FINE PARTICLES AND COARSE PARTICLES - CONCENTRATION RELATIONSHIPS RELEVANT TO EPIDEMIOLOGIC STUDIES

Fine particles and coarse particles are defined in terms of the modal structure of particle size distributions typically observed in the atm osphere. Differences between the various modes are discussed. The frac tions of fine and coarse particles collected in specific size ranges, such as total suspended particulate matter (TSP), PM10, PM2.5, and PM1 0-2.5, are shown. Correlations of 24-h concentrations of PM2.5, PM10, and PM10-2.5 at the same site show that, in Philadelphia and St. Louis , PM2.5, is highly correlated with PM10 but poorly correlated with PM1 0-2.5. Among sites distributed across these urban areas, the site-to-s ite correlations of 24-h PM concentrations are high for PM2.5 but not for PM10-2.5. This indicates that a PM measurement at a central monito r can serve as a better indicator of the community-wide concentration of fine particles than of coarse particles. The fraction of ambient ou tdoor particles found suspended indoors is greater for fine particles than for coarse particles because of the difference in indoor lifetime s. Consideration of these relationships leads to the hypothesis that t he statistical associations found between daily PM indicators and heal th outcomes may be the result of variations in the fine particle compo nent of the atmospheric aerosol, not of variations in the coarse compo nent. As a result, epidemiologic studies using PM10 or TSP may provide more useful information on the acute health effects of fine particles than coarse particles. Fine and coarse particles are separate classes of pollutants and should be measured separately in research and epide miologic studies. PM2.5 and PM10-2.5 are indicators or surrogates, but not measurements, of fine and coarse particles.