including susceptible subpopulations and several methodological issues concerning data analysis that need further study.

Exposure Assessment

Exposure assessment is of paramount importance in understanding the effects of ambient particles and developing cost-effective exposure-control strategies. As more is learned about the characteristics of particles that determine their toxicity, more information will be needed on exposures to particles considered to have toxic potential. The current studies will enable the scientists, policymakers, and other interested parties to understand better the factors that affect the relationship between personal exposure and outdoor concentrations of particles.

In its first report, the committee recommended the development of longitudinal studies in which groups of 10-40 persons would be evaluated at successive times to examine the relationship between their personal exposures to particles, as indexed by mass, and the corresponding outdoor concentrations. Those exposure investigations are intended to focus particularly on subpopulations that could be susceptible to the effects of PM exposures, such as the elderly, children, and people with respiratory or cardiovascular disease. The committee recommended that the exposure studies include measurements of PM 2.5, PM10, and gaseous copollutants, if appropriate. The committee expected the investigations to quantify the contribution of outdoor sources to personal and indoor exposures.

It appears that the developed study designs (such as repeated measurements of a small number of people) can address the key scientific questions. The studies that have been completed have identified some factors that influence relationships between outdoor air and personal exposure.

The committee expects the current longitudinal panel studies to be completed without difficulty. Although more time has been taken than expected in launching some of the currently funded panel studies, abundant personal and microenvironmental measurements have



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