somewhat more likely that reductions in industry output will be met by reductions in output at each affected facility rather than as a result of plant closures. However, closures of some marginal plants or poorly performing facilities are always possible.

To determine whether a rule is economically feasible, OSHA begins with two screening tests to consider minimum threshold effects of the rule under two extreme cases: (1) all costs are passed through to customers in the form of higher prices (consistent with a price elasticity of demand of zero), and (2) all costs are absorbed by the firm in the form of reduced profits (consistent with an infinite price elasticity of demand).

In the former case, the immediate impact of the rule would be observed in increased industry revenues. While there is no hard and fast rule, in the absence of evidence to the contrary, OSHA generally considers a standard to be economically feasible for an industry when the annualized costs of compliance are less than a threshold level of one percent of annual revenues. Retrospective studies of previous OSHA regulations have shown that potential impacts of such a small magnitude are unlikely to eliminate an industry or significantly alter its competitive structure, particularly since most industries have at least some ability to raise prices to reflect increased costs and, as shown in the PEA, normal price variations for products typically exceed three percent a year. Of course, OSHA recognizes that even when costs are within this range, there could be unusual circumstances requiring further analysis.

In the latter case, the immediate impact of the rule would be observed in reduced industry profits. OSHA uses the ratio of annualized costs to annual profits as a second check on economic feasibility. Again, while there is no hard and fast rule, in the absence of evidence to the contrary, OSHA has historically considered a standard to be economically feasible for an industry when the annualized costs of compliance are less than a threshold level of ten percent of annual profits. In the context of economic feasibility, the Agency believes this threshold level to be fairly modest, given that—as shown in the PEA—normal year-to-year variations in profit rates in an industry can exceed 40 percent or more. OSHA's choice of a threshold level of ten percent of annual profits is low enough that even if, in a hypothetical worst case, all compliance costs were upfront costs, then upfront costs would still equal seventy-one percent of profits and thus would be affordable from profits without resort to credit markets. If the threshold level were first-year costs of ten percent of annual profits, firms could even more easily expect to cover first-year costs at the threshold level out of current profits without having to access capital markets and otherwise being threatened with short-term insolvency.

In general, because it is usually the case that firms would able to pass on some or all of the costs of the proposed rule, OSHA will tend to give much more weight to the ratio of industry costs to industry revenues than to the ratio of industry costs to industry profits. However, if costs exceed either the threshold percentage of revenue or the threshold percentage of profits for an industry, or if there is other evidence of a threat to the viability of an industry because of the standard, OSHA will examine the effect of the rule on that industry more closely. Such an examination would include market factors specific to the industry, such as normal variations in prices and profits, international trade and foreign competition, and any special circumstances, such as close domestic substitutes of equal cost, which might make the industry particularly vulnerable to a regulatory cost increase.

The preceding discussion focused on the economic viability of the affected industries in their entirety. However, even if OSHA found that a proposed standard did not threaten the survival of affected industries, there is still the question of whether the industries' competitive structure would be significantly altered. For this reason, OSHA also examines the differential costs by size of firm.

b. Regulatory Flexibility Screening Analysis

The Regulatory Flexibility Act (RFA), Pub. L. No. 96-354, 94 Stat. 1164 (codified at 5 U.S.C. 601), requires Federal agencies to consider the economic impact that a proposed rulemaking will have on small entities. The RFA states that whenever a Federal agency is required to publish general notice of proposed rulemaking for any proposed rule, the agency must prepare and make available for public comment an initial regulatory flexibility analysis (IRFA). 5 U.S.C. 603(a). Pursuant to section 605(b), in lieu of an IRFA, the head of an agency may certify that the proposed rule will not have a significant economic impact on a substantial number of small entities. A certification must be supported by a factual basis. If the head of an agency makes a certification, the agency shall publish such certification in the Federal Register at the time of publication of general notice of proposed rulemaking or at the time of publication of the final rule. 5 U.S.C. 605(b).

To determine if the Assistant Secretary of Labor for OSHA can certify that the proposed silica rule will not have a significant economic impact on a substantial number of small entities, the Agency has developed screening tests to consider minimum threshold effects of the proposed rule on small entities. These screening tests are similar in concept to those OSHA developed above to identify minimum threshold effects for purposes of demonstrating economic feasibility.

There are, however, two differences. First, for each affected industry, the screening tests are applied, not to all establishments, but to small entities (defined as “small business concerns” by SBA) and also to very small entities (defined by OSHA as entities with fewer than 20 employees). Second, although OSHA's regulatory flexibility screening test for revenues also uses a minimum threshold level of annualized costs equal to one percent of annual revenues, OSHA has established a minimum threshold level of annualized costs equal to five percent of annual profits for the average small entity or very small entity. The Agency has chosen a lower minimum threshold level for the profitability screening analysis and has applied its screening tests to both small entities and very small entities in order to ensure that certification will be made, and an IRFA will not be prepared, only if OSHA can be highly confident that a proposed rule will not have a significant economic impact on a substantial number of small entities in any affected industry.

2. Impacts in General Industry and Maritime

a. Economic Feasibility Screening Analysis: All Establishments

To determine whether the proposed rule's projected costs of compliance would threaten the economic viability of affected industries, OSHA first compared, for each affected industry, annualized compliance costs to annual revenues and profits per (average) affected establishment. The results for all affected establishments in all affected industries in general industry and maritime are presented in Table VIII-11, using annualized costs per establishment for the proposed 50 μ g/m³ PEL. Shown in the table for each affected industry are total annualized costs, the total number of affected establishments, annualized costs per affected establishment, annual revenues per establishment, the profit rate, annual profits per establishment, annualized compliance costs as a percentage of annual revenues, and annualized compliance costs as a percentage of annual profits.

The annualized costs per affected establishment for each affected industry were calculated by distributing the industry-level (incremental) annualized compliance costs among all affected establishments in the industry, where costs were annualized using a 7 percent discount rate. The annualized cost of the proposed rule for the average establishment in all of general industry and maritime is estimated at $2,571 in 2009 dollars. It is clear from Table VIII-11 that the estimates of the annualized costs per affected establishment in general industry and maritime vary widely from industry to industry. These estimates range from $40,468 for NAICS 327111 (Vitreous china plumbing fixtures and bathroom accessories manufacturing) and $38,422 for NAICS 327121 (Brick and structural clay manufacturing) to $107 for NAICS 325510 (Paint and coating manufacturing) and $49 for NAICS 621210 (Dental offices).

Table VIII-11 also shows that, within the general industry and maritime sectors, there are no industries in which the annualized costs of the proposed rule exceed 1 percent of annual revenues or 10 percent of annual profits. NAICS 327123 (Other structural clay product manufacturing) has both the highest cost impact as a percentage of revenues, of 0.39 percent, and the highest cost impact as a percentage of profits, of 8.78 percent. Based on these results, even if the costs of the proposed rule were 50 percent higher than OSHA has estimated, the highest cost impact as a percentage of revenues in any affected industry in general industry or maritime would be less than 0.6 percent. Furthermore, the costs of the proposed rule would have to be more than 150 percent higher than OSHA has estimated for the cost impact as a percentage of revenues to equal 1 percent in any affected industry. For all affected establishments in general industry and maritime, the estimated annualized cost of the proposed rule is, on average, equal to 0.02 percent of annual revenue and 0.5 percent of annual profit.