We report a further follow-up of a birth cohort of 11 379 workers exposed to chrysotile. The cohort consisted of 10 939 men and 440 women, born 1891-1920, who had worked for at least a month in the mines and mills of Asbestos and Thetford Mines in Quebec. For all subjects, length of service and estimates of accumulated dust exposure were obtained, with a smoking history for the vast majority. Three methods of analysis, two based on the "man-years" methods, the other a "case-and-multiple-controls" approach, gave results consistent with one another and with previous analyses. By the end of 1975, 4463 men and 84 women had died. Among men, the overall excess mortality, 1926-75 was 2% at Asbestos and 10% at Thetford Mines, much the dustier region. The women, mostly employed at Asbestos, had a standardised mortality ratio (SMR) all causes, 1936-75) of 0.90. Analysis of deaths 20 years or more after first employment showed that in men with short service (less than five years) there was no discernible correlation with dust exposure. Among men employed at least 20 years, there were clear excesses in those exposed to the heaviest dust concentrations. Reanalysis in terms of exposure to age 45 showed definite and consistent trends for SMRs for total mortality, for lung cancer, and for pneumoconiosis to be higher the heavier the exposure. The response to increasing dose was effectively linear for lung cancer and for pneumoconiosis. Lung cancer deaths occurred in non-smokers, and showed a greater increase of incidence with increasing exposure than did lung cancer in smokers, but there was insufficient evidence to distinguish between multiplicative and additive risk models. There were no excess deaths from laryngeal cancer, but a clear association with smoking. Ten men and one woman died from pleural mesothelioma. If the only subjects studied had been the 1904 men with at least 20 years' employment in the lower dust concentrations, averaging 6.6 million particles per cubic foot (or about 20 fibres/cc), excess mortality would not have been considered statistically significant, except for pneumoconiosis. The inability of such a large epidemiological survey to detect increased risk at what, today, are considered unacceptable dust concentrations, and the consequent importance of exposure-response models are therefore emphasised.
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