Increasing Exposures to Compound Wildfire Smoke and Extreme Heat Hazards in California, 2011–2020

Climate change is simultaneously worsening wildfire and extreme heat events in California increasing the likelihood of exposure to compound hazards (CH). This study examines the exposure distribution of compound wildfire smoke and extreme heat in California, 2011–2020, and characterizes disproportionate population vulnerabilities. We obtained fine resolution temperature data (4-km) from GridMET and wildfire-influenced fine particulate matter (PM_2.5) estimates (3-km) from a combined metric of geostatistical modeled total PM_2.5 and satellite-detected wildfire smoke plumes. Estimates were aggregated to the ZIP-Code Tabulation Area (ZCTA) level and population weighted. Exposure days to CH and single hazards were defined using a 2-day exposure lag window with binary indicators for wildfire smoke and extreme heat. Daily exposure counts were summed by year and over the 10 years for descriptive mapping. Ten-year exposures were characterized by community factors and differences were tested using ANOVA. Exposures to compound wildfire smoke and extreme heat varied temporally and geographically, primarily driven by wildfire smoke. On average, ZCTAs experienced 3–4 CH days annually, peaking in 2020 (9.85 days). From the early (2011–2015) to later period (2016–2020), ZCTAs experienced 2.77 more annual CH days (95% CI: 2.62, 2.92; p < 0.0001). The number of ZCTAs exposed annually also increased. ZCTAs with persistently higher CH days had significantly higher proportions of minority populations, lower median incomes, and more urban characteristics. Our results show increasing and unequal exposure to compound wildfire smoke and extreme heat. These risks should be considered in mitigation strategies for climate-vulnerable populations.