MSHA Now Enforcing Lower Diesel Particulate Matter Limit

The Mine Safety and Health Administration has begun enforcing an exposure limit of 160 micrograms of total carbon (TC) per cubic meter of air (160TC µg/m3) and said it 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 diesel particulate matter (DPM). The limit applies to underground metal and nonmetal mines.

The limit has been 350TC. MSHA measures a miner's personal exposure to DPM by analyzing a sample for TC, which is a DPM surrogate obtained by combining elemental carbon (EC) and organic carbon (OC). In its May 2006 DPM final rule, MSHA said it would propose a rule to convert the 160 TC PEL to a comparable EC PEL prior to the effective date of May 20, 2008, if sufficient scientific data were available to support a proposed rule. But MSHA chose not to issue the proposed rule, instead offering 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 for EC, and TC on the miner's personal sample is greater than 160 micrograms per cubic meter of air times the error factor for TC. MSHA said it believes there is still insufficient evidence suggesting an appropriate conversion factor.

The DPM rulemaking record showed that exposure can't be validated by simply adding EC and OC of a TC sample because of possible interference from non-diesel exhaust sources such as tobacco smoke, drill oil mist, and ammonium nitrate/fuel oil vapors.

MSHA's notice of the decision to move ahead with enforcement mentions Kennecott Greens Creek Mining Company v. Mine Safety and Health Administration, 476 F.3d 946, 956 (DC Cir. 2007), which upheld the DPM standard and allowed MSHA to enforce either the 160 TC PEL or a converted elemental carbon PEL. The U.S. Court of Appeals for the District of Columbia Circuit also upheld MSHA's selection of TC and EC as appropriate surrogates for DPM. The latest available scientific evidence is "Relationship between Elemental Carbon, Total Carbon, and Diesel Particulate Matter in Several Underground Metal/Non-metal Mines," which was published in February 2007 (J. D. Noll; A. D. Bugarski; L. D. Patts; S. E. Mischler; L. McWilliams, Environ. Sci. & Technol., Vol. 41, No. 3) and concluded the variability of the TC-to-EC ratio increases below 230 TC and is high at 160 TC, so MSHA could not identify a single, constant conversion factor for EC at any level below 230 TC.

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