The health benefits analyses reviewed by the committee have depended heavily on the estimated mortality impacts of PM. To better understand the role of and uncertainties in exposure assessment for such benefits analyses, it is important to examine characteristics of the exposure assessments used in the epidemiological studies on which the PM mortality effects were based.
Two classes of study designs have been used to assess mortality effects: time-series and prospective cohort studies (Kinney 1999). The time-series studies examine day-to-day associations between citywide mean daily outdoor PM concentrations and citywide daily death counts. This approach addresses the relationship between acute exposure and health. For example, deaths on a given day are related to PM concentrations on the same day or on a few previous days. In contrast, the prospective cohort studies examine differences between cities in mortality among individuals followed over an extended period and the variations in annual (or longer) mean outdoor PM concentrations. These studies are believed to address the relationship between chronic exposure and mortality. (See the Concentration-Response Function section for a further discussion of time-series and cohort studies.)
Population exposures are assessed in both designs using outdoor citywide average PM concentrations derived from regulatory air-quality monitoring data collected from a small number of sites in each city. Uncertainties may arise in using a citywide average to represent exposures of persons at risk because of spatial variations in ambient concentrations across a city, differences in penetration of ambient air pollution indoors, and the wide range in activity patterns of persons at risk. However, in the single-city time-series studies, central-site fine-particle measurements have been shown to correlate well over time with average population personal exposures (Rojas-Bracho et al. 2000; Sarnat et al. 2000). These findings support the validity of daily ambient PM measurements in capturing variations over time in population exposures to fine particles and strengthen the reliability of benefits estimates of acute health effects that depend on ambient PM concentrations.
Less is known about the reliability of central-site, long-term average, ambient PM concentrations in characterizing variations between cities in