Asbestos Induce Pleural Mesothelioma

Asbestos is a recognized human carcinogen, causally related to pleural and peritoneal mesothelioma and to lung cancer (1). Mesothelioma is of particular interest, as it is a specific outcome of asbestos exposure and no other causal factor except for exposure to asbestos (and erionite, a naturally occurring mineral fiber found in Turkey) (2) has been established or even convincingly suspected. The vast majority of asbestos-induced mesotheliomas in the industrialized world is caused by occupational asbestos exposure, and occurs among workers engaged in extracting and manufacturing asbestos, or performing tasks involving contact with asbestos-containing materials (3). Concern used to be focused on the occupational environment, but it is now recognized that asbestos fibers are widely distributed in the general environment. Persons can be exposed to asbestos in different nonoccupational circumstances: living with asbestos workers, with regular exposure to soiled work clothes brought home; environmental exposure in the neighborhood of industrial sources (asbestos mines and mills, asbestos processing plants); passive exposure in buildings containing asbestos; and natural environmental exposure to geological sources (4). Studying the effects of nonoccupational asbestos exposure on mesothelioma risk is important because it could provide information about the nature of the exposure–response relationship that cannot be obtained from studies of workers whose exposures begin in adulthood, are limited to working hours, are typically much higher in concentration, and who are mainly male. Natural environmental exposure to geological sources is of special interest, since populations subjected to natural sources present specific temporal exposure characteristics: exposure can start during childhood and be lifelong, and it can occur around the clock, seven days a week. Evidence on environmental exposure of natural origin thus provides information about the effect of early exposure on cancer risk and on latency periods, susceptibility according to sex, or the potentially different effect of asbestos fiber types (chrysotile, or amphiboles such as crocidolite, amosite, or tremolite, which are considered to be more potent carcinogens for mesothelioma than chrysotile fibers). The main findings of studies in rural areas of Turkey (5), Greece (6), some Mediterranean islands (7–9), China (10), and New Caledonia (11), where occupational exposure to asbestos is rare, even nonexistent, show that asbestos exposure starting at birth does not seem to influence the latency period of mesothelioma. The data also indicate that susceptibility does not differ according to sex, and confirm that the much higher rates of mesothelioma among males in the industrialized countries are most probably due to sex differences in occupational exposure to asbestos. Studies of mesothelioma related to environmental exposure to geological sources of asbestos have yielded important findings. Am J Respir Crit Care Med Vol 172. pp 939–943, 2005 Internet address: www.atsjournals.org However, some important issues remain unresolved. Thus, it would be of utmost importance, from a scientific and public health point of view, to know whether exposure to low levels of asbestos is able to induce pleural mesothelioma. There is still controversy regarding this question (12, 13). While exposure in environmental settings is generally much lower than in occupational circumstances, the levels may not be negligible. In studies in which elevated risk of mesothelioma was demonstrated, people typically lived in close vicinity of naturally occurring asbestos sources, and may have had direct contact with asbestos, when whitewashing houses with material containing asbestos or working in polluted fields. It is thus likely that lifelong cumulative exposure may have been as high (if not higher) as in some occupational settings, but it was not—or not adequately—measured, and nonoccupational studies have not yet provided adequate answers