Regulatory Authorities

Occupational health and safety regulations are set in the U.S. at the federal level by two agencies within the Department of Labor:

  • Occupational Safety and Health Administration (OSHA) is responsible for general occupational environments
  • Mine Safety and Health Administration (MSHA) is responsible for mining

A table of Threshold Limit Values (TLV) for chemical substances and physical agents found in the workplace is published annually by the American Conference of Governmental Industrial Hygienists (ACGIH). The TLVs set by ACGIH are developed as recommendations or guidelines to be used in the practice of industrial hygiene, which establish the maximum ambient concentrations of toxic chemical substances. Formally, they are not legal standards. However, regulatory authorities follow the ACGIH recommendations in their law-making activities.

The scope of this article is limited to U.S. federal regulations. Diesel emission exposure or the use of diesel engines may be additionally regulated by occupational health or mining authorities in particular states.

Exposure Limits for Gases

The exposure limits for selected gaseous pollutants found in diesel exhaust are listed in Table 1. Limits set by OSHA are known as Permissible Exposure Limits (PEL). Both OSHA PELs and MSHA TLVs are legally enforceable limits. Limits shown in the column OSHA 88 were adopted as a final rule in 1988, but were later remanded by court and have no legal significance. The TLVs by ACGIH are industrial hygiene recommendations. All limits are 8 hour time weighted averages (TWA), unless marked as ceiling values.

Table 1. Exposure Limits for Gaseous Pollutants (ppmv, TWA, 8 hr)
Substance CAS# OSHA PEL OSHA 88* MSHA TLV ACGIH TLV
CO 630-08-0 50 35 50 25
CO2 124-38-9 5000 5000 5000 5000
NO 10102-43-9 25 25 25 25
NO2 10102-44-0 (C) 5 1d 5 3
HCHO 50-00-0 0.75     (C) 0.3 A2
SO27446-09-5525a / 2b2
* - not legal limits (PELs adopted in 1988 were later remanded by court)
a - for metal/nonmetal mines
b - for coal mines
d - 15-minute short term exposure limit (STEL)
(C) - Ceiling value
A2 - Suspected human carcinogen

Exposure Limits for Particulates

General Occupational Settings (Non-Mining)

There are no legal exposure limits for DPM in general occupational settings in the USA. An exposure limit for DPM was proposed by the ACGIH, but was withdrawn from the ACGIH Notice of Intended Changes (NIC) in 2003.

The first DPM exposure limit of 0.15 mg/m3 was proposed by ACGIH in its 1995-1996 NIC. It was later lowered 0.05 mg/m3 (as total diesel particulate matter). In its 2001 NIC, the ACGIH replaced it with a practically equivalent TLV of 0.02 mg/m3 as elemental carbon (the EC fraction typically constitutes about 40% of the total diesel particulate mass), but the DPM listing was withdrawn in 2003.

Underground Mining

On January 19, 2001, MSHA published final diesel regulations for underground metal/nonmetal (i.e., non-coal) and for underground coal mines. The metal/nonmetal mine rule adopts exposure limits for diesel particulates. MSHA has not introduced exposure limits for coal mines, due to the lack of a suitable DPM measuring method in the presence of coal dust.

The underground metal/nonmetal mine rule establishes a concentration limit for diesel particulate matter and requires mine operators to use engineering or work practice controls to reduce DPM exposure to that limit. The 2001 rule introduced two DPM limits: (1) an “interim” DPM concentration limit of 400 µg/m3 effective July 19, 2002 and (2) a final DPM concentration limit of 160 µg/m3 effective January 19, 2006. For the purpose of ambient sampling (according to NIOSH method 5040), DPM was defined as total carbon (TC). This definition includes both elemental and organic (i.e., hydrocarbon derived) carbon, and excludes inorganic ash and sulfates from the TLV.

The 2001 rule has been subject to legal challenges and negotiations between MSHA, mining industry, and labor, resulting in several amendments:

  • In 2005, the interim limit was changed from 400 µg/m3 of TC to 308 µg/m3 expressed as elemental carbon (EC), based on a TC:EC conversion factor of 1.3.
  • On May 18, 2006, MSHA promulgated a final rule which:
    • Changed the interim limit to 350 µg/m3 EC and its effective date to January 20, 2007.
    • Extended the effective date of the 160 µg/m3 TC standard to May 20, 2008.

The 400 µg/m3 TC and 350 µg/m3 EC limits are of a similar stringency, but the EC measurement eliminates potential non-diesel sources of organic carbon particulates, for instance oil mist.

The final DPM exposure limit of 160 µg/m3 TC will be also converted to an equivalent EC-based limit in a separate rule to be issued by MSHA.

Mining Diesel Engine Regulations

Engine Approval Program

In addition to setting exposure limits for air pollutants, as summarized above, MSHA has established other health protection measures in regards to the use of diesel engines underground. Diesel engines to be used in underground mining require a formal MSHA approval. The approval process involves emission testing on an engine dynamometer using the ISO 8178 C1 test cycle (mining engines are exempted from the EPA off-road emission standard and certification).

Based on the test results, ventilation rates are calculated for each approved engine model. The ventilation rates are computed as the quantity of additional ventilation air that has to be provided to the mine in order to dilute raw exhaust emissions to the respective TLV levels (Table 1 - MSHA). This mechanism ensures that mine operators increase the amount of ventilation as more engines are introduced underground. A particulate index (PI) is also calculated for DPM dilution to an arbitrary ambient level of 1 mg/m3. Since the PI has only an advisory role, the PI-based ventilation rates are not enforceable.

Coal Mining Regulations

MSHA coal mine diesel regulations published on January 19, 2001 set a specific engine emission limit of 2.5 g/hr of DPM for both permissible and non-permissible equipment. This limit will be phased in for an operation’s existing equipment inventory over a 48-month period (with an interim limit of 5 g/hr for non-permissible equipment), but new equipment must meet the emission limits sooner. Coal mine operators may use a combination of controls to comply with the emission limit.

It is expected that, in most cases, meeting the limit will require the use of particulate filters. Only the smallest engines will be able to meet the 2.5 g/hr limit with no emission aftertreatment. Permissible vehicles, which are equipped with water scrubbers or heat exchangers, are expected to use disposable, paper-based particulate filter cartridges, while non-permissible equipment would use catalytic particulate filters.

Compliance with the regulation will be determined based on two laboratory measurements: (1) engine DPM emission factor and (2) percentage DPM filtration efficiency of the particulate filter (or other emission control device) installed on the engine. The filtration efficiency must be sufficient to bring the engine DPM emission below 2.5 g/hr.

The engine emission factors are determined over the ISO 8178 8-mode cycle during MSHA engine approval testing. DPM emissions of currently approved mining engines are listed in the preamble to the regulation (page 5667 in the Federal Register of January 19, 2001).

In order to minimize the amount of laboratory tests, MSHA adopts a flexible approach for the testing of aftertreatment devices. Generally, aftertreatment devices should be tested on an approved mining engine of DPM emissions not higher than the engine on the machine the device is to be used, over the MSHA approval test cycle (ISO 8178 C1). However, other tests can be accepted at MSHA’s discretion. In particular, all filter efficiencies determined by the VERT program are automatically accepted for use in coal mines, regardless of the type of engine or test cycle. The rule does not explicitly specify the sulfur level of the fuel for filter testing.

Diesel Fuel Requirements

Coal mining regulations adopted in 1997 introduce the following diesel fuel requirements:

  • Maximum sulfur content 500 ppm wt.
  • Flashpoint above 100°F
  • Fuel additives have to be EPA registered

The January 2001 regulations introduce the same fuel sulfur (500 ppm) and additive (EPA registration) requirements for metal/nonmetal mines. The EPA “low sulfur” fuel (500 ppm S) is also used for engine approval testing.