associations between outdoor particulate-matter concentrations and various adverse health effects, including premature mortality, exacerbation of asthma and other respiratory-tract diseases, and decreased lung function. The biological basis of most of the associations is essentially unknown (at the ambient particulate levels at which the associations were observed). There is a great deal of uncertainty about the implications of the findings for risk management, due to the limited scientific information about the specific types of particles that might cause adverse health effects, the contributions of particles of outdoor origin to actual human exposures, the toxicological mechanisms by which the particles might cause adverse health effects, and other important questions. These questions are not presented here as a rationale for abandoning efforts to control public exposures to fine particulate matter and other pollutants, but they do indicate the critical need for better scientific knowledge to guide such efforts.

EPA has estimated that implementation of the new PM2.5 standards will prevent approximately 15,000 premature deaths per year in the United States. By law, EPA cannot consider compliance costs when setting these standards. EPA has announced, along with its timetable for implementing the new PM2.5 standards, that it contemplates no actual regulatory emission-control requirements to be implemented until well after the next scheduled review of the particulate-matter standards in the year 2002. In the meantime, Congress has mandated and has begun to fund a major program of research to reduce the key scientific uncertainties. In EPA's Fiscal 1998 appropriations, Congress provided $49.6 million for particulate-matter research—nearly twice the amount requested by EPA. Congress also directed the EPA administrator to arrange for this independent study by the National Research Council (NRC) to identify the most important research priorities relevant to setting NAAQS for particulate matter, to develop a conceptual plan for particulate-matter research, and, over the next 5 years, to monitor and evaluate research progress toward improved understanding of the relationship between particulate matter and its effects on public health.

In response to the request from Congress, the Committee on Research Priorities for Airborne Particulate Matter was established in



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