Joint Effects of Wildfire Smoke and Extreme Heat on Hospitalizations in California, 2011-2020

Wildfire smoke and extreme heat events are worsening in California, but their combined health effects are not well understood. This study estimates joint effects of extreme heat and wildfire smoke on hospitalizations in California, 2011-2020. We used a case crossover design with time-stratified controls and conditional logistic regression to estimate these effects at multiplicative and additive scales. Exposures were assessed for 16 combinations of exposure lags (0-3 days) for extreme heat and wildfire influenced fine particulate matter. Among over 28 million cases of all-natural cause morbidity, the majority were adults aged 65 and older (41.4%), English speakers (85.1%), and White, non-Hispanic (49.7%), mostly residing in urban areas (97.2%). The study found roughly 8% of respiratory morbidities (95% CI, 2.4%-13.8%) were attributable to the interaction of wildfire smoke and extreme heat. Significant joint effects were also observed for cardiovascular (5.5%) and renal morbidities (6.2%). Subgroup analyses revealed stronger effects: Respiratory (19.2%, 95% CI 6.5%-32.1%) and cerebrovascular morbidities (15.7%, 95% CI 4%-27.4%) were most pronounced in Black individuals; older adults (50-64 years) showed strong effects for renal morbidities (15.4%, 95% CI -1.6%-32.6%); and cardiovascular effects were highest among females (9.8%, 95% CI 2.9%-16.7%). Effects on all-natural cause morbidity were generally null. The interaction of wildfire smoke and extreme heat within a short exposure window (4 days) increases hospitalizations; highlighting the need for joint heat and wildfire smoke interventions that target populations at greater risk.