Trends in Airborne Particulate Matter in the United States

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Abstract
This article examines trends in population exposures to airborne particles in the United States for the period 1940–1990. Changes in national emissions from specified source categories and trends in ambient monitoring data are considered, both as national averages and for specific locations. To address the question of trends by particle size, sources of particulate matter (PM) are segregated into two groups: those that emit primarily fine particles derived from combustion and industrial processes, and those that emit the larger particles typically produced by materials handling and fugitive dust. Downward trends are seen in both groups at average rates from 2 to 5 percent per year. Ambient trend data are presented for total suspended particles and for fine particles (PM2.5); no consistent long-term network measurements are available for the latter, but data are combined from various sources to estimate the overall trends from about 1960 to 1990. Important exposure factors include the shift from winter to summer peaks in ambient PM concentrations and increased market penetration of residential air conditioning. The article concludes that there have been substantial improvements in population exposures to PM in all particle sizes during this period, primarily in the locations having the worst initial air quality. Epidemiological studies designed to study long-term or chronic health effects based on spatial contrasts must recognize these differential rates of air quality improvement if they are intended to derive realistic estimates of exposure-response functions.