A national multicity analysis of the causal effect of local pollution, NO 2 , and PM 2:5 on mortality

Abstract

BACKGROUND: Studies have long associated PM 2:5 with daily mortality, but few applied causal-modeling methods, or at low exposures. Short-term exposure to NO 2 , a marker of local traffic, has also been associated with mortality but is less studied. We previously found a causal effect between local air pollution and mortality in Boston. OBJECTIVES: We aimed to estimate the causal effects of local pollution, PM 2:5 , and NO 2 on mortality in 135 U.S. cities. METHODS: We used three methods which, under different assumptions, provide causal marginal estimates of effect: a marginal structural model, an instrumental variable analysis, and a negative exposure control. The instrumental approach used planetary boundary layer, wind speed, and air pressure as instruments for concentrations of local pollutants; the marginal structural model separated the effects of NO 2 from the effects of PM 2:5 , and the negative exposure control provided protection against unmeasured confounders. RESULTS: In 7.3 million deaths, the instrumental approach estimated that mortality increased 1.5% [95% confidence interval (CI): 1.1%, 2.0%] per 10 lg/m 3 increase in local pollution indexed as PM 2:5 . The negative control exposure was not associated with mortality. Restricting our analysis to days with PM 2:5 below 25 lg/m 3 , we found a 1.70% (95% CI 1.11%, 2.29%) increase. With marginal structural models, we found positive significant increases in deaths with both PM 2:5 and NO 2 . On days with PM 2:5 below 25 lg/m 3 , we found a 0.83% (95% CI 0.39%, 1.27%) increase. Including negative exposure controls changed estimates minimally. CONCLUSIONS: Causal-modeling techniques, each subject to different assumptions, demonstrated causal effects of locally generated pollutants on daily deaths with effects at concentrations below the current EPA daily PM 2:5 standard.