At the same time, EPA issued a new standard for ozone, another criteria pollutant, but ozone is not a focus of this study. The new PM standards were justified by EPA largely on the basis of epidemiologic studies that related ambient PM concentrations to various health effects, including premature mortality, exacerbation of asthma and other respiratory tract diseases, and decreased lung function. EPA has estimated that the new standards will prevent approximately 15,000 premature deaths per year in the United States. Many experts believe that substantially more scientific information is needed to determine whether the standards should be revised, and the Congress has mandated a major program of research to be conducted before that date. This NRC study addresses the request from Congress.
Particles originate from diverse sources, both natural and anthropogenic.
These include forest fires, wind erosion, agriculture, fossil fuel combustion, industrial manufacturing, and construction of building and roads. The particles can be emitted directly from those sources or formed in the atmosphere from gaseous emissions, such as sulfur dioxide. The variety of sources leads to substantial differences in the sizes and chemical composition of particles. For example, PM is composed mainly of sulfates in the eastern United States and organic carbon in the west. There is a lack of information about the sources and composition of the specific types of particles that are most important in causing adverse health effects, the levels of human exposure to such particles, and the mechanisms of toxicity.