The Effects of Co-Exposure to Extremes Heat and Particulate Air Pollution on Mortality in California

Abstract

Background: Extremes of heat and particulate air pollution threaten human health and are becoming more frequent due to climate change. Understanding the health impacts of the co-exposure to extreme heat and air pollution is urgent. Methods: Death certificate data for California from January 1, 2014, to December 31, 2019 were used to identify all-cause, cardiovascular, and respiratory mortality. A case-crossover study with time-stratified matching and conditional logistic regression was used to estimate the mortality association with acute co-exposures to extreme heat and PM2.5. Several percentile-based threshold values (i.e., 90th, 95th, 97th, and 99th) were used to define a day as an extreme exposure day. For each case day (date of death) and its control days, daily average PM2.5, maximum and minimum temperature were assigned based on decedent’s residence census tract. Age-stratified analyses were also conducted. Results: This study included 1.5 million all-cause, 0.5 million cardiovascular, and 0.14 million respiratory deaths. Based on a 99th percentile threshold, all-cause mortality risk increased 6.1% (95%confidence interval,CI: 4.1, 8.1) on extreme maximum temperature only days and 5.0% (95%CI: 3.0, 8.0) on extreme PM2.5 only days, compared to non-extreme days. Risk increased 21.0% (95%CI: 6.6, 37.3) on days with exposure to both extreme maximum temperature and PM2.5. Increased risk of cardiovascular and respiratory mortality on extreme co-exposure days were 29.9% (95%CI: 3.3, 63.3) and 38.0% (95%CI: -12.5, 117.7), respectively, and were more than the sum of individual effects of extreme temperature and PM2.5 only. A similar pattern was observed for co-exposure to extreme PM2.5 and minimum temperature; effect estimates were larger among those over age 75 years. Conclusion: Short-term exposure to extreme heat and air pollution alone were individually associated with increased risk of mortality, but their co-exposure had larger effects beyond the sum of their individual effects. Keywords: extreme co-exposure, heat, air pollution, mortality