Mesothelioma Cancer - Causes of Mesothelioma
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.
Mesothelioma Cancer
The evaluation of IPCS/WHO in 1986 (7) was: In the general population the risks of mesothelioma and lung cancer attributable to asbestos cannot be quantified reliably and are probably undetectably low.
Cigarette smoking is the major etiological factor in the production of lung cancer in the general population. The risk of asbestosis is virtually zero. However, the latest IPCS /WHO evaluation in 1998 (31) stated that no threshold had been identified for carcinogenic risks from chrysotile asbestos.
There is an almost general consensus that no threshold exists for amphiboles. There is still a controversy as to whether there is a threshold, or at least a practical threshold, for chrysotile. Studies are limited to only two industrial cohorts with relatively pure exposure to chrysotile fibres containing sufficient high quality data for exposure-response analysis.
These studies include the Quebec miners and millers (47-53, 59) and South Carolina textile workers (37-40). Table 4 shows standard mortality from lung cancer in Quebec miners and millers (48), 1976-1988, in relation to exposure accumulated up to the age of 55 years, and the lung cancer mortality by cumulative exposure in South Carolina workers (39) employed between 1940 and 1990.
There is no indication of a trend in standard mortality over 7 lowest categories of exposure of miners and millers (<10 -<990 f/ml yrs). The standard mortality was elevated at the three highest levels, i.e. at the cumulative exposure of more than 990 f/ml yrs. A completely different result was obtained in South Carolina textile workers.
There was a consistent increase in the risk of lung cancer with increasing cumulative exposure in all the exposure categories of cumulative exposure more than 2.7 f/ml yrs. The proportional mortality from mesothelioma in the Quebec cohort was only 0.45% (33 deaths among 7,312 workers) by end of 1988.
Comparing the very high slope of 0.021 per f/ml yr in textile workers with the very low slope of 0.0005 per f/ml yr in Quebec miners and millers, the authors of the last exposure response analysis (40) attribute this large difference to the considerably higher proportion of carcinogenic long fibres in the textile production.
It was on the basis of the results obtained in Quebec workers that the authors (48, 50, 53) concluded that chrysotile was not the cause of lung cancer, except at very high levels of exposure above 25-30 f/ml, well above current exposure even under poor conditions. Can the finding that there was no trend in standard mortality over 7 lowest exposure categories of miners and millers be taken as the basis for the conclusion that there is a practical threshold for chrysotile (49)?
The situation with mesothelioma is somewhat different. The standard mortality rates in several countries show an increasing trend. The results of some evaluations caused panic. British (14, 19), French (17), New Zealand (15), and the US (12,18) data projected thousands of deaths per year of mesothelioma in the decades to come.
As a considerable proportion of diagnosed mesothelioma was believed to be the consequence of exposure to asbestos fibres, there is a tendency to attribute all these deaths to the effects of these fibres without an objective proof and without differentiating the type of fibres.
It is worth noting that the description of mesothelioma in literature preceded the exploitation of asbestos (59) and that other causes of mesothelioma have also been described (60). The role of Simian virus SV40 in the development of human mesothelioma has recently received more attention.
Some authors (60) assume that SV40 may contribute to the development of human mesotheliomas that occur in people not exposed to asbestos.
However, they state that the available epidemiological data are insufficient to explain the role that SV40 may have played in contributing to the increased incidence of mesothelioma currently recorded. Other authors (18,61,62) propose that asbestos and SV40 may be cocarcinogens.
The latency period for the development of mesothelioma is between 30 and 50 years, so that the current mesothelioma deaths are predominantly the consequence of exposure to mixtures of chrysotile and amphiboles in the far past when the exposure levels were incomparably higher than those of today. It is impossible to evaluate whether the current (considerably lower) exposures to pure chrysotile would bring about similar consequences.