Rapid EHR Surveillance Helps Protect High-Risk Patients During Heat Waves

Heat waves, defined as prolonged periods of excessively high temperature, drive sharp increases in emergency department demand, yet their clinical impact is often quantified only months later. This delay can hinder timely staffing, targeted outreach, and protection of high-risk patients, including older adults and those with kidney disease. A new study shows that near-real-time analysis of de-identified electronic health records can reveal heat-related burdens within days, offering a rapid surveillance approach to support operational decisions during the same heat season..

The approach was led by the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health and the Department of Epidemiology at Boston University School of Public Health, in collaboration with Truveta. It uses de‑identified electronic health records from Truveta member health systems to monitor emergency care during extreme heat. The analysis covered 8.1 million emergency department visits from the summers of 2023 through 2025 across 19 eastern U.S. states.

Investigators linked daily maximum temperatures to state‑level counts of visits for all causes, heat‑related illnesses, kidney diseases and psychiatric conditions. They then estimated the increase in visits on hotter days and the number attributable to extreme temperatures during major heat waves in June and August 2025. Because the electronic health record data were available within one week of each encounter, the system enabled rapid situational awareness while the heat season was still underway.

During the June 2025 heat wave, an estimated 9.2% of all-cause emergency department visits across the 19 states, or about 6,986 additional encounters, were attributable to heat. In August 2025, heat accounted for 8.3% of all-cause visits, or about 6,351 excess encounters, equivalent to roughly one in 10 visits on extreme heat days. Nearly 30% of kidney disease-related visits during heat-wave periods were linked to high temperatures. Across April to August in 2023–2025, warmer days were associated with higher utilization for all causes, kidney diseases, and psychiatric conditions, with increases of 6.3%, 19.5%, and 8.8%, respectively. The findings were published in Nature Health on August 31, 2026.

The authors report that near-real-time emergency department surveillance could validate heat warnings and interventions such as opening cooling centers, guide urban greening and cooling infrastructure toward the most affected areas, and inform worker protections and tailored public health messaging. The same framework could evaluate new measures within a single season by comparing visit patterns before and after implementation. The findings support investment in secure data systems and integration of rapid surveillance into heat action plans and climate resilience efforts, with particular attention to older adults and people with kidney disease.

"These two heat waves alone were responsible for over 13,000 excess emergency department visits—a surge that requires real-time visibility and rapid response. These data underscore the importance of investment in timely population health surveillance and corresponding intervention strategies in the era of increasing extreme heat," said Mary Willis, senior author and assistant professor of epidemiology and environmental health at BUSPH.

"As climate-related health threats become more frequent and severe, timely evidence is increasingly essential for effective public health response. Timely real-world data allow researchers to rapidly assess the health impacts of extreme heat across diverse populations and geographies, providing a clearer picture of risk. This approach has the potential to transform how we monitor, understand and respond to climate-related health emergencies," said Brianna Cartwright, director and lead of Truveta Research at Truveta and coauthor on the paper.

Related Links
Harvard T.H. Chan School of Public Health
Boston University School of Public Health
Truveta