Numerous epidemiological studies, discussed at length in HEI-AR, have compared the incidence of lung cancer and mesothelioma in various situations. Moreover, the ratio of mesothelioma/lung cancer numbers has been compared according to fibre types. However, the observed discrepancies between cohorts may be due to the fact that it was retrospectively difficult to know exactly what types of asbestos have been used in these plants.
McDonald et al. had the opportunity to evaluate the incidence of mesothelioma deaths in different contrasted cohorts:
i) workers exposed only to crocidolite during gas mask manufacture;
ii) miners exposed to pure chrysotile;
iii) workers in a textile manufacture exposed to a mixture of chrysotile, amosite and
crocidolite.
The proportional mortality due to mesothelioma (per 1000) increased from pure chrysotile to pure crocidolite, being intermediate for the mixed fibres. We can deduce from
comparing different cohorts that those where workers had been exposed to commercial amphiboles (crocidolite, amosite, anthophyllite), or to a mixture of amphiboles with chrysotile, were associated with a high mortality rate from mesothelioma (20 to 86 per thousand), compared with the much lower proportional mortalityratios for workers exposed to pure chrysotile.
The presence of fibres in the parietal pleural tissue, where mesothelioma usually starts, has rarely been studied. Recently, Boutin et al. have demonstrated the presence of a significant amount of fibres in biopsies of parietal pleura.
The electron microscopic analysis of tissue samples obtained through thoracoscopy has shown high focal concentrations of asbestos, mainly long crocidolite fibres and also, in one case, tremolite fibres in a man possibly environmentally exposed in Corsica. The hypothesis of a focal accumulation of long durable fibres in close contact with the target mesothelial cells is worthy of serious examination.
Although a majority of studies considered that mesothelioma was more related to significant exposure to amphibole fibres (++) than to chrysotile (+ ou -), we must mention that there are some exceptions or some disagreement:
i) Woitowitz and Rôdelsperger published a different opinion when reporting a study of
615 samples of respirable air collected by the German Institute of Occupational
Safety.
In the friction lining industry, only chrysotile was present in 611 samples; by contrast, crocidolite was discovered only four times in a single plant between 1981 and 1982. Thus, the authors reported to have observed an increased incidence of mesothelioma cases among car mechanics who have been exposed only to chrysotile, with a mean dose of less than 1 f/mL-years.
ii) Smith and Wright published a paper entitled ‘Chrysotile asbestos is the main cause
of pleural mesothelioma’, which is not convincing.
iii) The mathematical model used recently by Nicholson and Raffn on cancer due to
asbestos in the USA and Denmark reached the conclusion that the ‘data speak strongly
that much of the mesothelioma risk is predominantly from exposure to chrysotile’.
However, in the context of mesothelioma, in France, an electron microscopic analysis of lung tissue samples showed that crocidolite fibres were found at higher concentrations than chrysotile in mesothelioma cases compared with lung cancer patients or control cases (cardiac surgery). Edward et al. made the same observation.
McDonald et al. had the opportunity to evaluate the incidence of mesothelioma deaths in different contrasted cohorts:
i) workers exposed only to crocidolite during gas mask manufacture;
ii) miners exposed to pure chrysotile;
iii) workers in a textile manufacture exposed to a mixture of chrysotile, amosite and
crocidolite.
The proportional mortality due to mesothelioma (per 1000) increased from pure chrysotile to pure crocidolite, being intermediate for the mixed fibres. We can deduce from
comparing different cohorts that those where workers had been exposed to commercial amphiboles (crocidolite, amosite, anthophyllite), or to a mixture of amphiboles with chrysotile, were associated with a high mortality rate from mesothelioma (20 to 86 per thousand), compared with the much lower proportional mortalityratios for workers exposed to pure chrysotile.
The presence of fibres in the parietal pleural tissue, where mesothelioma usually starts, has rarely been studied. Recently, Boutin et al. have demonstrated the presence of a significant amount of fibres in biopsies of parietal pleura.
The electron microscopic analysis of tissue samples obtained through thoracoscopy has shown high focal concentrations of asbestos, mainly long crocidolite fibres and also, in one case, tremolite fibres in a man possibly environmentally exposed in Corsica. The hypothesis of a focal accumulation of long durable fibres in close contact with the target mesothelial cells is worthy of serious examination.
Although a majority of studies considered that mesothelioma was more related to significant exposure to amphibole fibres (++) than to chrysotile (+ ou -), we must mention that there are some exceptions or some disagreement:
i) Woitowitz and Rôdelsperger published a different opinion when reporting a study of
615 samples of respirable air collected by the German Institute of Occupational
Safety.
In the friction lining industry, only chrysotile was present in 611 samples; by contrast, crocidolite was discovered only four times in a single plant between 1981 and 1982. Thus, the authors reported to have observed an increased incidence of mesothelioma cases among car mechanics who have been exposed only to chrysotile, with a mean dose of less than 1 f/mL-years.
ii) Smith and Wright published a paper entitled ‘Chrysotile asbestos is the main cause
of pleural mesothelioma’, which is not convincing.
iii) The mathematical model used recently by Nicholson and Raffn on cancer due to
asbestos in the USA and Denmark reached the conclusion that the ‘data speak strongly
that much of the mesothelioma risk is predominantly from exposure to chrysotile’.
However, in the context of mesothelioma, in France, an electron microscopic analysis of lung tissue samples showed that crocidolite fibres were found at higher concentrations than chrysotile in mesothelioma cases compared with lung cancer patients or control cases (cardiac surgery). Edward et al. made the same observation.
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