Diesel Particulate Matter Exposure of Underground Metal and Nonmetal Miners

AGENCY:

Mine Safety and Health Administration (MSHA), Labor.

ACTION:

Notice of enforcement of DPM final limit; withdrawal of intent to issue a proposed rule.

SUMMARY:

This notice informs the public of MSHA's decision to implement the diesel particulate matter (DPM) final permissible exposure limit (PEL) of 160 micrograms of total carbon (TC) per cubic meter of air (160_TC g/m³). MSHA has developed a practical sampling strategy to account for interferences from non-diesel exhaust sources when TC is used as a surrogate for measuring a miner's exposure to DPM. The Agency will begin enforcement of the 160 TC limit under existing 30 CFR 57.5060(b)(3) on May 20, 2008. MSHA will post details of its sampling strategy on the Agency's DPM Single Source Page prior to enforcement. The sampling strategy is based on the best available scientific evidence and will be specific to each mine.

DATES:

Effective Date: May 20, 2008.

FOR FURTHER INFORMATION CONTACT:

Patricia W. Silvey, Director, Office of Standards, Regulations and Variances at silvey.patricia@dol.gov (E-mail), 202-693-9440 (Voice), or 202-693-9441 (Fax).

SUPPLEMENTARY INFORMATION:

A. Background

MSHA measures a miner's personal exposure to DPM by analyzing the sample for a DPM surrogate, TC. TC is the sum of elemental carbon (EC) and organic carbon (OC). The 160 TC limit was promulgated in the 2001 final rule “Diesel Particulate Matter Exposure of Underground Metal and Nonmetal Miners” which was published in the Federal Register on January 19, 2001 (66 FR 5706) and amended on June 6, 2005 (70 FR 32868) and May 18, 2006 (71 FR 28924).

When the Agency published the 2006 final rule, MSHA stated its intent to issue a proposed rule to convert the 160 TC PEL to a comparable EC PEL prior to the effective date of May 20, 2008, provided sufficient scientific data were available to support a proposed rule. MSHA is not issuing a proposed rule to uniformly convert the 160 TC limit to a comparable EC limit. Instead, MSHA provides a protocol for calculating a location specific adjustment for situations in which the EC on the miner's personal sample is less than 160 micrograms per cubic meter of air times the error factor (EF) for EC, and TC on the miner's personal sample is greater than 160 micrograms per cubic meter of air times the EF for TC. The decision not to issue a uniform conversion factor is based on MSHA's assessment that there is still insufficient evidence suggesting an appropriate conversion factor, and the latest available scientific evidence regarding the relationship between TC and EC at levels as low as 160 TC. MSHA will continue to monitor and encourage research in this field.

The DPM rulemaking record established that a miner's exposure could not be validated simply by adding the EC and OC of a TC sample due to the potential for non-diesel exhaust sources to deposit on the OC part of the sample and interfere with the MSHA sample analysis. These interferences include environmental tobacco smoke, drill oil mist, and ammonium nitrate/fuel oil (ANFO) vapors. When measuring EC, interferences are not a factor in assuring the accuracy of the sample analysis.

Currently, MSHA determines a miner's exposure to the PEL of 350_TC μg/m ³ (350 TC) by conducting an EC analysis to validate that the miner's overexposure to TC is not the result of interferences. In each analysis, MSHA incorporates an error factor to account for variability in sampling and analysis resulting from such things as pump flow rate, filters, and the NIOSH Analytical Method 5040. If the TC measurement is above 350 TC micrograms times the error factor for TC, MSHA looks at the EC measurement from the sample obtained through the NIOSH Analytical Method 5040, and multiplies EC by a conversion factor of 1.3 to produce a statistically valid estimate of what the TC result is without interferences. MSHA issues a citation when the EC measurement times the multiplier is above 350 micrograms times the error factor for EC. The 1.3 multiplier that MSHA uses to estimate TC (i.e., EC × 1.3 = estimated TC) is the median value of all TC to EC ratios obtained from valid TC samples (i.e., without OC interferences) collected by MSHA during the 31-Mine Study, and it is consistent with NIOSH's determination that TC is 60-80% EC.

In the 2006 final rule (71 FR 28924, May 18, 2006), MSHA retained the 2001 final limit of 160 TC but determined that it should be phased in over a two-year period and stated that:

Consequently, on May 20, 2006, the initial final limit will be 308 micrograms of EC per cubic meter of air (308_EC μg/m³), which is the same as the existing interim limit; on January 20, 2007, the final limit will be reduced by 50 micrograms and will be a TC limit of 350_TC μg/m ³; and on May 20, 2008, the final limit of 160_TC μg/m ³ will become effective. Note that the 350_TC μg/m ³ final limit and the 160_TC μg/m ³ final limit are established as TC-based limits in this final rule. (Id. at 28934).

Also in the 2006 final rule, MSHA discussed its concerns regarding the relationship between TC, EC and OC at lower concentrations and its intent to conduct a separate rulemaking to determine the most appropriate way to convert the 160 TC PEL to a comparable EC PEL by stating:

Moreover, we intend to convert the final limits of 350_TC μg/m ³ and 160_TC μg/m ³ in a separate rulemaking by January 2007. As we said in the 2005 NPRM, if we do not complete this rulemaking by that time, we will use the EC equivalent as a check to validate that an overexposure to the 350_TC μg/m ³ final limit is not the result of interferences. This enforcement policy, which is based on the Second Partial Settlement Agreement and data in the rulemaking record, would be the same that we used to implement the 400_TC μg/m ³ interim limit before we converted it to 308_EC μg/m ³ in the June 2005 final rule. Whereas we have evidence that we can obtain an accurate sample analysis of the final limit of 350_TC μg/m ³, there is no evidence in the rulemaking record suggesting that the 1.3 conversion factor is appropriate for substantially lower limits, such as the final limit of 160_TC μg/m ³. (Id. at 28976).

Although in the 2006 final rule MSHA acknowledged the limitations of sampling a miner's exposure to TC and preferred EC rather than TC as a DPM surrogate, the Agency did not conclude that TC could not be used as an appropriate surrogate for measuring a miner's exposure to DPM. In addition, the court decision in Kennecott Greens Creek Mining Company v. Mine Safety and Health Administration, 476 F.3d 946, 956 (DC Cir. 2007), upholding the DPM standard, allows MSHA to enforce either the 160 TC PEL or a converted elemental carbon (EC) PEL. The court upheld MSHA's selection of TC and EC as appropriate surrogates for DPM. See Id. at 956.

Subsequent to the DPM court decision, MSHA decided to wait for further scientific evidence regarding whether MSHA could reasonably convert the 160 TC PEL using a fixed conversion factor such as the 1.3 conversion factor currently used. The latest available scientific evidence is the study titled “Relationship between Elemental Carbon, Total Carbon, and Diesel Particulate Matter in Several Underground Metal/Non-metal Mines” which was published on February 1, 2007 (J. D. Noll; A. D. Bugarski; L. D. Patts; S. E. Mischler; L. McWilliams, Environ. Sci. & Technol., Vol. 41, No. 3: February 1, 2007, 710-716). The authors concluded that the variability of the TC-to-EC ratio increases below 230 TC and is high at 160 TC. Therefore, MSHA could not identify a single, constant conversion factor for EC at any level below 230 TC.

In March 2007, MSHA hired an outside expert with experience in DPM sampling methodology and analysis to advise the Agency in developing an enforcement strategy for accurately determining a miner's exposure to TC. The expert also reviewed the latest available data to attempt to devise a scientific method for converting the 160 TC PEL to a comparable EC PEL. The expert was unable to recommend such a method. As an alternative to developing a conversion factor, the expert recommended sampling strategy options for the Agency's consideration in enforcing the DPM final limit in a September 2007 report. MSHA was reviewing the expert's recommendations when it published its December 10, 2007 Semi-Annual Regulatory Agenda in which the Agency continued to state its intent to propose a rule to convert the 160 TC limit. MSHA now has determined that insufficient data exist to proceed with further rulemaking to convert the DPM final limit using a single, constant conversion factor, such as the 1.3 factor currently used for EC for all mines.

B. Notice of Enforcement of DPM Final Limit

MSHA has developed an enforcement strategy for implementation of the DPM 160 TC PEL beginning May 20, 2008. MSHA will continue to determine a miner's exposure to DPM based on a single personal sample taken over the miner's full shift as specified in existing 30 CFR § 57.5061 of the DPM standard. MSHA will use an EC analysis and appropriate sampling methods to ensure that a citation for a miner's overexposure to the 160 TC PEL is valid and not the result of interferences.

C. Reason for Withdrawal of Intent To Issue a Proposed Rule

MSHA is withdrawing its intent to issue a proposed rule to convert the 160 TC PEL because it has determined that insufficient data exist to support such a rule, and because it has determined that the enforcement strategy it will begin to use on May 20, 2008, is an accurate and effective way of enforcing the DPM standard. This enforcement strategy will provide effective health protections for miners at underground metal and nonmetal mines. In light of MSHA's enforcement action, this notice does not reduce health protections for underground metal and nonmetal miners.

Diesel Particulate Matter Exposure of Underground Metal and Nonmetal Miners is withdrawn from the Regulatory Agenda. This document does not preclude future agency action that MSHA may find to be appropriate.