A follow-up study of miners exposed to unusual concentrations of quartz

Since the 1950s, British Coal’s Pneumoconiosis Field Research (PFR) programme has been the principal source of epidemiological data on coalworkers’ pneumoconiosis in the UK. Many analyses of PFR data have concluded that exposure to respirable mixed coalmine dust generally, rather than to its quartz content specifically, was the primary determinant of both incidence and prevalence of coalworkers’ simple pneumoconiosis. Results on the specific role of quartz have been more equivocal, but the most detailed investigations have suggested that increasing the amount of quartz, mass for mass, in a specific dust increases the pneumoconiosis risk. However there has, until recently, been little information on the quantitative relationship between exposure to quartz and risks of pneumoconiosis, and associated lung function deficits.In 1980 the IOM reported that quartz concentrations in certain faces of a Scottish colliery reached unusually high levels for a period in the 1970s, due to unusual geological conditions. In the main, Scottish collieries tended to experience less pneumoconiosis than some other regions in Britain, but by 1978, a number of men had developed signs of radiological progression. The epidemiological results, and data from animal inhalation experiments, suggested that the response to the dust was directly related to its quartz content.This report describes a follow-up study of the population of men who worked and were surveyed by the PFR in this colliery during the 1970s. The study was undertaken to ascertain the current respiratory health status of these men, and to examine the quantitative relations between history of exposure to respirable quartz and risk of chest radiological and lung functional abnormality. The men were traced and invited to attend for an examination at which were taken a postero-anterior chest radiograph, a questionnaire on respiratory symptoms and smoking habits, a history of employment since leaving the colliery, and anthropometric and lung function measurements. A total of 551 men were seen between late 1990 and Spring 1991, representing 53% of the surviving population of men employed at this colliery in the 1970s.Extensive data were available on these men’s exposures to respirable dust and quartz throughout their work at the colliery. The assessment of exposures for the PFR was based on detailed and frequent measurements of dust concentrations in different occupational groups, and carefully maintained records of the times each man spent working in each of these groups. A wide range of quartz concentrations was observed, but for some periods, certain face occupations experienced concentrations of respirable quartz over 10 mg.m-3.For the present study, these concentration data were brought together, supplemented with data from the histories taken at the follow-up survey, and summarised as exposures to respirable dust and quartz in inter-survey periods of about four or five years in length, in order to permit some examination of temporal trends underlying any exposure-response relationships.The chest radiographs were examined and classified according to the ILO (1980) scheme by three experienced readers. For analysis, profusions of small opacities were summarised by the median of the three readings. On this basis, the observed prevalence of small opacities 2/1 or greater was 8.6% overall, but reached 20 to 40% in the groups with the highest quartz exposures.Logistic regression analyses of the profusion of small opacities recorded for the radiographs from the follow-up survey showed strong evidence of association between risks of displaying pneumoconiotic abnormalities and the exposures experienced during work at the colliery. The correlation between an individual’s exposures in different inter-survey periods, induced by continuity of occupational group across time, requires some caution in interpreting regression results. Comparison of the estimated regression coefficients and their statistical significance from a large number of models suggested that the strongest association was with exposures in the periods 1970 through 1974 to 1978, followed by 1964 to 1970; and that the association was stronger with the estimated exposure to the quartz fraction of the respirable dust, than with its non-quartz component. The specific association with the periods in the 1970s, and the association with quartz in particular, showed most strongly in analyses where the response was a small opacities profusion of 2/1 or higher. There was no evidence of an association with exposures prior to the third PFR survey in early 1964, and little evidence for any association with coalmine exposures accumulated after 1978.The results of these analyses have been used to calculate preliminary risk estimates, which give an approximate but useful indication of the magnitude of the risks involved. For example, a man aged 60 at survey and with fifteen years’ exposure to an average quartz concentration of 0.1 mg.m3 is predicted to have a risk of approximately 3% of showing small opacities of category 2/1 or greater at follow-up. With fifteen years’ exposure to 0.3 mg.m3, the prediction is a risk of 22%.There was a strong association between radiological abnormalities at follow-up and time spent working in certain occupational groups known to have had very high concentrations of respirable quartz. It has not, however, been possible as yet to identify all of the occupational groups associated with increased risks of developing pneumoconiotic abnormalities, and further work is needed here.The rapid progression observed in the radiological abnormalities, their relationship with quartz exposure estimates, and the strength of their relationship with lung function confirm suggestions that the effects of this unusual exposure are more similar to those of classical silicosis than to the patterns of pneumoconiosis observed in coalworkers. In turn, these findings are in line with two separate, though possibly complementary, theories: (1) that freshly fractured quartz may be much more aggressive in its effects than quartz whose surfaces are heavily contaminated with other ions; (2) that the unusual intensities (concentrations) of the quartz exposures experienced by some men induced proportionally higher risks in relation to exposure than those resulting from cumulative exposures at more usual quartz concentrations, conceivably by inducing a disproportionately severe and persisting inflammatory response.