Comments on the Mine Safety and Health Administration's Proposed Rule on Diesel Exposure

October 14, 2003 

Dave D. Lauriski
Assistant Secretary of Labor for Mine Safety and Health
1100 Wilson Boulevard
Arlington, VA 22209-3939

Re: Proposed Rule on Diesel Particulate Matter Exposure of Underground Metal and Nonmetal Miners 

Dear Assistant Secretary Lauriski: 

Public Citizen opposes the Mine Safety and Health Administration’s (MSHA’s) August 14, 2003 Notice of Proposed Rulemaking (Proposed Rule),[1] which would undo existing legal protections for the over 17,000 miners[1] laboring in metal and non-metal mines who are exposed to diesel particulate matter (DPM), a long-recognized cause of lung cancer. While the industry may chafe at requirements to protect workers, the current regulations are based in solid science and, according to MSHA’s own estimates, represent a relatively minor cost to industry. There is simply no excuse for undoing them. 

On January 19, 2001, MSHA published a Final Rule governing miner exposure to DPM.[2] The rule established an Interim Concentration Limit of 400 *g/m3 of total carbon, with which industry was supposed to be compliant by July 2002, and a Final Concentration Limit of 160 *g/m3, scheduled to take effect in January 2006.[3] The rule was the product of an extended regulatory process going all the way back to 1992, when the agency first proposed to lower DPM exposure significantly.  During the intervening years, industry had multiple opportunities to raise its concerns about the proposed standard.  Now, it seems, very similar concerns are being directed at fresh ears and, despite a body of scientific literature that has grown in size, sophistication and certainty about the carcinogenic properties of DPM, the agency is proposing to reverse much of what has been accomplished (see Table below). 

The first step in the undoing of the Final Rule was MSHA’s agreement, after extensive discussions with industry and labor, not to issue citations for noncompliance with the Interim Concentration Limit until July 19, 2003, a decision that was announced in a notice in the Federal Register on July 18, 2002.[4] This had the effect of negating a crucial element of the Final Rule, without public comment. After the expiration of that agreement in July 2003, MSHA replace it with a Compliance Guide.[5] The Compliance Guide effectively defers enforcement of the Interim Concentration Limit, allowing mine operators to substitute the use of personal protective equipment for compliance with the Interim Concentration Limit “where controls are infeasible.”[5]  With respect to the Interim Concentration Limit, the Proposed Rule echoes the Compliance Guide and, ominously, promises that the Final Concentration Limit will be revisited “in the near future”[1] in the form of a separate rulemaking procedure. Step-by-step, worker protections are being peeled back. 

Beyond its non-enforcement of the Interim Concentration Limit and its plan to “revisit” the Final Concentration Limit, the agency in its Proposed Rule is now proposing several other dangerous modifications to the Final Rule. The Final Rule contemplates occasional extensions to the Final (not the Interim) Concentration Limit for technological reasons only; this is to be accomplished through an application to the Secretary.  In contrast, the Proposed Rule would permit extensions to the much-more-lenient Interim Concentration Limit, recognize both technological and economic causes for exemptions and allow District Managers to issue extensions. The Final Rule required that engineering or work practice controls be used to reduce DPM to the Interim or Final Concentration Limits. In contrast, the Proposed Rule permits a mine operator to substitute less-protective respirators if that operator can convince an MSHA District Manager that it is not feasible for that operator, technologically or economically, to comply with that limit. Finally, whereas the Final Rule permitted measurements of DPM through personal, occupational or area samples, the Proposed Rule permits personal samples only, reducing inspector flexibility. Based on prior experience, there is reason to be concerned that workers will be shifted to lower-exposure tasks on sampling days; in such a circumstance, the personal sample would be inappropriately low, whereas occupational or area samples would measure the worker’s typical exposure. 

A long series of studies conclusively demonstrates the potential of DPM to cause lung cancer.[6] In the Final Rule, MSHA exhaustively reviewed the then-available studies. The agency identified 47 epidemiologic studies, dating back to 1957, which had examined the risk of lung cancer among people exposed to DPM on the job (usually miners, railroad workers or truck drivers). Of the 47 studies, 41 showed some association between DPM and lung cancer; this finding was statistically significant in 25 studies. Importantly, the studies reviewed by MSHA found elevated lung cancer risks at DPM levels significantly below current exposures in U.S. mines and even at levels below the Final Concentration Limit. The remaining six studies showed some negative association between DPM and lung cancer, but only one study reached statistical significance. However, that study[7] did not have a minimum period of exposure or latency, had a relatively youthful cohort, did not have detailed exposure histories and did not adjust for the “healthy worker effect.”[8] In the Final Rule, the agency concluded that the study “contributes little or no information on the potential health effects of long-term dpm exposures and that whatever information it does contribute does not extend to effects, such as cancer, expected in later life.” The Proposed Rule adds three additional studies to the 47, all of which were positive. For reasons unclear to us, a relatively recent study by Larkin, et al., involving over 55,000 railroad workers, has not been included in the Final Rule or the Proposed Rule. That study showed an excess risk of lung cancer of 44% for those with the longest histories of exposure.[9]

In addition, in the Final Rule the agency identified two meta-analyses, which analyze and statistically combine the results of the epidemiologic studies. These found statistically significant increased risk of lung cancer among DPM-exposed workers of 30% - 40%.

The leading institutions in carcinogenesis have also concluded that DPM causes lung cancer. The International Agency for Research on Cancer and the World Health Organization have concluded that diesel exhaust is probably carcinogenic in humans and the National Toxicology Program has stated that diesel exhaust is reasonably anticipated to be a human carcinogen. As long ago as 1988, the National Institute for Occupational Safety and Health recommended that DPM be regarded as a probable or potential human carcinogen. 

MSHA’s own risk assessment in the Final Rule (not revised in the Proposed Rule) found that, depending on the level of exposure and the estimate of risk assumed, the excess risk of lung cancer death (compared to no exposure to DPM), based on a working lifetime of exposure to then-current levels of DPM, was as high as 800 per 1,000 workers. This means that as many as 80% of workers so exposed could die from lung cancer as a result of DPM exposure. These risks are considerably greater than the 1 per 1,000 worker excess risk standard established in the benzene case as sufficient to require government regulation. Moreover, the risk assessment concluded that a reduction from then-current exposure levels to 160 *g/m3 would prevent between 68 and 620 lung cancer deaths per 1,000 metal and nonmetal miners over a 45-year working lifetime. Clearly, any further delay in enforcement will take a heavy toll in workers’ lives. 

Predictably, some in the mining industry are using their financial resources to challenge, among other things, the feasibility of engineering controls and other measures to reduce miners’ exposure to DPM. Despite these claims, measurements collected by MSHA over the last year and published with the Proposed Rule confirm the agency’s findings that greatly lowered concentration limits are feasible.  MSHA reports that during the so-called “baseline” study (conducted from October 2002 to March 2003), the median DPM concentration was 209 *g/m3, substantially below the 400 *g/m3 Interim Concentration Limit, even though most mine operators have not yet implemented any controls. Only 16% of measurements exceeded the Interim Concentration Limit. Most trona[10] measurements are already compliant with the Final Concentration Limit. Furthermore, MSHA reports that of 31 mines selected by the industry and sampled for DPM prior to the baseline study, “five mines were already in compliance with the interim concentration limit, and another two mines were already in compliance with the final concentration limit.”[1] MSHA would be wise to heed the words of the Appellate Court ruling supporting the Occupational Safety and Health Administration’s 1974 vinyl chloride health standard: “the Secretary is not restricted by the status quo.”[11]  

In the Final Rule, MSHA estimated the annual cost of complying with the rule for the entire industry at $25.1 million, or about $128,000 per mine. Data since collected by the agency suggest that the cost of compliance may be lower. Using the same methodology, but incorporating specific data from these same 31 mines, the annual cost of complying with the Final Rule was estimated at $103,000 per mine. This represents 0.18% of annual revenues at these mines. Thus, meeting the requirement of the Final Rule is certainly economically feasible for this industry as a whole. 

The Mine Act explicitly states that:   

No mandatory health or safety standard promulgated under this subchapter shall reduce the protection afforded miners by an existing mandatory health or safety standard.[12]

The changes contemplated in the Proposed Rule, as well as the presumed weakening of the Final Concentration Limit yet to be proposed, appear to be inconsistent with that mandate. 


Peter Lurie, M.D., M.P.H.
Deputy Director  

Sidney M. Wolfe, M.D.
Public Citizen’s Health Research Group


1 Federal Register, Vol. 68, 48668-48721, August 14, 2003.

2 Federal Register, Vol. 66, 5706-5910, January 19, 2001.

3 In these comments, all exposure concentrations are for measurements of Total Carbon, not Elemental Carbon or Diesel Particulate Matter.

4 Federal Register, Vol. 67, 47296–47299, July 18, 2002.

5 Mine Safety and Health Administration. Metal and Nonmetal Interim Diesel Particulate Matter (DPM) Standard: Compliance Q&As. Final version (August 5, 2003). Available at:

6 These comments focus only on the propensity of DPM to cause lung cancer and not on its association with other conditions, including bladder cancer and cardiovascular, pulmonary and immunological toxicity.

7 Christie DG, Brown AM, Taylor RJ, Seccombe MA, Coates MS. Mortality in the New South Wales coal industry, 1973-1992. Medical Journal of Australia 1995;163:19-21.

8 The healthy worker effect occurs when the mortality rate for workers is artificially lowered compared to the general population because people too ill to work are removed from the workforce.

9 Larkin EK, Smith TJ, Stayner L, Rosner B, Speizer FE, Garshick E. Diesel exhaust exposure and lung cancer: adjustment for the effect of smoking in a retrospective cohort study. American Journal of Industrial Medicine 2000;38:399-409.

10 A mineral that is a source of sodium

11 Society of the Plastics Industry, Inc. v. Occupational Safety and Health Administration, 509 F.2d 1301 (2d Cir. 1975).

12 30 U.S.C. 811(a)(9).


Final Rule (January 2001)

Compliance Guide (July 2003)

Proposed Rule (August 2003)

Interim Concentration Limit

400 ug/m3

Took effect in July 2002.

Operators can get exemption

Operators can get exemption

Final Concentration Limit

160 ug/m3

Takes effect in January 2006.

MSHA says it will re-propose rule to change the final limit

MSHA says it will re-propose rule to change the final limit


Operator may file an application with the Secretary for an extension if, for technological reasons, the mine needs additional time to comply with the Final concentration limit.

No mine may receive more than one extension, and it cannot exceed 2 years.

Formal application not required. Extension granted by the District Manager if, for technological and/or economic reasons, the mine needs additional time to comply with the Interim concentration limit.

Operator may file an application with the District Manager for an extension if, for technological and/or economic reasons, the mine needs additional time to comply with the Interim concentration limit.

Allows an unlimited number of extensions.

Sampling Methods

Personal, occupational or area

Personal samples only

Personal samples only