The importance of width in asbestos fiber carcinogenicity and its implications for public policy.

Evidence from human epidemiology, experimental animal implantation and inoculation studies, and lung burden studies show that fibers with widths greater than 1 micron are not implicated in the occurrence of lung cancer or mesothelioma. Furthermore, it is generally believed that certain fibers thinner than a few tenths of a micrometer must be abundant in a fiber population in order for them to be a causative agent for mesothelioma. These conclusions are fully consistent with the mineralogical characteristics of asbestos fibers, which, as fibrils, have widths of less than 1 micron and, as bundles, easily dissagregate into fibrils. Furthermore, the biological behavior of various habits of tremolite shows a clear dose-response relationship and provides evidence for a threshold between fiber width and tumor experience in animals. Public policy in regulating mineral fibers should incorporate this knowledge by altering the existing federal asbestos fiber definitions to reflect it.