MESOTHELIOMA IN QUEBEC CHRYSOTILE MINERS AND MILLERS - EPIDEMIOLOGY AND ETIOLOGY

In a cohort of some 11 000 men born 1891-1920 and employed in the Queb ec chrysotile production industry, including a small asbestos products factory, of 9780 men who survived into 1936, 8009 are known to have d ied before 1993, 38 probably from mesothelioma-33 in miners and miller s and five in factory workers. Among the 5041 miners and millers at Th etford Mines, there had been 4125 deaths from all causes, including 25 (0.61%) from mesothelioma, a rate of 33.7 per 100 000 subject-years; the corresponding figures for the 4031 men at Asbestos were eight out of 3331 (0.24%, or 13.2 per 100 000 subject-years). At the factory in Asbestos, where all 708 employees were potentially exposed to crocidol ite and/or amosite, there were 553 deaths, of which five (0.90%) were due to mesothelioma; the rate of 46.2 per 100 000 subject-years was 3. 5 times higher than among the local miners and millers. Six of the 33 cases in miners and millers were in men employed from 2 to 5 years and who might have been exposed to asbestos elsewhere; otherwise, the 22 cases at Thetford were in men employed 20 years or more and the five a t Asbestos for at least 30 years. The cases at Thetford were more comm on in miners than in millers, whereas those at Asbestos were all in mi llers. Within Thetford Mines, case-referent analyses showed a substant ially increased risk associated with years of employment in a circumsc ribed group of five mines (Area A), but not in a peripherally distribu ted group of ten mines (Area B); nor was the risk related to years emp loyed at Asbestos, either at the mine and mill or at the factory. Ther e was no indication that risks were affected by the level of dust expo sure. A similar pattern in the prevalence of pleural calcification had been observed at Thetford Mines in the 1970s. These geographical diff erences, both within the Thetford region and between it and Asbestos, suggest that the explanation is mineralogical. Lung tissue analyses sh owed that the concentration of tremolite fibres was much higher in Are a A than in Area B, a finding compatible with geological knowledge of the region. These findings, probably related to the far greater bioper sistence of amphibole fibres than chrysotile, have important implicati ons in the control of asbestos related disease and for wider aspects o f fibre toxicology. (C) 1997 British Occupational Hygiene Society. Pub lished by Elsevier Science Ltd.