Work‐related mesothelioma in Québec, 1967–1990

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Abstract
Prior surveys of malignant mesothelioma in Québec have noted that almost all the excess in occupational exposure related mesothelioma was in the manufacture and industrial application of asbestos rather than in the minign and milling operations. To evaluate the current status of malignant pleural mesothelioma in the Québec workforce, we reviewed all cases of pleural mesothelioma seen and accepted by the Québec Workman's Compensation Board (CSST) for work related compensation of industrial disease. We identified 120 cases, 7 of whom were females. They were of an average age of 59 ± 8.5 yrs (sd) (range 42–84); they were exposed to asbestos dust in the workplace for an average of 26 ± 14.3 yrs (range 0.5–50). The cases were subdivided into 3 groups according to workplace asbestos exposures. There were 49 cases originating in the mines and mills of the Québec Eastern Township region (primary industry, group 1), 50 cases from the manufacture and industrial application sector (secondary industry, group 2), and 21 cases from industries where asbestos was not a major work material, often an “incidental” material (tertiary industry, group 3). Group 1 was of an average age of 62 ± 8 years, exposed to asbestos dust 31 ± 14 years and the distribution of exposure time was as follows: 15% cases with ≤ 10 year‐exposure and 77% ≥ 25 year‐exposure. In group 2, the age was significantly lower at 57 ± 9 years; the exposure time was also significantly lower at 22 ± 14 years, and the distribution of exposure time differed from the above (29% cases with ≤ 10 year‐exposure and 48% ≥ 25 year‐exposure). In group 3, the average age was 58 ± 7 years, the exposure time was also significantly lower at 28 ± 12 years and the distribution of exposure time differed from the above (33% cases with ± 10 year‐exposure and 62% ≥ 25 year‐exposure). Analyses of the yearly incidence of new cases in each group documented the general incremental trend in all groups, with the sharpest rises in group 3. In the mining towns of Thetford and Asbestos, the incidence of mesothelioma was proportional to the workforce, thus suggesting that the tremolite air contamination, which is 7.5 × higher in Thetford, may not be a significant determinant of the disease in these workers. The incidence of the disease in these chrysotile miners and millers was 62.5 cases per million per year for the 1980–1990 period, well above the rate for the North American population, estimated at between 2.5 to 15 cases per year per million adult males. In conclusion, we observed an increasing number of work‐related mesothelioma in the 1967–1990 period in Québec. The incidence of pleural mesothelioma in chrysotile miners and millers, although not as high as in the crocidolite workers, is well above the North American male rate. Comparative analyses of incidence of the disease in the 2 mining towns suggest that tremolite contamination may not be a determining factor in these chrysotile workers. Whereas in the primary industry, the majority of cases are found in long‐term workers, in the secondary and tertiary sectors, the fraction of cases with short exposures is twice as high. The incidence of work‐related mesothelioma in the tertiary sector appears to be increasing more rapidly than in other sectors, and constituted 33% of all cases in the last 4year period of our study.