Short-term exposure to particulate matter constituents and mortality in a national study of U.S. urban communities

Background: Although the association between PM_2.5 mass and mortality has been extensively studied, few national-level analyses have estimated mortality effects of PM_2.5 chemical constituents. Epidemiologic studies have reported that estimated effects of PM_2.5 on mortality vary spatially and seasonally. We hypothesized that associations between PM_2.5 constituents and mortality would not vary spatially or seasonally if variation in chemical composition contributes to variation in estimated PM_2.5 mortality effects. Objectives: We aimed to provide the first national, season-specific, and region-specific associations between mortality and PM_2.5 constituents. Methods: We estimated short-term associations between nonaccidental mortality and PM_2.5 constituents across 72 urban U.S. communities from 2000 to 2005. Using U.S. Environmental Protection Agency (EPA) Chemical Speciation Network data, we analyzed seven constituents that together compose 79-85% of PM_2.5 mass: organic carbon matter (OCM), elemental carbon (EC), silicon, sodium ion, nitrate, ammonium, and sulfate. We applied Poisson time-series regression models, controlling for time and weather, to estimate mortality effects. Results: Interquartile range increases in OCM, EC, silicon, and sodium ion were associated with estimated increases in mortality of 0.39% [95% posterior interval (PI): 0.08, 0.70%], 0.22% (95% PI: 0.00, 0.44), 0.17% (95% PI: 0.03, 0.30), and 0.16% (95% PI: 0.00, 0.32), respectively, based on single-pollutant models. We did not find evidence that associations between mortality and PM_2.5 or PM_2.5 constituents differed by season or region. Conclusions: Our findings indicate that some constituents of PM_2.5 may be more toxic than others and, therefore, regulating PM total mass alone may not be sufficient to protect human health.