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.

Climate change is expected to exacerbate the urban heat island effect in cities worldwide, increasing the risk of heat-related morbidity and mortality. Solar reflective “cool pavement” is one of several mitigation strategies that may counteract the negative effects of the urban heat island effect. An increase in pavement albedo results in less heat absorption, which results in reduced surface temperatures (Tsurface). Near surface air temperatures (Tair) could also be reduced if cool pavements are deployed at sufficiently large spatial scales, though this has never been confirmed by field measurements. This field study is the first to conduct controlled measurements of the impacts of neighborhood-scale cool pavement installations. We measured the impacts of cool pavement on albedo, Tsurface, and Tair. In addition, pavement albedo was monitored after installation to assess its degradation over time. The field site (~0.64 km2) was located in Covina, California; ~30 km east of Downtown Los Angeles. We found that an average pavement albedo increase of 0.18 (from 0.08 to 0.26) corresponded to maximum neighborhood averaged Tsurface and Tair reductions of 5 °C and 0.2 °C, respectively. Maximum Tsurface reductions were observed in the afternoon, while minimum reductions of 0.9 °C were observed in the morning. Tair reductions were detected at 12:00 local standard time (LST), and from 20:00 LST to 22:59 LST, suggesting that cool pavement decreases Tair during the daytime as well as in the evening. An average albedo reduction of 30% corresponded to a ~1 °C reduction in the Tsurface cooling efficacy. Although we present here the first measured Tair reductions due to cool pavement, we emphasize that the tradeoffs between Tair reductions and reflected shortwave radiation increases are still unclear and warrant further investigation in order to holistically assess the efficacy of cool pavements, especially with regards to pedestrian thermal comfort.

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