Human cancer risk from ingested asbestos: a problem of uncertainty.

Although studies of populations exposed to high concentrations of asbestos fibers in drinking water appear attractive as research opportunities to define an asbestos-related risk, they are unlikely to provide a definitive answer to the question of whether asbestos in drinking water is associated with an elevated risk of malignancy. Consider, for example, a hypothetical 10-yr analysis of the deaths in a city of 1 million persons whose water supply is contaminated with 100 million fibers per liter (f/L) (a gross overestimate of any real situation). From the estimates in the asbestos criteria document (1), ingestion of this water over a 70-yr period would give rise to an added risk of death 3.3 x 10-3 per person. Tb estimate the number of asbestos-related deaths in this population, using the above risk data, assume that the average residence time of those deceased in the contaminated area is 14 yr and that the distribution of residence times follows an exponential function, exp {t/14}. This distribution will certainly overestimate residence times compared with actual populations. Nearly half the census tracts in the U.S. EPA-sponsored San Francisco study, for example, showed more than 53% of the residents moving within 5 yr (2). Assume also that 7 yr is required for the risk of asbestos malignancy to manifest itself, as is the case for lung cancer. (See Fig. 1 for the expression of the relative risk of lung cancer in insulators.) This would appear to be so for gastrointestinal