A National Multicity Analysis of the Causal Effect of Local Pollution, NO2, and PM2.5 on Mortality

• Published in: Environmental health perspectives Vol. 126; no. 8
• Main Authors: Schwartz, Joel, Fong, Kelvin, Zanobetti, Antonella
• Format: Journal Article
• Language: English
• Published: Research Triangle Park National Institute of Environmental Health Sciences 01.08.2018; Environmental Health Perspectives
• Subjects: Air pollution; Boundary layers; Cities; Confidence intervals; Epidemiology; Estimates; Expected values; Exposure; Fatalities; Gene expression; Modelling; Mortality; Nitrogen dioxide; Outdoor air quality; Particulate matter; Planetary boundary layer; Pollutants; Pollution index; Public health; Short term; Structural models; Technology transfer; Wind speed; United States > US
• ISSN: 0091-6765; 1552-9924
• DOI: 10.1289/EHP2732

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 NO2 , 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 NO2 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 NO2 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 10μg / 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 25μg / 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 NO2 . On days with PM 2.5 below 25μg / 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.