Toxicological insight into low-level exposure to chrysotile

The disease potential of exposure to high levels of airborne respirable chr ysotile fiber is well known, although the spectrum of diseases consequent u pon exposure is, to a degree, controversial. Exposure is known to cause asb estosis and probably, also, lung cancer, particularly in cigarette smokers, but chrysotile seems unlikely to be responsible for mesothelioma. This spe ctrum of diseases results predominantly from heavy occupational exposure. E xposure to low levels of chrysotile, either occupationally or environmental ly, and particularly to the levels found in the urban environment, does not produce attributable disease within the limitations of the epidemiological method. Nor does it produce any histopathological changes in the lungs. If it were not for the fact that chrysotile is unequivocally accepted as a ca rcinogen, then the risk associated with such low-level exposure would have been assessed as insignificant. However, carcinogens are judged by regulato ry authorities to have no threshold of effect. Since this hypothesis cannot be tested, authorities rely on the precautionary principle in the hope tha t this will reduce the level of cancer in the population. However, there is at present little evidence that even urban levels of mineral fibers pose a real risk to human health. The near-hysterical rush to clear asbestos from buildings to reduce even this immeasurably small risk may cause more human exposure to mineral fibers and not less.