(see Chapter 3). The portfolio is designed to maximize growth in critical knowledge as expeditiously and cost effectively as possible through interactive components and iterative stages, providing a continued stream of scientific evidence on a series of hierarchical topics to fill key gaps in the committee's overall research framework.

The committee's portfolio spans a 13-year horizon, from 1998 to 2010, the estimated closure date for scientific input to EPA's anticipated 2012 review of particulate-matter standards. This portfolio, if aggressively and properly implemented by EPA, should result in substantial new information for the next scheduled EPA review of the particulate-matter standards in the year 2002. By that time, critical information would be expected regarding the most biologically important components and characteristics of particulate matter, the toxicological mechanisms through which they act, and how well the data from ambient air monitors represent the actual exposure of people to particulate matter, especially for the most potentially susceptible subpopulations. It is noteworthy that EPA's current timetable (Table 1-1) for the implementation of the new PM2.5 standards does not envision the implementation of actual control requirements until approximately 2012. The committee believes that the 1998-2010 time span is an interval over which a comprehensive research program could be reasonably anticipated to produce findings that would materially address key uncertainties in a deliberate plan that emphasizes both early and longer-term results. But the pace of research cannot always be adjusted to match the schedule of a regulatory decisionmaking process, which is determined mainly by law, policy, and administrative factors. Some of the committee's recommended research topics (e.g., effects of controlled exposures of susceptible subpopulations to particulate matter) can be quickly and efficiently addressed, with the possibility of definitive findings within a few years. Other research topics (e.g., long-term effects of air pollution on morbidity and mortality) will require substantially more time for resolution. Consequently, there needs to be a commitment to begin some research immediately, to sustain research on the longer-term questions, and to implement and maintain surveillance of the health effects of particulate matter as the new standards are implemented.

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