Quebec
A large cohort of nearly 11,000 chrysotile miners, millers, and factory workers who worked in Asbestos and Thetford, Quebec, has been intensively studied. Thirty-six mesotheliomas were found for those first employed at the Asbestos mine/mill (8 cases), Asbestos factory (5 cases), and two Thetford Mines companies (23 cases), and there were two more cases that did not meet inclusion criteria (Liddell et al., 1997; also see Nicholson et al., 1979; McDonald et al., 1997). Exposures to fibers of amosite, crocidolite, tremolite, and chrysotile in the Canadian asbestos districts are well documented (case and Sebastien, 1987; Churg, 1998; Herman and Crump, 2003, section 3), even though tremolite was not detected in a chrysotile mixed sample from eight mines (Frank et al., 1998). Airborne exposures are indicated by lung tissue results of mesothelioma cases in Canada (McDonald et al., 1989; Sebastien et al., 1989; Churg et al., 1993; McDonald et al., 1997). Eleven mesotheliomas were observed in a population-based study of women (over 220,000 person-years) residing in Asbestos (1 peritoneal mesothelioma) and Thetford Mines (6 definite or probable and 4 possible pleural mesothelomas). Of the pleural cases, the mean cumulative exposure to asbestos was 226.1 f/ml-yr, and 5 of them worked in the asbestos industry. Ambient asbestos fibers were chrysotile contaminated with tremolite ranging from 0.1 to 3 f/ml before 1970. The mesothelioma incidence rates were 67.5 per million person-years in the Thetford area and 13.7 per million personyears in the Asbestos area. The authors explained that the greater risk in the Thetford district could have been due to the higher level of contamination of the chrysotile with tremolite in some of Thetford's mines (Camus et al., 2002; case et al., 2002a).