Asbestos Exposure - Mesothelioma, Lung Cancer, Fibrosis
7.1.2 Lung cancer and mesothelioma
It has been suggested that in the absence of pulmonary fibrosis, lung cancer cannot be attributed to asbestos exposure regardless of fibre type; however, there is also evidence to the contrary. For example, in a recent case-control study, there was evidence of a statistically significant increase in risk of lung cancer without radiological signs of fibrosis (Wilkinson et al., 1995). The question remains the subject of active controversy (Hughes & Weill, 1991; Henderson et al., 1997). Results of cohort studies of workers almost exclusively exposed to chrysotile asbestos and considered by the Task Group to be most relevant to this evaluation are summarized in Table 23 and described in section 7.1.2.1. Studies that contribute less to our understanding of the effects of chrysotile, due primarily to concomitant exposure to amphiboles or to limitations of design and reporting, are presented in section 7.1.2.2. Information most relevant to characterization of risk (i.e. exposure-response assessment) is emphasized.
Assessment of exposure response for mesothelioma is complicated in epidemiological studies by factors such as the rarity of the disease, the lack of mortality rates in the populations used as reference and problems in diagnosis and reporting. In many cases, therefore, cruder indicators of risk have been developed, such as absolute numbers of cases and death and ratios of mesothelioma over lung cancers or total deaths. The mesothelioma/lung cancer ratio in particular is highly variable depending on the industry and the nature and intensity of asbestos exposure, in addition to a number of factors not related to asbestos exposure. Data on mesothelioma occurrence in occupational cohorts should, therefore, be cautiously interpreted.
For the studies reviewed here, the number of mesothelioma deaths is reported, together with the percentage over total deaths (Table 23). It should be noted, however, that additional cases of mesothelioma have been reported in workers from the factories included in the studies reported in Table 23 who were not included in the original cohort studies. However, in the absence of information on the numbers of workers at risk, such reports do not contribute to quantification of risk.
7.1.2.1 Critical occupational cohort studies - chrysotile
a) Mining and milling
Mortality from lung cancer and mesothelioma has been studied extensively in miners and millers of Quebec and in a smaller operation at Balangero in northern Italy. In 1966, a cohort of some 11 000 men and 440 women, born between 1891 and 1920, who had worked for one month or more in chrysotile production in Asbestos and Thetford Mines and 400 persons employed in a small mixed asbestos products factory in Asbestos, Canada, was identified. The cohort, which has now been followed up to 1988, was selected from a register compiled of all workers, nearly 30 000, ever known to have been employed in the industry. The factory workers were included because there was frequent and often unrecorded movement between the plant and the mine and mill. Apart from a failure to trace 9% of the cohort, most after less than 12 months' employment before 1930, losses have amounted to well under l%. The intensity of exposure was estimated for each cohort member by year, based on many thousand midget impinger dust particle counts and, more recently, membrane filter fibre counts.
The most relevant analyses of this cohort are those published by McDonald et al. (1980) and McDonald et al. (1993), and in a preliminary fashion by Liddell (1994). In the first of these reports, where 4463 men had died, the standardized mortality ratio (SMR) for men 20 or more years after first employment, assessed against provincial rates, was 1.09 for all causes and 1.25 for lung cancer. There was no excess mortality for lung cancer in men employed for less than 5 years, but at 5 years and above there were clear excesses. Based on analysis by cumulative exposure up to age 45, there was a linear relationship with lung cancer risk.