UC Davis

Wildfires and extreme heat events are inherently linked; shared climatic drivers are simultaneously worsening both hazardous events in California. Populations likely experience exposure to a combination of these hazards, i.e., compound hazards; yet there is little known about this public health threat. Studies indicate wildfire smoke and extreme heat, independently, elevate risks for morbidity and mortality. In addition, epidemiologic evidence gives reason to suspect compound wildfire smoke and extreme heat could exacerbate health outcomes and present a greater threat to public health.This dissertation aims to examine multiple facets of compound wildfire smoke and extreme heat in California. Chapter one assesses the distribution of compound wildfire smoke and extreme heat exposures in California from 2011-2020. The exposure assessment uses a clinically meaningful definition of exposure to estimate populations with disproportionate vulnerabilities. Chapter two then estimates the joint effects of exposure to wildfire smoke and extreme heat within a short-term exposure lag window. Chapters two and three both utilize a statewide payer’s database to examine all-natural cause, cardiovascular, cerebrovascular, renal, and respiratory morbidity impacts. In Chapter three, exposures are examined at different time periods to evaluate temporally compounding wildfire smoke and extreme heat in two ways; that is, effect modification of extreme heat-related morbidity by wildfire smoke exposure weeks to months prior, and vice versa. Methods include descriptive mapping and exposure assessment to identify disproportionately exposed populations. Joint effects and effect modification are estimated using time-stratified case crossover design with conditional logistic regression models. Each chapter investigates different periods of exposure and characterizes findings by individual or community level factors, such as race and ethnicity, income, education, and rural and urban classifications. These studies utilize secondary datasets from trusted agencies in environmental health monitoring and exposure assessment, including public health surveillance. Altogether, the findings from this dissertation work provide evidence on the public health impacts of compound wildfire smoke and extreme heat. Exposures to compound wildfire smoke and extreme heat are increasing and their health impacts are synergistic, even for exposures across different periods of time. Individuals across California have been impacted by compound wildfire smoke and extreme heat in recent years; however, exposures were consistently highest within the Central Valley. In addition, health impacts were disproportionate, often by race and ethnic groups. This work improves characterization of vulnerable populations to help target joint mitigation of compound hazard risks.