that point to potentially susceptible subpopulations within the general population can be used as part of the basis to develop laboratory animal models and toxicological studies. Toxicological results, in turn, may help to identify biologically important constituents or characteristics of particulate matter or potentially susceptible subpopulations. This information can then be used to focus on human exposures to biologically important aspects of particulate matter. An iterative process involving interpretation of evidence from toxicological and exposure studies will lead to the selection of the metrics of exposure for designing future epidemiological studies on the health effects of particulate matter and other pollutants. The process should also lead to a better understanding of source-concentration-exposure-dose-response relationships through the application of successive generations of analytical tools for the most biologically important components or characteristics of particulate matter and gaseous copollutants. Although each of the research topics discussed below was evaluated individually, the committee recognized and addressed the fundamental interdependence of the individual elements. In the research investment portfolio presented in Chapter 5, the committee integrates these interdependent issues into a set of year-by-year timing recommendations and funding priorities for research. A truly integrated research strategy has rarely been used to investigate environmental problems, and it will require a major shift in current approaches to filling knowledge gaps and building toward a coherent understanding of the particulate-matter problem.

In addition to their scientific value, the research priorities described below are also expected to strengthen the basis of evidence for establishing allowable emission rates for the chemical components and precursors of particulate matter that are biologically important. This evidence base will be essential for designing and implementing effective control strategies for particulate matter in outdoor air through state implementation plans and for developing other mitigation approaches, including educational activities, voluntary emissions reductions, and product improvements for reducing indoor particulate-matter emissions that are not currently regulated.

Each research priority is presented with a description that includes