Estimating the causal effect of annual PM(2.5) exposure on mortality rates in the Northeastern and mid-Atlantic states

BACKGROUND: Dozens of cohort studies have associated particulate matter smaller than 2.5 µm in diameter (PM(2.5)) exposure with early deaths, and the Global Burden of Disease identified PM(2.5) as the fifth-ranking mortality risk factor in 2015. However, few studies have used causal modeling techniques. We assessed the effect of annual PM(2.5) exposure on all-cause mortality rates among the Medicare population in the Northeastern and mid-Atlantic states, using the difference-in-differences approach for causal modeling. METHODS: We obtained records of Medicare beneficiaries 65 years of age or more who reside in the Northeastern or mid-Atlantic states from 2000 to 2013 and followed each participant from the year of enrollment to the last year of follow-up. We estimated the causal effect of annual PM(2.5) exposure on mortality rates using the difference-in-differences approach in the Poisson survival analysis. We controlled for individual confounders, for spatial differences using dummy variables for each ZIP code and for time trends using a penalized spline of year. RESULTS: We included 112,376,805 person-years from 15,401,064 people, of whom 37.4% died during the study period. The interquartile range (IQR) of the annual PM(2.5) concentration was 3 µg/m(3), and the mean annual PM(2.5) concentration ranged between 6.5 and 14.5 µg/m(3) during the study period. An IQR incremental increase in PM(2.5) was associated with a 4.04% increase (95% CI = 3.49%, 4.59%) in mortality rates. CONCLUSIONS: Assuming no omitted predictors changing differently across ZIP codes over time in correlation with PM(2.5), we found a causal effect of PM(2.5) on mortality incidence rate.