DUST EXPOSURE AND LUNG-CANCER IN QUEBEC CHRYSOTILE MINERS AND MILLERS

A large cohort of men born between 1891 and 1920 and employed for at l east a month in the chrysotile producing industry of Quebec has been u nder study since 1966. These men were followed from first employment ( the earliest in 1904) to 1992, by which time over 8000 had died, 657 f rom lung cancer. The current study is of 488 cases of lung cancer form erly employed at three places, viz. a major complex, here called Compa ny 3, in the region of Thetford Mines (243 cases), the mine and mill i n the town of Asbestos (206) and a small asbestos-products factory in the same town (39). For each case, four referents were sought by rando m selection from among survivors to a greater age, after matching on p lace of employment, age of starting work, smoking habit and date of bi rth. This process was highly successful, although six cases had less t han four referents. For each man (the 488 cases with 1941 referents) a cid for each calendar year of employment, we obtained the fraction of the year worked at various levels of intensity, assessed in 13 'dust c ategories' of mpcf (million particles per cubic foot). We then calcula ted how many years each man spent at these various levels; these Sears , adjusted for the length of the working week (66 h until 1937; 48 h 1 938-1949; and 40 h 1950-1985), were accumulated up to ten years before the death of the case. The men were classified according as they vr e re non-or ex-smokers, or smokers, of cigarettes. For each man at Compa ny 3 and one referent for each, his years of work in a central area of five mines and in a peripheral area of ten mines were differentiated; contamination of the chrysotile by fibrous tremolite was known to be much greater in the central than in the peripheral area. Case-referent comparisons, within place of employment, were made by conditional log istic regression. As anticipated from earlier subject-years analyses, lung cancer risks were found to be negligible for years worked in dust categories 1 and 2 (averaging 0.5 and 2 mpcf), regardless of place; a s the upper limit of category 1 is considerably higher than permitted nowadays, the lung cancer risk from exposure to chrysotile at permitte d levels can be taken as extremely small. Patterns of exposure-respons e for higher categories were irregular. At Company 3, some risks appea red elevated for years spent in the higher dust categories: 3-4, 5-7, 8-10 and 11-13, with averages around 9, 20, 36 and 92 mpcf, respective ly. For categories 3-4 :Ind 8-10, the odds ratios were high for some o r all work in the central area, but minimal for Sears spent in the per ipheral area only. Odds ratios were fairly low for cigarette smokers w ho worked in categories 5-7 and also for years spent in the highest ca tegories (11-13). At the mine and mill in Asbestos, all risks were low except for years worked by non-and ex-smokers in categories 7-13 (ca. 40 mpcf). There were no increased risks at the factory. It was known from the subject-years analyses that most of the excess had occurred a t Company 3, but it is now clear that for all practical purposes it wa s confined to the central area there, probably due largely to fibrous tremolite and in dust conditions of at least dust category 3. The aver age of this category was 7 mpcf or very roughly 24 fibres/ml, about tw o orders of magnitude higher than today's hygiene standards. (C) 1998 British Occupational Hygiene Society. Published by Elsevier Science Lt d.