The Effects of Coexposure to Extremes of Heat and Particulate Air Pollution on Mortality in California: Implications for Climate Change

Abstract

Rationale: Extremes of heat and particulate air pollution threaten human health and are becoming more frequent because of climate change. Understanding the health impacts of coexposure to extreme heat and air pollution is urgent. Objectives: To estimate the association of acute coexposure to extreme heat and ambient fine particulate matter (PM_2.5) with all-cause, cardiovascular, and respiratory mortality in California from 2014 to 2019. Methods: We used a case-crossover study design with time-stratified matching using conditional logistic regression to estimate mortality associations with acute coexposures to extreme heat and PM_2.5. For each case day (date of death) and its control days, daily average PM_2.5 and maximum and minimum temperatures were assigned (0- to 3-day lag) on the basis of the decedent's residence census tract. Measurements and Main Results: 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 PM_2.5-only days, compared with nonextreme days. Risk increased by 21.0% (95% CI, 6.6-37.3) on days with exposure to both extreme maximum temperature and PM_2.5. Increased risk of cardiovascular and respiratory mortality on extreme coexposure days was 29.9% (95% CI, 3.3-63.3) and 38.0% (95% CI, -12.5 to 117.7), respectively, and were more than the sum of individual effects of extreme temperature and PM_2.5 only. A similar pattern was observed for coexposure to extreme PM_2.5 and minimum temperature. Effect estimates were larger over age 75 years. Conclusions: Short-term exposure to extreme heat and air pollution alone were individually associated with increased risk of mortality, but their coexposure had larger effects beyond the sum of their individual effects.

Keywords: air pollution; all-cause; coexposure; mortality; temperature.

Figure 1.

Monthly distribution of the number of co-occurrence of extreme temperature and fine particulate matter (PM_2.5) days. P = percentile; T_max = maximum temperature; T_min = minimum temperature.

Figure 2.

Distributions of daily (A) maximum temperature (T_max); (B) minimum temperature (T_min); and (C) fine particulate matter (PM_2.5) on days with exposure above different percentile thresholds over the study period from January 2014 through December 2019. P = percentile.

Figure 3.

Estimated excess risk of all-cause mortality (and 95% confidence interval [CI]) on (A) co-occurrence of extreme maximum temperature (T_max) and fine particulate matter (PM_2.5) days compared with nonextreme days; (B) co-occurrence of extreme minimum temperature (T_min) and PM_2.5 days compared with nonextreme days. Results are shown for same-day exposure (lag-0) to 3 days before exposure (lag-3). ER = excess risk.

Figure 4.

Estimated excess risk of cardiovascular mortality (and 95% confidence interval [CI]) on (A) co-occurrence of extreme maximum temperature (T_max) and fine particulate matter (PM_2.5) days compared with nonextreme days; (B) co-occurrence of extreme minimum temperature (T_min) and PM_2.5 days compared with nonextreme days. Results are shown for same-day exposure (lag-0) to 3 days before exposure (lag-3). ER = excess risk.

Figure 5.

Estimated excess risk of respiratory mortality (and 95% confidence interval [CI]) on (A) co-occurrence of extreme maximum temperature (T_max) and fine particulate matter (PM_2.5) days compared with nonextreme days; (B) co-occurrence of extreme minimum temperature (T_min) and PM_2.5 days compared with nonextreme days. Results are shown for same-day exposure (lag-0) to 3 days before exposure (lag-3). ER = excess risk.