the time-series studies, the researchers assumed that outdoor particle concentrations serve as a valid surrogate of personal exposures to ambient particles. Previous findings from monitoring studies had suggested that personal exposures differ from ambient concentrations because of particle sources in key indoor microenvironments (Dockery and Spengler 1981; Ozkaynak et al. 1993; Ozkaynak et al. 1996). In addition, most of these investigations found weak associations, often not statistically significant, between personal exposure and ambient concentrations when assessed cross-sectionally (at different locations for different people). However, these conclusions were based on a relatively small number of studies, which were originally designed to determine population exposure distributions rather than to examine the degree of association between personal exposures and ambient concentrations. For interpreting the time-series studies of air pollution and health, an understanding of the pattern of association between ambient concentrations and personal exposures over time was needed.

To address this knowledge gap, the committee recommended that longitudinal panel studies of personal exposure to PM be conducted (NRC 1998). In such studies, particle and gaseous copollutant exposures of groups of individuals would be measured at successive points in time to examine the relationship between personal exposures and the corresponding ambient concentrations. Further, these studies would attempt to identify factors influencing the observed relationships. The recommended exposure assessment studies would include not only healthy individuals but also emphasize individuals susceptible to the effects of particle exposures, including persons with chronic obstruction pulmonary disease (COPD), cardiovascular disease, and asthma, as well as children and older adults. As a result of the committee’s recommendations, a large number of particle exposure studies were conducted in several cities in the United States with different climatic conditions and air pollution mixtures. Studies were also conducted in Europe and South America. Below we summarize the major findings that have emerged from either the initial or the completed analyses of the collected data.

Relationship Between Personal Exposures and Ambient Concentrations

Results from the recent panel studies support the hypothesis that ambient PM2.5 concentrations are significant predictors of corresponding personal exposures, over time, for the investigated cohorts (Ebelt et al.



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