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Pneumoconiosis in Coal Miners: NIOSH Research and Surveillance
Michael D. Attfield Epidemiological Investigations Branch Division of Respiratory Disease Studies National Institute for Occupational Safety and Health
The National Institute for Occupational Safety and Health (NIOSH) administers two programs that permit the Institute to monitor the effectiveness of the respirable dust standard of 2 mg dust/m3 air in preventing new occurrences of coal workers' pneumoconiosis (CWP). The National Coal Workers Health Surveillance Program was mandated by the Mine Health and Safety Act of 1969; it offers free periodic x-ray examinations to all underground coal miners in the United States. The program, which is mandatory for the operator but voluntary for the miner, was begun in 1970. The National Coal Study (NCS) is a long-term epidemiologic study, limited to workers in a selected group of mines with various seam heights, mining methods, coal types, and geographic locations. The following is a report of some of the results from these two programs.
Posteroanterior x-ray examinations are offered through the National Coal Workers Health Surveillance Program. The x-ray films are taken at over 250 NIOSH-certified facilities and interpreted by two or more physicians, also certified by NIOSH. Examinations in the NCS may include, in addition, a lateral x-ray film, lung-function tests, a respiratory-symptom questionnaire, and a record of work history, smoking history, and demographic characteristics.
Categorization of X rays for pneumoconiosis is undertaken using international classification schemes (see the ILO U/C 1971 scheme ð1tå, for example). Films are read for opacities of both round and irregular types. Rounded opacities fall into three grades of abnormality: category 1, in which the normal lung markings are still visible; category 2, in which the lung markings are partially obscured; and category 3, in which the opacities are so numerous that the normal markings are totally obscured. The most advanced stage of CWP is progressive massive fibrosis (seen as large opacities). Final determinations, made for the purpose of reporting and analysis, are obtained by combining the information from the multiple readings according to defined schemes. Final determinations of category 1 or greater are used to identify CWP in the following presentation.
These studies, which are divided into approximately concurrent "rounds," have involved over 300,000 x-ray examinations. The first round began August 19, 1970, when the original regulations governing the examinations were published, and ended on December 30, 1971(2t). The second round began when revised regulations became effective July 27, 1973, and ended March 31, 1975. The third round, which began on August 1, 1978, ended December 15, 1980.
The first-round data from the NCS, collected from 9,078 miners, showed an overall CWP prevalence of 30% and revealed clear trends that corresponded with observations made elsewhere (3,4t). The profusion and severity of combined opacities were related to both tenure and type of job in the mine (2t). Furthermore, prevalence of CWP decreased with geographic location of mine, in an east-to-west trend (2t). This trend supports the theory that the disease is more severe where the coal is older and contains more fixed carbon and less volatile matter, an observation made also in Europe (5,6t).
Results from the second round of the NCS (over 9,300 miners) demonstrated a pattern of results very similar to those of the first round regarding job, tenure, and geographic region (7t). However, in the second round the overall prevalence of small opacities appeared sharply reduced, from 30% to 8%. This reduction was caused in part by different criteria for the interpretation of x-ray films and by an influx of new, minimally exposed miners into the study, which diluted the apparent prevalence. No evidence was seen of a large exodus of miners with CWP. The overall prevalence of CWP in the third round, based on data for over 5,000 miners, was 5%, and the pattern of results was similar to that seen in the earlier rounds (Table 1). The prevalence of massive fibrosis was initially reported to be about 2% (2t), although later surveys indicate lower levels. Particular interest has been focused on the results for miners who have been on the job for 4A510 years. Since these miners worked predominantly under strict dust standards, few cases of CWP were expected.
There are several problems inherent in interpreting these results. First, participation of eligible coal workers in both the National Coal Miners Health Surveillance Program and the NCS is currently low; participation in the NCS was 98% in the first round, 74% in the second, and 60% in the third. Another difficulty common to all cross-sectional studies of persons in the workplace, is the increased likelihood of including only healthier individuals in the study. This "healthy worker" effect may bias findings toward underreporting ill health. Third, the inconsistencies in reading x-ray films illustrate the need for caution in interpreting radiologic findings and the need for careful design and control of reading trials. A fourth difficulty is the relatively short time between the first- and third-round surveys. The 10- to 12-year period may be too short for adequate examination of incidence for a condition such as CWP with a long period of development.
Data from the surveillance program should also be interpreted cautiously. A clear distinction must be made between the radiographic results of surveillance (i.e., screening for health status) and those read for more intensive epidemiologic studies. The criteria for defining CWP from the multiple readings differ and are more conservative (in favor of protecting the health of the miner) for the screening program (those from the National Coal Workers Health Surveillance Program). They may therefore tend to indicate greater prevalences of abnormality than do the readings from the epidemiologic research program (the NCS). In addition, the low participation and limited period of study may have tended to reduce reported prevalences.
Further analysis in the NCS is being focused on the group of miners who began work around the time of the first round of the study. Miners in this group worked first when the dust standard was 3 mg dust/mðcp6å3ðcp9,11å air (June 1970-December 1972) and then 2 mg/mðcp6å3ðcp9,11å (December 1972-present). The data used to set up the U.S. standards indicate that little pneumoconiosis should be expected to occur in this group. If the NCS data reveal a considerably higher attack rate than that expected, the standard and its method of enforcement may need to be reevaluated. (Analysis of the British data ð5,8tå from which the U.S. dust standard is derived indicates an attack rate of 2%-3% over 10 years in category 0/1.)
Because of these difficulties in the interpretation of the data currently available, future NIOSH surveys may monitor groups of miners from the first round and will involve longer periods of investigation. The merits of this approach include attention to specific groups of interest; the follow-up of former miners, which would eliminate any "healthy worker effect;" and the improvement in participation that would result from personal involvement rather than group contact.
Both the National Coal Workers Health Surveillance Program and the NCS benefit coal miners directly through the detection of CWP. As mandated by the Mine Health and Safety Act, miners who have x-ray evidence of CWP can choose to transfer to a less dusty area of the mine without any reduction in pay. Overall, the results indeed indicate that the dust regulations have been of benefit to coal miners, although it is not yet clear as to whether the regulations are sufficient to prevent CWP. Miners who have progressive massive fibrosis are notified that they qualify immediately for black-lung benefits. As a long-term goal, analysis of both these cohorts may provide useful information on the level of exposure that predisposes to CWP and on the appropriateness of the current dust standard and its method of enforcement.
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