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

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 (PM2.5) on all-cause and cause-specific mortality. We correct the endogeneity of PM2.5 by applying the control function approach and explore transboundary externalities of all-source PM2.5 and wildfire-caused PM2.5. We use a county-year balanced panel dataset covering 2,992 United States counties from 2001 through 2011. Results show that extreme heat and air pollution exacerbate each other and jointly increase mortality. Specifically, a one 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, fatalities from circulatory system diseases, respectively. A one SD increase in PM2.5 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 PM2.5 leads to a substantial underestimation of PM2.5 risk. In addition, our instrumental variable strategy offers evidence of spillover effects from PM2.5 and wildfires.