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Canada Releases Chrysotile Asbestos Risk Report
By asbestoshub | May 17, 2009
The Annals of Occupational Hygiene has published a long-awaited report about a debate on the risks of chrysotile, initiated by Health Canada.
The report discusses two meta-analyses of asbestos epidemiological studies for which exposure–response relationships could be estimated, specifically for the risks of chrysotile asbestos mined in Canada. These were Hodgson and Darnton (2000) (here designated H&D) and Berman and Crump (B&C) and general conclusions are as follows:
(1) Chrysotile often occurs in association with tremolite, which is an amphibole and much more persistent in the lung, and it is reasonable to expect that presence of a small amount of tremolite may increase the apparent hazard of the ‘chrysotile’. H&D did not try to discriminate chrysotile cohorts according to the amount of tremolite, so the risk assigned by H&D to chrysotile may include an unknown contribution from tremolite. B&C used available information on the percentage of tremolite present, which is often incomplete, to try to isolate and remove the effect of tremolite from that of chrysotile, so insofar as this was successful, B&C’s estimates are for pure chrysotile without tremolite.
(2) For each cohort, H&D derived an average exposure and an average disease risk and used the results from different cohorts to estimate overall dose–effect relationships. B&C used the disease and exposure information on different subcohorts within each study and estimated the exposure–risk relationship within each study. This meant that H&D could use studies for which there was only one average exposure given, and their single aggregate exposure estimate per cohort was less vulnerable to misclassification of individuals. On the other hand, B&C’s approach enabled them to more systematically adjust for different background lung cancer rates in the cohorts.
(3) In line with classic asbestos risk assessments, B&C assumed linear relationships between disease and exposure, even though they observed some non-linearity for both lung cancer and mesothelioma, but for mesothelioma they assumed that risk depended on the cube of the time since first exposure. In contrast, H&D estimated the best-fitting non-linear relationships. The main difference between the linear and non-linear models shows at low exposures, where there are few epidemiological data to distinguish between them and where the risks are low.
(4) H&D distinguished between pleural and peritoneal mesotheliomas. B&C pooled the two mesotheliomas as one disease.
(5) B&C’s estimates include the Charleston cohort (see below), but H&D’s figures in the table exclude it. They stated the Charleston estimates separately as ‘exceptional’ (unexplainably high) but possible.
Topics: Exposure |
