Heat Sends Far More Californians to the Emergency Room Than Official Counts Suggest

Extreme heat is quietly filling emergency departments across California, and the burden is falling hardest on the communities least equipped to cope. A new study published in PLOS Climate by Marisa Donnelly of the University of California, Davis, Brooke Lappe of Emory University, Meredith Milet of the California Department of Public Health, and Jason Vargo analyzed nearly 130 million emergency department visits recorded between 2005 and 2017, and found that heat was responsible for a far larger share of illness than official surveillance statistics have suggested. The research also reveals a stark inequity: residents of the state’s least healthy neighborhoods faced roughly four times greater risk of heat-attributable emergency visits for physical illness, and about twice the risk for mental health and all-cause visits, compared with those living in the healthiest conditions.

The scale of the dataset is what makes the analysis unusual. Most studies of heat and health have focused on single cities, limited age groups, or narrow sets of diagnoses. This study, by contrast, drew on statewide emergency department records from the California Department of Health Care Access and Information, covering every visit in the state over a 13-year span. The researchers sorted the visits into three clinical groupings: heat-related physical illness, which includes septicemia, fluid and electrolyte disorders, congestive heart failure, renal failure, urinary tract infections, and heat stroke; heat-related mental health visits, spanning diagnoses from anxiety and mood disorders to schizophrenia, substance use, and intentional self-injury; and all-cause visits, meaning every emergency department encounter regardless of diagnosis. In total, the physical illness grouping contained more than 13.8 million visits and the mental health grouping more than 15.7 million.

To connect those visits to temperature, the team used daily gridded mean temperature data at 4-kilometer resolution from the PRISM Climate Group, a statistical mapping system that blends observations from federal, state, and local weather station networks with a digital elevation model to account for California’s notoriously complex topography, including rain shadows, temperature inversions, slope aspect, and coastal proximity. Using Google Earth Engine, the researchers performed a spatial join that computed the area-weighted mean of all grid cells intersecting each ZIP code polygon on each day. Over the study period, the statewide average temperature was 16.2 degrees Celsius, with ZIP code averages ranging from 12.6 to 18.1 degrees across the state’s six Health Officer Regions, which span everything from the cool, foggy Bay Area to the scorching agricultural interior of Central California.

The statistical engine of the study was the distributed lag nonlinear model, or DLNM, a framework widely used in environmental epidemiology that can simultaneously capture two features of the temperature-health relationship: its nonlinearity and its timing. In the first stage, the researchers fitted a quasi-Poisson time-series regression for each individual ZIP code, modeling the exposure-response curve with a quadratic B-spline whose three internal knots were placed at the 10th, 75th, and 90th percentiles of each location’s temperature distribution. The lag-response curve, which describes how risk evolves after exposure, was modeled with a natural cubic spline over a three-day window, reflecting evidence from prior heat-health studies that most excess risk occurs within zero to three days of exposure. Alternative specifications with longer lags and different spline complexity were compared using the Akaike Information Criterion, and the final model produced smooth, epidemiologically plausible curves, with risk peaking within a day of exposure and declining toward zero by day three.

Because small ZIP codes with few emergency visits produce noisy estimates, the second stage pooled the location-specific results with a multivariate meta-regression and derived best linear unbiased predictions for each ZIP code, allowing sparsely populated areas to borrow statistical strength from larger, climatically similar neighbors. Local average temperature and temperature range were included as meta-predictors, since local climate is known to modify how temperature affects health. From these fitted curves, the team calculated each ZIP code’s minimum morbidity temperature, the point at which heat-related illness is minimized, and used it as the reference for computing attributable fractions: the proportion of emergency visits that would not have occurred had temperatures stayed at that optimum.

The headline numbers are striking. Statewide, 5.1 percent of heat-related physical illness visits, 5.4 percent of heat-related mental health visits, and just under 1 percent of all-cause emergency visits were attributable to heat. Regionally, Central California and Los Angeles carried the largest heat-attributable fractions for physical illness, at 6.5 percent and 6.1 percent respectively, while Los Angeles, Southern California, and Central California led for mental health visits. The Rural North and Greater Sierra-Sacramento regions, despite their cooler average temperatures, had the largest fractions of all-cause heat-attributable visits, a pattern the authors suggest may reflect lower acclimatization and weaker adaptive capacity in communities unaccustomed to extreme heat.

The most consequential findings, however, concern social inequality. To measure it, the researchers turned to the California Healthy Places Index, or HPI, a composite metric developed by health departments and community organizations that ranks every census tract on a 0-to-100 scale based on 23 social determinants of health most strongly associated with life expectancy at birth. These indicators are grouped into eight policy-action domains covering the economy, education, transportation, housing, social resources, the built and clean environment, and health-care access. Low-scoring communities are marked by poverty, unemployment, overcrowded and cost-burdened housing, limited car ownership, long commutes, sparse tree canopy, higher pollution, and low insurance coverage. When the researchers divided ZIP codes into HPI quartiles, a clear gradient emerged: as community conditions improved, emergency visit rates and heat-attributable rates fell for all three outcome categories. ZIP codes in the lowest quartile held 27 percent of the state’s population but contributed 33 percent of physical illness visits, 32 percent of mental health visits, and 35 percent of all-cause visits, and had 2.2 times more total emergency visits than the healthiest quartile.

Expressed as rates, the disparity becomes even sharper. Statewide, heat-attributable visits averaged 141 per 100,000 people annually for physical illness, 165 for mental health, and 317 for all causes. In the lowest HPI quartile, the rate ratios relative to the healthiest quartile were 4.3 for physical illness, 2.1 for mental health, and 2.6 for all-cause visits, with confidence intervals that did not overlap across quartiles. Central California recorded the highest physical illness rate at 218 visits per 100,000, a rate ratio of 3.4 relative to the Bay Area, while the Rural North posted the highest mental health and all-cause rates at 234 and 852 per 100,000. The authors point to mechanisms embedded in the HPI itself: limited access to parks, beaches, shade, and transportation, combined with less air conditioning and fewer green spaces, can raise both heat exposure and vulnerability, while higher stress and reduced access to health services may amplify the physiological and psychological toll.

The study also exposed a systematic blind spot in official statistics. When the researchers compared their population-weighted county estimates with crude heat-associated emergency visit rates in Tracking California, the state’s environmental public health tracking network, their estimates were on average seven times higher, with a range of 2.3 to 18 times. Tracking California relies on heat-specific diagnosis codes appearing in medical records, which the authors argue captures only direct attribution and misses the many heat-related illnesses that present as cardiac, renal, or psychiatric emergencies. Correlations between the two data sources were strong for physical outcomes but weak and statistically insignificant for mental health, suggesting that the psychological burden of heat is the most undercounted of all.

With climate projections pointing to more frequent, intense, and longer-lasting heat waves, the authors frame their results as a conservative baseline rather than a worst case, noting that the study period ends before California’s recent record-breaking heat. They argue that extreme heat action plans, such as California’s 2022 state resilience strategy, could be made more effective by directing investments toward the ZIP codes this analysis identifies as having both unhealthy conditions and high heat-attributable rates: more trees and shaded areas, public air-conditioned spaces, cooling stations, cool roofs and pavement, and housing-focused measures such as weatherization incentives, subsidies for cooling technologies, and utility assistance. Because the study assigned visits to residential ZIP codes, it excluded people experiencing homelessness, who are among the most heat-vulnerable, and the authors caution that humidity and air pollution were not controlled. Even so, the message is unambiguous: heat is not an equal-opportunity hazard, and protecting the communities with the fewest resources for healthy living is both an equity imperative and the most efficient way to reduce the strain that rising temperatures place on emergency care.

Subject of Research: Inequities in heat-attributable emergency department visits across California communities

Article Title: Inequities in heat-related emergency department visits in California from 2005 – 2017

Article References: Donnelly, M., Lappe, B., Milet, M., & Vargo, J. (2026). Inequities in heat-related emergency department visits in California from 2005 – 2017. PLOS Climate, 5(10), e0000586. https://doi.org/10.1371/journal.pclm.0000586

Image Credits: AI Generated

DOI: 10.1371/journal.pclm.0000586

Keywords: extreme heat, emergency department visits, California, health equity, Healthy Places Index, distributed lag nonlinear model, climate change, mental health, social determinants of health, heat-attributable fraction, public health surveillance, acclimatization