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.

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.

Worsening wildfire seasons in recent years are reversing decadal progress on the reduction of harmful air pollutants in the US, particularly in Western states. Measurements of the contributions of wildfire smoke to ambient air pollutants, such as fine particulate matter (PM_2.5), at fine resolution scales would be valuable to public health research on climate vulnerable populations and compound climate risks. We estimate the influence of wildfire smoke emissions on daily PM_2.5 at fine-resolution, 3 km, for California 2011–2020, using a geostatistical modeled ambient PM_2.5 estimate and wildfire smoke plume data from NOAA Hazard Mapping System. Additionally, we compare this product with the US Environmental Protection Agency (EPA) daily and annual standards for PM_2.5 exposure. Our results show wildfires significantly influence PM_2.5 in California and nearly all exceedances of the daily US EPA PM_2.5 standard were influenced by wildfire smoke, while annual exceedances were increasingly attributed to wildfire smoke influence in recent years. This wildfire-influenced PM_2.5 product can be applied to public health research to better understand source-specific air pollution impacts and assess the combination of multiple climate hazard risks.

California’s agricultural industry, an “essential industry” during the COVID-19 pandemic, required support to understand and implement changing public health knowledge and regulations in the workpl...

Background: To prevent wildfires, electric utilities may preemptively de-energize power lines (public safety power shutoffs [PSPS]). These PSPS events may unintentionally harm health, particularly for those with pre-existing conditions like chronic obstructive pulmonary disease (COPD) or who rely on electricity-dependent medical equipment. Health impacts may worsen further if a wildfire occurs despite the power shutoff. To assess the effect of PSPS events, wildfire fine particulate matter (PM _2.5 ) exposure, and their co-occurrence on cause-specific healthcare utilization. Methods and findings: We identified PSPS events in California from 2013 to 2019, categorizing magnitude based on number of people without power by zip code. We leveraged zip code-level average daily wildfire PM _2.5 estimates and respiratory, COPD-specific, cardiovascular, and psychiatric emergency department visits and unscheduled hospital admissions among adults aged 20+ from the California Department of Health Care Access and Information. Using a time-stratified case-crossover design, we estimated the impact of same-day and lagged PSPS events alone, wildfire PM _2.5 alone, and their co-occurrence on healthcare utilization, adjusting for daily temperature. Severe PSPS events were associated with elevated odds of same-day respiratory encounters (OR = 1.06, 95% CI: 1.02, 1.11) and lag _0–3 effects were similar. These associations were stronger for COPD. Compared to no PSPS or wildfire exposure, joint PSPS and 10 μg/m ³ higher wildfire PM _2.5 was associated with higher odds of respiratory encounters (1.16 [95% CI: 1.10, 1.22]), with evidence of multiplicative interaction. Associations were null for cardiovascular and psychiatric encounters. Limitations include exposure misclassification, because we assigned exposure at the residential zip code level, aggregating circuit-level data and not accounting for individual behavior or the potential for PSPS events to impact wildfire PM _2.5 exposure. Conclusions: PSPS events may increase risk of respiratory events requiring acute care utilization, especially when they co-occur with wildfire PM _2.5 .