Asbestos Risks -
Malignant Mesothelioma, asbestos lung cancer
What is Mesothelioma?
"Mesothelioma (malignant)"
Mesothelioma is a rare form of cancer. It affects the linings of the cavities around the lungs, stomach, and heart. It is caused by inhaling asbestos fibers, but the cancer usually does not appear until 10 to 40 years after a person first inhales asbestos.
Asbestos Risks
I wish to single out the problem of asbestos-induced cancers due to exposure to friction materials. In the Final Rule of 1989 (1), EPA attributes up to 144 projected cases of cancer to exposure to friction materials. These risks account for the majority of all risks in the Final Rule. These risk assessments were obtained using exposure-response relationships for cancer in different industries and in populations exposed to different asbestos materials of which the friction materials in only one.
In their study of more than 13,500 workers manufacturing friction materials in the period 1942-1980, Berry and Newhouse (67) found little excess cancer and the only excess mortality comprised 10 deaths from pleural mesothelioma, out of which 8 at least partly due to exposure to crocidolite.
The slope for increased lung cancer risk was only 0.00058 fibres /ml years. McDonald and co-workers (68) found practically no lung cancer risk and no mesothelioma in the group of long-term workers and in higher exposure categories in their study of more than 3,500 men employed in the manufacturing of friction products in the period 1938 Ð 1958. The slope for increased lung cancer risk was practically zero. The authors interpreted the results as 'doubtful whether there was any significant lung cancer excess".
I strongly disagreed (57) with the approach to the estimation of the projected number of cancers using the mean of slopes derived in all studies, of which only two (by far the lowest) were obtained in the friction products exposures. The population with expected exposure of asbestos fibres are garage mechanics, because of their work on the maintenance and repair of automobile asbestos containing brakes and clutches.
In a large case-control survey of all cases of mesothelioma diagnosed by pathologists in the USA and Canada during a defined period, McDonald (69) observed a substantial excess risk of mesothelioma in many occupations with exposure to asbestos, and particularly to amphiboles, but no excess was observed in the category of garage mechanics.
In 1988 (70), I analyzed all the available literature regarding asbestos risk in vehicle manufacture, maintenance and repair, and concluded that, provided good work practices are followed and no amphiboles are used, detectable risks in vehicle maintenance and repair are not to be expected.
As in 1991 (56) and 1993 (57), I still disagree with the EPA's approach to the estimation of the projected number of cancers due to exposure to friction materials by using a mean slope of 11 studies (1,36), of which only two (having by far the lowest slopes) were obtained in the friction products exposure.
It is hardly justifiable to estimate risks due to exposure to one type of fibre population by using the slopes obtained in exposure to completely different fibre populations, while being fully aware of the large variations among the slopes. This approach has resulted in an ungrounded overestimation of the projected number of cancers in exposure to friction materials.
Unfeasibility of practical application of risk assessments
As early as in 1988 and later in 1993, I pointed to the implications and practical unacceptability of the results of some well-known published asbestos risk estimates (55,57). Table 9 shows my calculations of exposure limits for asbestos in the atmosphere derived from some of these risk assessments.
A 1986 WHO Expert Meeting proposed the lifetime risk estimate for smokers (mesothelioma: 12 x 10 5, lung cancer: 16 x 10 5 as upper limits of the number of expected deaths per 100,000 population) at an assumed airborne asbestos fibre concentration of 500 f/m3 (71). Assuming that the acceptable risk, used for carcinogens in the WHO Water Quality Guidelines (72), is 1 x 10 5, the calculated exposure limit is 18 fibres per cubic meter of air.
Taking the risk estimate of 13.5 x 10 5 for nonsmokers and using the same acceptable risk (1 x 10 5), the obtained exposure limit is 37 fibres per cubic meter. Confronted with prevalent concentrations found in the air of rural areas with no specific asbestos sources (up to 100 f/m3) (7), these exposure limits seem to suggest that in areas without any specific source of asbestos emission, a nearly 6-fold reduction of current asbestos levels would be required, which is practically impossible to achieve.
Table 9 also illustrates that an exposure limit of 45 asbestos fibres per cubic meter can be derived from the asbestos risk estimate published in the WHO Air Quality Guidelines of 1987 (11 x 10 5 for a population with the hypothetical proportion of 30% smokers) (73). This value is lower or as low as the concentrations found in rural areas without specific asbestos emission. The table also shows prevalent asbestos fibre concentrations in urban areas (from fewer than 100 to 10,000 per cubic meter) (7).
People exposed to asbestos fibers are at risk of developing an asbestos lung disease. Asbestos lung disease typically has a high fatality rate. People with asbestosis are more likely to develop asbestos lung cancer; smokers exposed to asbestos are 55 times more likely to develop asbestos lung cancer than non-smokers.