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NIOSH Respiratory Diseases Research Program

Evidence Package for the National Academies' Review 2006-2007

NIOSH Programs > Respiratory Diseases > Evidence Package > 3. Interstitial Lung Diseases > 3.1 Respiratory Diseases Induced by Coal Mine Dust

3.1a) Verification that the Enacted Coal Mine Dust Standard Protects Miners

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Issue

The PEL for respirable coal mine dust adopted for U.S. workers in 1969 was solely derived from British epidemiologic research. Although individuals with simple CWP (i.e., without progressive massive fibrosis) are often asymptomatic (compared to those without pneumoconiosis), they are at much greater risk of developing the potentially disabling and life-threatening progressive massive fibrosis, with the risk increasing with increasing severity of simple pneumoconiosis. In 1969, the prevailing wisdom was that if severe simple CWP could be eliminated, progressive massive fibrosis would also be prevented. British research had derived a dust exposure-response curve of predicted working lifetime (35 years) risk of category two or greater simple CWP against level of dust exposure. At 2 mg/m3, the exposure-response curve indicated zero lifetime risk, implying ultimate prevention of progressive massive fibrosis. This value was adopted (by MSHA’s predecessor, MESA) in 1969 as the PEL for dust exposures in underground coal mines, but few of the assumptions behind the 2 mg/m3 PEL had been validated for the U.S. coal mining situation.

Approach

RDRP focused on the following objectives to validate a respiratory dust standard for coal miners:

  • Extent of disease - through a comprehensive survey across the country using standardized methodology, derive reliable and complete baseline data for establishing and confirming disease status in the U.S.
  • Validity of the existing standard - through epidemiological research, RDRP sought to verify the appropriateness of the British exposure-response data for the U.S. situation and recommend changes if needed.
  • Mechanisms of disease - explore, examine, and validate knowledge and assumptions about disease causation pertinent to the U.S. situation.

RDRP focused epidemiologic research on repeated cross-sectional studies of essentially the same group of coal mines. All working coal miners at the selected mines were invited to participate in medical examinations, which included chest radiograph, lung function test, and a questionnaire on smoking, work history, and demographic information. We obtained data on dust exposures from MSHA, and used these data with pre-1969 data to calculate estimated cumulative dust exposures for each miner. We then used the exposure data to derive exposure-response findings for CWP and lung function outcomes.

After the third round of cross-sectional surveys undertaken by RDRP, we concluded that the cross-sectional design was providing increasingly less-relevant information. Accordingly, for the fourth round (1985-1988) RDRP changed to a longitudinal (follow-up) design. Workers who attended the first two rounds of surveys and who were young enough in 1985 to still be at work were selected for medical follow-up. An important component of this study was the inclusion of ex-miners, thereby permitting assessment of any bias in past studies that were restricted to active miners (i.e., fit enough to remain at work). As before, medical outcomes were investigated for exposure-response using miner exposure estimates developed from MSHA data in partnership with collaborating researchers at the University of Washington.

The main emphasis in RDRP’s coal-related research has been on underground miners, not only because of their greater numbers but because dust exposures were generally much greater underground. However, because the health of surface coal miners is also of concern, limited epidemiological research took place on surface miners between 1970 and 1990. Because the main outcome of concern among surface miners was found to be silicosis among rock drillers, these and later studies are presented under silicosis (A3-1).

By the early 1990s, the depth and extent of knowledge gained through epidemiological studies was deemed sufficient for RDRP to develop a criteria document for coal mine dust. At this point, the focus of field work in coal mining moved largely from epidemiologic investigations to surveillance and prevention activities.

From the very beginning, RDRP initiated extensive laboratory research on etiologic and causative factors, and mechanisms using toxicological, physiologic, and pathologic approaches. A major part of this research involved the Coal Workers’ Autopsy Study data, a repository of tissue and slide samples and associated information, (some of this laboratory-based work concerning the effects of silica exposure and genetics of silicosis is presented in chapter 3.2c).

Findings from RDRP research showed that coal miners with long work tenure in the mines had considerable levels of disease. These findings supported the pre-1969 observations on disease prevalence, and so justified the need for strict dust control in the mines. The results also pinpointed areas where miners appeared at greater risk, indicating locations, jobs, and tasks for which particular emphasis should be placed on reducing dust exposures. In addition, the epidemiologic research, culminating in new exposure-response models for U.S. coal miners, indicated that the original information and assumptions employed in the setting of the U.S. standard were inadequate. In particular, it was shown that, contrary to the zero risk assumed earlier, simple pneumoconiosis could develop at dust levels lower than 2 mg/m3. It was also shown that progressive massive fibrosis could develop much more often than originally suspected from early stages of simple pneumoconiosis. Thus, the strategy originally employed for prevention of severe pneumoconiosis was shown to be somewhat flawed.

Our findings also indicated that miners were at different risks of disease development depending on coal rank. Anthracite miners were clearly at greater risk of CWP than bituminous miners, a finding that has been reported in other countries. We confirmed a finding in British miners that the risk of progressive massive fibrosis was significant among those with even mild CWP. This indicates that early disease development may be more problematic than we thought previously and the current U.S. coal mine dust limit is not completely protective.

Outputs and Transfer

This new information from RDRP research was incorporated, along with findings on non-pneumoconiotic disease outcomes, in a NIOSH criteria document “Occupational Exposure to Respirable Coal Mine Dust” (A3-2), which recommended a new lower limit for coal mine dust exposures, along with other measures to prevent disease occurrence. Many of our research findings on pneumoconiosis were cited by MSHA in a “Notice of Proposed Rule” published in July, 2000, “Determination of Concentration of Respirable Coal Mine Dust” (A3-3). In February 1998, based on findings reported in the coal mine dust criteria document and further joint analyses, MSHA and NIOSH co-published a Final Notice of Joint Finding (concerning the accuracy of single-shift sampling of coal mine dust) that was accompanied by a notice describing MSHA’s final policy on “Coal Mine Respirable Dust Standard Noncompliance Determinations” (A3-4).

Over the life of the program, 37 peer-reviewed papers, book chapters, and reports have been published by RDRP authors on field studies research and associated methodology relating to CWP morbidity and mortality and their relationship to dust exposure or its surrogates. Of the 37, 30 percent were published after 1995, reflecting the de-emphasis on epidemiological research in favor of surveillance and prevention in the current program following publication of the coal mine dust criteria document (A3-5). RDRP publications included a very important pair of papers which showed exposure-response data for CWP modeled against cumulative dust exposure and how the cumulative dust exposures for coal miners were generated (1, 2; A3-6, A3-7). Together, the two papers on these topics have been cited over 59 times in the scientific literature. Among surface coal miners, a critical result was the finding of severe silicosis among rock drillers. On the basis of this finding, our researchers published several peer-reviewed papers and produced a NIOSH Alert warning employers and employees of the severe risk of improper operation of drilling equipment.

Publications based on laboratory research studies accounted for just under half of the total publications by authors in RDRP on lung health and associated risks among coal miners (52 of 123). Of these, 28 were published after 1995 (A3-8). Mechanistic studies were of greatest interest, a paper on the effect of free radicals on disease development was cited in the scientific literature 53 times (3, A3-9) and a paper on cellular response to coal and silica dust was cited 38 times (4, A3-10). Another noteworthy study examined the effect of clay coatings on ameliorating the toxicity of silica in coal mining for pneumoconiosis development (5, A3-11); the method and initial findings were referenced in World Health Organization’s (WHO) IARC 1997 monograph on the evaluation of carcinogenic risks to humans and are also included in the 2000 “Encyclopedia of Analytical Chemistry.”

A 2006 count of citations of RDRP research on pneumoconiosis in coal miners revealed over 1100 distinct entries. This is probably an undercount because papers published in the early years are not well represented in citation indices. RDRP authors have written the definitive descriptions of CWP in major text books.

RDRP assisted MSHA in its investigation into fraud in dust sampling in underground mines (The Special Inspection Program) by providing an analysis of the inspector-gathered dust samples. The findings indicated that small mines put their employees at greater risk than did large companies. In general, our researchers have maintained close ties with enforcement colleagues and are currently collaborating with MSHA on targeted efforts to prevent CWP.

RDRP scientists were consultants and contributors to the Secretary of Labor’s Advisory Committee on the Elimination of Pneumoconiosis among Coal Miners, a selected group of experts that met in 1996 and provided recommendations to MSHA for the elimination of pneumoconiosis among coal miners.

Intermediate Outcomes

MSHA convened an advisory committee following receipt of the coal mine dust criteria document and has responded to at least some of the advisory committee's recommendations. These MSHA responses include: expanding federal sampling beyond once annually to 75 percent of the nation's coal mines; initiating a new program of monthly spot inspections at mines identified as having difficulty complying with the respirable dust standards; initiating a nationwide awareness program on the hazards associated with exposure to excessive levels of respirable coal mine dust and quartz dust and on ways to prevent lung diseases; revising criteria used for targeting mines for increased health enforcement emphasis; and finalizing Federal Register notices to enable inspectors to make noncompliance determinations based on the results of a single-sample measurement. MSHA continues to rely upon the coal mine dust criteria document for guidance (http://www.msha.gov/S&HINFO/NUMOADV/ISSU1&2.HTM [External link]).

Similarly, RDRP epidemiologic research results were employed by MSHA in a recent proposed rule (A3-4), while information on RDRP epidemiologic and laboratory research was recently provided to MSHA in support of the possible adoption of the personal dust monitors on a routine basis in mines for enhancing detection of overexposures to coal mine dust. As part of a recent National Academies review of NIOSH Mining Research Program in February 2006, senior MSHA leadership pointed out that “MSHA’s partnership with NIOSH is more important than ever,” with NIOSH providing “in depth scientific knowledge” and “long-term scientific research capability” (A3-12).

Progress Towards End Outcomes

By revealing the full extent of occupational respiratory disease morbidity and mortality, both in terms of severity (functional impairment and premature death) and breadth (pneumoconiosis and obstructive airways disease), RDRP through its research contributed together with many others, including MSHA and the coal mining industry, to reducing dust exposures in underground coal mines, a leading indicator of potential success in preventing pneumoconiosis and other occupational respiratory diseases (Figure 6). In turn, reduced dust levels have been accompanied by a marked drop in pneumoconiosis prevalence among active miners over the past three decades (chapter 3.1c, and “Work-Related Lung Disease (WoRLD) Surveillance Report” A3-13).

Figure 6: Respirable coal mine dust concentrations as measured for high-risk occupations monitored in the MSHA operator sampling program, and prevalence of Coal Workers' Pneumoconiosis among participants in the Coal Workers' Health Surveillance Program with 30 or more years of tenure in underground coal mining, 1970-2005.

Figure 6

 

What’s Ahead

Discussed together with “What’s Ahead” in chapter 3.1b, “Tracking Disease Occurrence in Coal Miners.”

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