Study finds ER visits rise sharply with hotter days in California

Using data covering all deaths, emergency department (ED) visits, hospitalizations, and daily temperatures in California from 2006 to 2017, researchers from the University of California San Diego and Stanford University reported that hot and cold days influence illness and deaths differently in California. The findings were published online in the journal Science Advances on July 30, 2025.

“Heat can harm health even when it doesn’t kill,” said Carlos Gould, assistant professor at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego and first author of the study. “Warmer temperatures were consistently associated with more trips to the emergency department, so studies and planning that only consider mortality miss a big slice of the burden.”

Varied health impact by age

The study found that emergency room visits, which reflect a wider range of health impacts across age groups, rise sharply with hotter days. Conditions like injuries, mental health issues, and poisonings show clear increases with heat but are not major causes of death, so they are often missed in studies that focus only on mortality.

“Age plays a critical role in shaping health risks from temperatures,” said Gould. “Older adults are particularly vulnerable to cold temperatures, whereas younger adults and children are more affected by heat.”

Understanding who is affected, how, and at what temperatures is critical for planning appropriate responses to protect health.
Marshall BurkeAssociate Professor of Environmental Social Sciences

While California may see fewer cold‑related deaths as the state experiences fewer extreme cold days, that benefit will be partly offset by more trips to the emergency room as a result of more extreme heat. Researchers suggest that health policy needs to account for differences to address temperature-related impacts in the full population – hospitals, insurers, and public health agencies should prepare for heavier heat demand and tailor warnings and resources to different age groups.

“Understanding who is affected, how, and at what temperatures is critical for planning appropriate responses to protect health,” said study co-author Marshall Burke, associate professor of environmental social sciences at the Stanford Doerr School of Sustainability. “This is true with or without climate change, but a warming climate makes it more important and alters who is exposed to what.”

Economic and social burden of climate change

Health care spending in the United States on chronic disease alone is estimated to exceed $3 trillion annually, which accounts for 17.6% of U.S. gross domestic product, according to the National Health Expenditure Accounts.

Using projections based on moderate climate change scenarios through 2050, researchers estimate California will see around 53,500 fewer deaths overall due to less cold weather – saving approximately $30 billion annually. However, this is partially offset by an estimated additional 1.5 million heat-driven emergency department visits, costing an extra $52 million annually in health care spending.

“We often think about only the most extreme health impacts of heat waves: deaths. This work is showing that many things that we may not think about being sensitive to extreme heat are, like poisonings, endocrine disorders, injuries, and digestive issues,” said study co-author Alexandra Heaney, assistant professor at the Herbert Wertheim School of Public Health. “We need to focus on the full spectrum of health impacts when we think about heat waves, now and in the future.”

This story was adapted from a press release originally published by UC San Diego.

Burke is also an associate professor (by courtesy) of Earth system science; deputy director at the Center on Food Security and the Environment; and a senior fellow at the Freeman Spogli Institute for International Studies, the Stanford Woods Institute for the Environment, and the Stanford Institute for Economic Policy Research. Stanford co-authors of the study include Sam Heft-Neal, a senior research scholar at the Center on Food Security and Environment; Eran Bendavid, a professor of medicine (primary care and population health) and of health policy, and a senior fellow at the Woods Institute for the Environment and (by courtesy) at the Freeman Spogli Institute for International Studies; Christopher W. Callahan, a postdoctoral scholar in the Department of Earth System Science in the Doerr School of Sustainability; and Mathew V. Kiang, an assistant professor of epidemiology and population health. Co-author Josh Graff Zivin is affiliated with UC San Diego and the National Bureau of Economic Research.

This research was supported by the Robert Wood Johnson Foundation, Stanford’s Center for Population Health Sciences, the National Institutes of Health, and the National Institute of Environmental Health Sciences.

This story was originally published by Stanford Doerr School of Sustainability.