Rationale: There are unexplained geographical and seasonal differences in the short-term effects of fine particulate matter (PM 2.5) on human health. The hypothesis has been advanced to include the possibility that such differences might be due to variations in the PM 2.5 chemical composition, but evidence supporting this hypothesis is lacking. Objectives: To examine whether variation in the relative risks (RR) of hospitalization associated with ambient exposure to PM 2.5 total mass reflects differences in PM 2.5 chemical composition. Methods: We linked two national datasets by county and by season: (1) long-term average concentrations of PM 2.5 chemical components for 2000-2005 and (2) RRs of cardiovascular and respiratory hospitalizations for persons 65 years or older associated with a 10-mg/m 3 increase in PM 2.5 total mass on the same day for 106 U.S. counties for 1999 through 2005. Measurements and Main Results: We found a positive and statistically significant association between county-specific estimates of the short-term effects of PM 2.5 on cardiovascular and respiratory hospitalizations and county-specific levels of vanadium, elemental carbon , or nickel PM 2.5 content. Conclusions: Communities with higher PM 2.5 content of nickel, vanadium, and elemental carbon and/or their related sources were found to have higher risk of hospitalizations associated with short-term exposure to PM 2.5. Statistically significant associations between short-term exposure to airborne particulate matter (PM) and mortality and morbidity have been reported in numerous multicity studies (1-6). Several studies also found that health effect estimates vary substantially across communities and seasons (4, 7, 8). The findings of these studies support the hypothesis that geographical and seasonal heteroge-neity of community-specific relative rates could be explained by differences in the chemical composition of PM 2.5 (PM with aerodynamic diameter <2.5 mm). Animal and human toxicological studies support this hypothesis (9-12). However, empirical population-based evidence supporting this hypothesis is lacking. Understanding the basis of the variation in PM effects is critical to further characterize the biological mechanisms of toxicity and to move toward more focused regulatory approaches for PM (13). We investigated whether particular PM 2.5 chemical components are responsible for observed geographical and seasonal variation in the short-term association of PM 2.5 with hospital admissions (4, 7). We also performed a similar analysis based on effect estimates for PM 10 and mortality (see online supplement). METHODS We analyzed whether community-specific estimates of the impact of PM on health risk (cardiovascular and respiratory hospital admissions and mortality) were higher or lower in communities or seasons with particular PM 2.5 chemical composition, as indicated by the fraction of PM 2.5 total mass that is a particular component (e.g., elemental carbon [EC]). We estimated county-and season-specific relative risks (RR) of cardiovascular and respiratory hospitalization associated with a 10-mg/m 3 increase in PM 2.5 total mass on the same day for 106 U.S. counties for the years 1999 through 2005 (Figures 1 and 2). Counties were selected based on data availability for PM 2.5 total mass and chemical components and on having a population of 200,000 or more persons to allow for sufficient sample size and to allow a distribution of counties across the United States. The population criterion results in more urban counties. We conducted similar analysis for PM 10 and total nonaccidental mortality in 100 U.S. communities for 1987 through 2000 (see Figure E1 in the online supplement) (8). All county-and season-specific effect estimates were adjusted for day of the week, seasonality, and long-term trends based on a smooth function of a variable representing time by including these variables in the county-specific regression models. We adjusted for daily temperature and dew point temperature and for the previous 3 days' temperature and dew point temperature. Details of the methods are provided elsewhere (4, 7, 8). We generated a national database of PM 2.5 chemical component concentrations from February 2000 to December 2005 based on data obtained from the U.S. Environmental Protection Agency (USEPA) (14). We calculated county-and season-specific averages of PM 2.5 chemical components that were demonstrated to contribute a substantial fraction of PM 2.5 total mass (14) or to have been implicated as potentially toxic in earlier research (11, 15-18) (Table 1). Detailed information on the spatial and temporal variation of the PM 2.5 chemical components is provided elsewhere (14). We then calculated the fraction of PM 2.5 total mass for each component by season and county. Chemical composition data were available for 106 of the 200 AT A GLANCE COMMENTARY Scientific Knowledge on the Subject Although airborne particulate matter (PM) has been linked to adverse human health effects, the chemical constituents that cause harm are unknown. The relationship between PM and health varies seasonally and regionally , as does the particle's chemical composition. What This Study Adds to the Field This work provides evidence that the chemical composition of PM affects its toxicity. In places and during seasons when PM had higher fractions of nickel, vanadium, and elemental carbon, the risks of hospital admission associated with PM with aerodynamic diameter < 2.5 mm were higher.
Published Oct 7, 2008
Bell, M. L., Ebisu, K., Peng, R. D., Samet, J. M., & Dominici, F. (2009). Hospital Admissions and Chemical Composition of Fine Particle Air Pollution. Am J Respir Crit Care Med, 179, 1115–1120. https://doi.org/10.1164/rccm.200808-1240OC