The Impacts of Heat and Air Pollution on Mortality in the United States

Abstract Extreme heat events stress the body and can result in fatalities, especially for those with underlying health problems. Air pollution is another threat to health and is an important confounder of extreme heat risks. However, previous empirical studies that have addressed the joint health impacts of air pollution and heat rarely considered the endogeneity and spillover effects of air pollution. To fill this research gap, this article investigates the interconnected impacts of extreme heat and fine particulate matter (PM 2.5 ) on all-cause and cause-specific mortality. We correct the endogeneity of PM 2.5 by applying the control function approach and explore transboundary externalities of all-source PM 2.5 and wildfire-caused PM 2.5 . We use a county-year balanced panel dataset covering 2992 U.S. counties from 2001 through 2011. Results show that extreme heat and air pollution exacerbate each other and jointly increase mortality. Specifically, a 1-standard-deviation (SD) increase in the heat index results in 0.60% (95% confidence interval: 0.26%–0.97%), 2.14% (1.34%–2.94%), and 0.86% (0.41%–1.34%) more all-cause fatalities, fatalities from respiratory system diseases, and fatalities from circulatory system diseases, respectively. A 1-SD increase in PM 2.5 results in 5.75% (3.61%–7.90%), 6.99% (3.01%–11.15%), and 2.93% (0.66%–5.28%) additional fatalities, respectively. Failure to consider the endogeneity of PM 2.5 leads to a substantial underestimation of PM 2.5 risk. In addition, our instrumental variable strategy offers evidence of spillover effects from PM 2.5 and wildfires. Significance Statement This study illustrates how extreme heat events combined with air pollutants threaten health. This article investigates the interconnected impact of extreme heat and air pollution using data from 2992 United States counties over the 2001–11 period. Results indicate that extreme heat and air pollution jointly increase mortality. Results also show that wind-driven pollution from other counties and wildfires increase mortality.