The Committee considered the health risks posed by nonoccupational airborne exposures to asbestos and other natural or synthetic asbestiform fibers. The issue is important because many people may be exposed to these materials, although at relatively low levels. The EPA asked the NAS to undertake a study with two goals: to evaluate the human health risks associated with nonoccupational exposure to asbestiform fibers, with emphasis on inhalation of outdoor and indoor air, and; to determine the extent to which the physical-chemical properties of the fibers may be associated with the development of various human diseases and the extent to which such information may be incorporated into assessing health risks resulting from exposure to the fibers. The committee found that much more information is available about asbestos than about the other materials of concern. Nonoccupational exposure to asbestiform fibers in air present a risk to human health. The extent of the risk is highly uncertain, depending on the nature and amount of exposure and other factors. The committee made several recommendations, i.e., systematic monitoring and characterization of asbestiform fibers with standardized methods should be undertaken on nonoccupational environments, including urban, rural, indoor, and outdoor locations where exposure may be of special concern; cessation of cigarette smoking should be encouraged.