The largest epidemiological study in the asbestos industry was that of the miners and millers (those who crush the rock and extract the fibres) in Southern Quebec. Some 11,000 males born between 1891 and 1920 who worked in the industry were studied. Inevitably the researchers lost track of some of the men – there had been 1 or 2 world wars in their lifetime and some only worked for a few weeks and moved on. In total 9780 men were traced up to 1992. 1992. Of these, 8009 (82%) were known to have died: the most common causes of death were heart disease and stroke, which accounted for 3305 deaths (41.3%), followed by lung cancer (657, 8.2%), 38 died from mesothelioma, 1205 from other malignant disease, 108 from pneumoconiosis and 561 from other non-malignant respiratory diseases (excluding tuberculosis). Most of the cancer deaths could be attributed to smoking. The mines where they worked were grouped around either the town of Asbestos or the town of Thetford Mines.
Of the 38 deaths from mesothelioma in the cohort, 33 were in miners and millers—25 from Thetford Mines and 8 from Asbestos, and the remaining 5 were in an associated asbestos products factory. Those from Thetford mines were mostly from the areas found to be contaminated with tremolite ( a fibrous mineral similar to crocidolite). These men, born 1891-1920, had worked through years of very high dust exposure yet with no discernible effect on mortality from lung cancer below an accumulated exposure of about 1000 (fibres/ml) x years and no case of mesothelioma among over 4000 men employed for less than 2 years. The authors concluded that their studies of the Quebec chrysotile miners and millers consistently showed little evidence of a cancer risk except at very high levels of exposure.
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