personal exposures, especially for potentially susceptible subpopulations and individuals?

Compliance with the National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) is ascertained by measuring ambient concentrations of PM at monitoring sites. With regard to the health effects of air pollution, the risks depend on personal exposure—that is, the exposures received by people in the various specific places, conceptualized as microenvironments, where they spend time. Total personal exposure represents the time-weighted average of particle concentrations in the microenvironments where people spend their time. Exposures to particles generated by outdoor sources take place not only outside but also in indoor environments where the particles penetrate. Indoor particle sources, such as cigarette smoking, thus might contribute substantially to total personal exposure to particles. Research carried out in regard to this topic addresses the relationship of monitoring data for ambient air with personal exposures to PM and gaseous copollutants. Data on this relationship are needed not only for healthy people but also for those persons who are particularly susceptible to air pollution and at greatest risk for experiencing adverse effects. Such persons are referred to collectively as a “susceptible subpopulation” and are further addressed under topic 8 later in this chapter.

Before 1997, the majority of time-series analyses of morbidity and mortality data relied on ambient air pollution concentrations as measures of exposure. A critical assumption of these investigations was that ambient PM concentrations serve as surrogates of corresponding personal exposures to ambient particles. Previous findings from personal monitoring studies suggested that human exposures differ from ambient concentrations due to the contributions of microenvironmental sources (Dockery and Spengler 1981; Ozkaynak et al. 1993; Ozkaynak et al. 1996a). In addition, most of these investigations found statistically insignificant and weak associations between personal exposures and ambient concentrations when assessed cross-sectionally (that is, at different locations). However, these conclusions were based on a small number of studies that were originally designed