The impact of PM2.5 on mortality in older adults: evidence from retirement of coal-fired power plants in the United States

Background Evidence of causal relationship between mortality of older adults and low- concentration PM2.5 remains limited. Objectives This study investigates the effects of low-concentration PM2.5 on the mortality of adults older than 65 using the closure of coal-fired power plants in the Eastern United States as a natural experiment. Methods We investigated power plants in the Eastern United States (US) that had production changes through unit shutdown or plant retirement between 1999 and 2013. We included only non-clustered power plants without scrubbers and with capacities greater than 50 MW. We used instrumental variable (IV) and difference-in-differences (DID) approaches to estimate the causal impact of PM2.5 concentrations on mortality among Medicare beneficiaries. We compared changes in monthly age-adjusted mortality before and after the retirement of coal-fired plants between the treated and control counties; we accounted for annual wind direction in our selection of treated and control counties. In the models, we initially included only county and monthly fixed effects, and then adjusted for covariates including: 1) only weather variables (temperature, dew point, pressure); and 2) weather variables and socio-economic variables (median household income and poverty rate). Results The monthly age-adjusted mortality rate averaged across all plants was approximately 423 per 100,000 (SD = 69) and was higher for males than females. Mean PM2.5 concentrations across all counties were 12 mu g/m(3) (SD = 3.78). Using the IV method, we found that reductions in PM2.5 concentrations significantly decreased monthly mortality among older adults. IV results show that a 1-mu g/m(3) reduction in PM2.5 concentrations leads to 7.17 fewer deaths per 100,000 per month, or a 1.7% lower monthly mortality rate among people older than 65 years. Using the DID approach, we found that power plant retirement significantly decreased: 1) monthly PM2.5 levels by 2.1 mu g/m(3), and 2) monthly age-adjusted mortality by approximately 15 people per 100,000 (or 3.6%) in treated counties relative to control counties. The mortality effects were higher among males than females and its impact was the greatest among people older than 75 years. Conclusion These findings provide evidence of the effectiveness of local, plant-level control measures in reducing near-plant PM2.5 and mortality among U.S. Medicare beneficiaries.