A National Multicity Analysis of the Causal Effect of Local Pollution, , and on Mortality

Studies have long associated $$ with daily mortality, but few applied causal-modeling methods, or at low exposures. Short-term exposure to $$, 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.

We aimed to estimate the causal effects of local pollution, $$, and $$ on mortality in 135 U.S. cities.

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 $$ from the effects of $$, and the negative exposure control provided protection against unmeasured confounders.

In 7.3 million deaths, the instrumental approach estimated that mortality increased 1.5% [95% confidence interval (CI): 1.1%, 2.0%] per $$ increase in local pollution indexed as $$. The negative control exposure was not associated with mortality. Restricting our analysis to days with $$ below $$, 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 $$ and $$. On days with $$ below $$, we found a 0.83% (95% CI 0.39%, 1.27%) increase. Including negative exposure controls changed estimates minimally.

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 $$ standard. https://doi.org/10.1289/EHP2732.