Low-Concentration PM2.5 and Mortality: Estimating Acute and Chronic Effects in a Population-Based Study

BACKGROUND: Both short- and long-term exposures to fine particulate matter (<= 2.5 mu m; PM2.5) are associated with mortality. However, whether the associations exist at levels below the new U.S. Environmental Protection Agency (EPA) standards (12 mu g/m(3) of annual average PM2.5, 35 mu g/m(3) daily) is unclear. In addition, it is not clear whether results from previous time series studies (fit in larger cities) and cohort studies (fit in convenience samples) are generalizable. OBJECTIVES: We estimated the effects of low-concentration PM2.5 on mortality. METHODS: High resolution (1 km x 1 km) daily PM2.5 predictions, derived from satellite aerosol optical depth retrievals, were used. Poisson regressions were applied to a Medicare population (>= 65 years of age) in New England to simultaneously estimate the acute and chronic effects of exposure to PM2.5, with mutual adjustment for short- and long-term exposure, as well as for area-based confounders. Models were also restricted to annual concentrations < 10 mu g/m(3) or daily concentrations < 30 mu g/m(3). RESULTS: PM2.5 was associated with increased mortality. In the study cohort, 2.14% (95% CI: 1.38, 2.89%) and 7.52% (95% CI: 1.95, 13.40%) increases were estimated for each 10-mu g/m(3) increase in short- (2 day) and long-term (1 year) exposure, respectively. The associations held for analyses restricted to low-concentration PM2.5 exposure, and the corresponding estimates were 2.14% (95% CI: 1.34, 2.95%) and 9.28% (95% CI: 0.76, 18.52%). Penalized spline models of long-term exposure indicated a larger effect for mortality in association with exposures >= 6 mu g/m(3) versus those < 6 mu g/m(3). In contrast, the association between short-term exposure and mortality appeared to be linear across the entire exposure distribution. CONCLUSIONS: Using a mutually adjusted model, we estimated significant acute and chronic effects of PM2.5 exposure below the current U.S. EPA standards. These findings suggest that improving air quality with even lower PM2.5 than currently allowed by U.S. EPA standards may benefit public health.