Under the Clean Air Act, particulate matter (PM) is one of the major air pollutants for which National Ambient Air Quality Standards (NAAQS) are to be established on the basis of the scientific evidence on risks to human health. The . Environmental Protection Agency (EPA), other federal and state government agencies, and nongovernment organizations are conducting a major multiyear research effort to improve scientific understanding of airborne PM and its effects on human health. An overall objective is to reduce uncertainties in the scientific evidence used to set the NAAQS for airborne PM in the United States. At the request of Congress and EPA, the National Research Council's Committee.
