1. Suggested causes of mesothelioma other than mineral fibres Apart from mineral fibres, radiation (for example, through the contrast medium Thorotrast) has been suggested to cause human mesothelioma. However, in a large retrospective study of more than 250 000 women treated for mammary carcinoma, a quarter of them initially with radiotherapy, no association between radiation and mesothelioma could be found.22 As can be seen from table 2, other causes or contributing factors have also been suggested
2. Viruses can cause mesotheliomas in animals, but this has not been described in humans. DNA sequences associated with Simian virus 40 (SV 40) transforming factors have been reported in a high proportion of mesotheliomas from some countries.32 This suggests that there is a connection between SV40, which was a contaminant of live polio vaccines in 1959–61, and later development of mesothelioma. Thus, SV 40 might be a cofactor to asbestos in some patients with mesothelioma, but the results have not been confirmed and are still disputed
3. Incidence of mesothelioma There is a large variation in the incidence of mesothelioma in diVerent countries and in most places a steadily rising number of cases with time. In table 3, the incidence or mortality from mesotheliomas in diVerent countries at various times can be seen. As mortality for practical puprposes is the same as the incidence for this disease, both figures have been used in the table. Some of the diVerences between the countries are probably due to diagnostic diYculties, but most of the variations can be explained by the use of asbestos in the particular society some decades earlier
4. There are authors who claim that the presumed background level must be very low, and retrospective searches for the tumour in the medical literature yield no convincing cases of mesothelioma before 1946,61 although such negative evidence is of questionable value. McDonald and McDonald, in a recent review, estimated the background level to be 1–2/ million/year; they came to this figure by extrapolating backwards from epidemiological studies from various countries.62
5. Another example is the formerly active crocidolite mine at Wittenoom, Western Australia. At least 5000 people lived in the township of Wittenoom without working in the mines, and in 1993 27 cases of mesothelioma had occurred among these people.6 48 84 It has been estimated that 1.1% of child residents and 1.9 % of the female residents of Wittenoom have died or will die from mesothelioma, whereas among the workforce this figure would be 6%.6