14 June 2005

The US Department of Labor’s Mine Safety and Health Administration (MSHA) has issued a final rule limiting the exposure to diesel particulate matter (DPM) in underground metal and nonmetal (i.e., non-coal) mines.

The rule revises the interim DPM concentration limit measured in total carbon (TC) to a comparable, permissible exposure limit measured in elemental carbon (EC). The EC metric has been considered more accurate than the TC measurement, which can also include carbon from non-diesel sources, for instance oil mist. The former interim level was 400 µg/m3 of TC. The new interim level is 308 µg/m3 as EC.

The rule setting DPM exposure limits in metal/non metal mines was originally adopted by MSHA on 19 January 2001. The regulation established two DPM exposure limits: (1) the interim limit of 400 µg/m3 from 20 July 2002, and (2) a final limit of 160 µg/m3 from 20 January 2006. Both limits were expressed as TC, according to NIOSH method 5040.

Following a challenge of the 2001 rule by the mining industry, negotiations started between MSHA, mine operators, organized labor, and other stakeholders. A partial settlement was reached in 2003, when MSHA proposed to change to TC-based interim exposure limit to an equivalent limit based on elemental carbon—a change that has now become final.

Neither the 2003 proposal nor the final rule include any specific numbers for the final exposure limit (originally 160 µg/m3 TC). “MSHA believes that evidence in the current DPM rulemaking record is inadequate for MSHA to make determinations regarding revision to the final DPM limit,” says MSHA in the current rule. Thus, it remains an open question whether the final limit is to be delayed, relaxed, or dropped.

Source: MSHA (final rule | DPM page)