Mesotheliomas - Amphiboles Exposure, Chrysotile Mines
Although the odds ratio for lung cancer associated with exposure to "asbestos" has been estimated in many case-control studies, the studies have not been in general able to distinguish between chrysotile and amphibole exposure, and are therefore less informative for the present evaluation (see, for example, Kjuus et al., 1986). In a multisite case-control study from Montreal, Canada, however, exposures to chrysotile and to amphiboles were separated, although exposure to amphiboles was not controlled for in the analysis on exposure to chrysotile (Siemiatycki, 1991). In this study, the occupational history of male cases (aged 35-70) of cancer at 20 sites and of 533 population controls was evaluated by a team of industrial hygienists and chemists to assess exposure to 293 agents. Overall, the lifetime prevalence of exposure to chrysotile was 17%, and that of exposure to amphiboles, 6%.
The main occupations involving exposure to chrysotile that were considered were motor vehicle mechanics, welders and flame cutters, and stationary engineers. When lung cancer cases (N=857) were compared with cases of all other types of cancers, the odds ratio (OR) of any exposure to chrysotile was 1.2 (90% CI=1.0-1.5; 175 exposed cases), and that of 10 or more years of exposure with at least 5 years of latency ("substantial exposure") was 1.9 (90% CI 1.1-3.2; 30 exposed cases). Corresponding ORs of exposure to amphiboles were 1.0 and 0.9. The OR of exposure to chrysotile was higher for oat cell carcinoma than for other types of lung cancer. Twelve cases of mesothelioma were included in this study. The OR of any exposure to chrysotile was 4.4 (90% CI=1.6-11.9; 5 exposed cases) and that of substantial exposure was 14.6 (90% CI=3.5-60.5; 2 cases). Corresponding ORs of exposure to amphiboles were 7.2 (90% CI=2.6-19.9; 4 cases) and 51.6 (90% CI=12.3-99.9; 2 cases).
Based on analyses of mortality of workers with mixed exposures to chrysotile and amphiboles in the United Kingdom, by far the greatest proportion of mesotheliomas occurs in users of asbestos-containing products, rather than those involved in their production. In the United Kingdom, all death certificates that mention mesothelioma have been recorded since 1968, and 57 000 workers subject to the 1969 Asbestos Regulation or the 1984 Asbestos (Licensing) Regulations have been followed-up. Analyses of these data have led to the following conclusions:
Asbestos exposure caused approximately equal numbers of excess deaths from lung cancer (749 observed, 549 expected) and mesotheliomas (183 deaths) within the occupations covered by the 1969 and 1984 regulations (OPCS/HSE, 1995). 2. Only a few (5%) of British mesothelioma deaths were among workers in regulated occupations (Peto et al., 1995). The majority of deaths occurred in unregulated occupations in which asbestos-containing products are used, particularly in the construction industry. The risk was particularly high among electricians, plumbers and carpenters as well as among building workers.
Extensive case-control studies of 668 cases of mesothelioma as ascertained through pathologists were conducted by McDonald & McDonald (1980) throughout Canada (1960-1975) and the USA (in 1972). Relative risks were as follows: insulation work, 46.0; asbestos production and manufacture, 6.1; heating trades (other than insulation), 4.4. Four subjects were men who had been employed in Quebec chrysotile mines and three were children of employees; no other subjects had lived in the mining area. In some 12 listed occupations, there was no excess of cases over controls, e.g., garage work, carpentry, building maintenance.
Begin et al. (1992) analysed 120 successful claims for pleural mesothelioma submitted to the Quebec Workman's Compensation Board during 1967-1990. Of these, 49 cases occurred among workers in the mining and milling industry, 50 in the manufacturing and industrial application sector and 21 in other types of industry. The miners and millers were thought to be primarily exposed to chrysotile, while the rest were believed to be exposed to mixtures of amphiboles and chrysotile. The numbers of cases ascertained by Begin et al. via the compensation system were consistent with the numbers of incident mesotheliomas observed in miners and millers but grossly underestimated the recorded frequency of mesothelioma in the other industrial sectors (McDonald & McDonald, 1993).
In other large population-based case-control studies of mesothe-lioma (see, for example, Bignon & Brochard, 1995), it was not possible to separate the effect of chrysotile from that of amphiboles.
Attempts have been made by three groups of investigators to assess the contribution of chrysotile to mesothelioma risk by considering the duration of its use compared with other fibres. These analyses were based, in part, on models for the risk of mesothelioma associated with exposure to various forms of asbestos, which have been widely used by regulatory agencies in the USA, such as the Consumer Product Safety Commission (1987), the Environmental Protection Agency (1986) and the Occupational Safety and Health Administration (1986). Formulae for these models are similar (see, for example, the HEI report) and will not be described here in detail. The analyses include studies of insulation workers (Nicholson & Landrigan, 1994) and railroad machinists in the USA (Mancuso, 1988), and cement workers in Denmark (Raffn et al., 1989). Although the authors of these studies suggest the occurrence of mesothelioma prior to the widespread introduction of amphiboles into industries, there is unresolved controversy about the reliability of the data on which these conclusions are based. Motor mechanics who repair asbestos-containing brakes and clutches can be exposed to chrysotile, as this is by far the predominant fibre used in this application. Exposures can occur during removal of wear debris from brake and clutch assemblies and during grinding of new linings (Rohl et al., 1976; Rodelsperger et al., 1986). Cases of mesothelioma have been reported among brake mechanics (Langer & McCaughey, 1982; Woitowitz & Rodelsperger, 1991; Woitowitz & Rodelsperger, 1992).
In two case-control studies of mesotheliomas, there was no excess risk among garage workers or mechanics (Teta et al., 1983; Woitowitz & Rodelsperger, 1994). In the latter study, there were two control groups; one was based on hospital cases undergoing lung resection, in most instances because of lung cancer, and the other was from the general population. The authors noted that confounding due to asbestos exposure in other occupations limited their ability to detect mesothelioma risks among car mechanics.
The proportional mortality for mesothelioma among British motor mechanics was reported to be lower than the national average (PMR = 0.40) (OPCS/HSE, 1995). The extent to which all motor mechanics were exposed to friction products was not defined.