Mesothelioma: cases associated low dose exposures

Abstract Objectives—To estimate the importance of low dose exposure to asbestos on the risk of mesothelioma. Methods—A review of the literature. Results and conclusions—There is no evidence of a threshold level below which there is no risk of mesothelioma. Low level exposure more often than not contains peak concentrations which can be very high for short periods. There might exist a background level of mesothelioma occurring in the abscence of exposure ot asbestos, but there is no proof of this and this “natural level” is probably much lower than the 1–2/million/year which has been often cited. (Occup Environ Med 1999;56:505–513) Keywords: low exposure; asbestos; mesothelioma Mesothelioma is an incurable disease which is almost exclusively due to inhalation of asbestos fibres. Asbestos has been extensively used in industry and construction in the 20th century, especially during and after the second world war, and even if the mineral is no longer used in most rich western countries the total world production remains high. There is a worldwide pollution with asbestos, as indicated by the finding of the mineral in samples of Greenland ice1 and on the Yorkshire Moors,2 and every citizen in the world has been exposed to some extent. Consequently, asbestos fibres can be found in most lungs at necropsy.3 It is thus understandable that there is concern about the risk of mesothelioma for the general population. However, it should be remembered that mesothelioma is a rare disease with incidence in industrialised countries ranging from 1 to 5/million/year among women and values for men 5–10 times higher (see table 3). Even in cohorts with a very heavy exposure to asbestos most people will die from other causes. In people with certified asbestosis—that is, with a heavy exposure—up to 10% will develop mesothelioma; among insulators in the United States and Canada, also a heavily exposed group, 9.3% of the deaths have been due to this disease; and in amphibole miners in South Africa or Australia, this figure is 2–4% (table 1). Clearly, with exposure concentrations several magnitudes lower, as occurs in the general population, the risk is very small, often impossible to measure. A discussion of the risks from low exposure must include the dose-response curve; the existence or non-existence of a threshold, and thus a background concentration; and should try to define low exposure and estimate to what degree that really means a low concentration. From conflicting findings and opinions attempts must be made to make a meaningful conclusion. The diVerent types of asbestos seem to differ considerably in their ability to cause mesotheliomas. Chrysotile is considered by many authors to be a weak carcinogen in humans,11 whereas the two amphiboles crocidolite and tremolite are much more dangerous according to many studies.12 The third of the more important amphiboles, anthophyllite, was long considered not to cause mesothelioma, but such tumours have now been reported although the risk seems to be small.13 There is, none the less, a minority opinion that chrysotile is in fact responsible for most of the pleural mesotheliomas in society14 or should at least be considered to carry the same risk.15 This discussion, however, falls outside the present review and is not important for the conclusions drawn here.