Electronic Health Records Reveal Heat Wave Toll in Near-Real Time

For decades, public health officials confronting a dangerous heat wave have faced a frustrating paradox: the information they needed most urgently arrived last. Traditional studies of heat and health have depended on insurance claims data that typically surface six to eighteen months after an extreme heat event, long after the summer has ended and the opportunity to protect vulnerable communities has passed. A new study published on August 31, 2026 in Nature Health argues that this delay is no longer inevitable. By tapping into de-identified electronic health records from hospitals across the Eastern United States, researchers demonstrated that the health toll of a heat wave can be quantified within days, while the crisis is still unfolding and interventions can still make a difference.

The research was led by teams at 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 the health data company Truveta. Their approach represents a proof of concept for what the authors describe as near-real time surveillance of climate-related health harms. Rather than waiting for administrative claims to be processed and released, the team drew directly on clinical records generated at the moment patients walked into emergency departments, transforming routine medical documentation into actionable intelligence for an ongoing climate event.

The scale of the analysis was substantial. The researchers examined 8.1 million emergency department visits recorded during the summers of 2023 through 2025, drawing on de-identified electronic health records contributed by Truveta member health systems spanning 19 Eastern U.S. states. They linked daily maximum temperatures to state-level counts of emergency department visits for all causes, heat-related illnesses, kidney diseases, and psychiatric conditions. From these connections, they estimated how much visit rates rose on hotter days and calculated how many visits during the June and August 2025 heat waves could be attributed to extreme temperatures.

The findings quantify, with unusual immediacy, just how heavily heat weighs on emergency care. During the June 2025 heat wave, approximately 9.2 percent of all-cause emergency department visits were attributable to heat, corresponding to roughly 6,986 additional visits across the 19 states studied. The August 2025 heat wave produced a similar burden, with about 8.3 percent of all-cause visits attributable to heat, or approximately 6,351 additional visits. Taken together, the two events were responsible for more than 13,000 excess emergency department visits, a surge that the authors argue demands real-time visibility and rapid response rather than retrospective accounting.

Perhaps the most striking pattern emerged in kidney disease. Nearly 30 percent of kidney disease-related emergency department visits during heat wave periods were attributable to heat, a proportion that underscores how profoundly high temperatures stress the body’s fluid and thermoregulatory systems. Across the entire warm season from April through August in 2023 to 2025, warmer temperatures were associated with a 6.3 percent increase in all-cause emergency department utilization, a 19.5 percent increase in visits for kidney diseases, and an 8.8 percent increase in visits for psychiatric conditions. These associations held across three consecutive summers, suggesting a consistent and predictable relationship between rising temperatures and acute health care demand.

One finding in particular carries weight for how health agencies evaluate their own programs. Intuition might suggest that heat-related emergency visits would ease as summer progresses, as residents acclimatize and heat advisories become routine. The data showed only a modest decline between the June and August heat waves. According to Amruta Nori-Sarma, the study’s lead author and deputy director of Harvard Chan C-CHANGE, this has important implications for real-time evaluation of locally implemented adaptation programs. If adaptation measures such as cooling centers or outreach campaigns are working, their effect should be visible in emergency department patterns within the same season, and if they are not working, agencies can adjust before the next event rather than the next year.

The practical applications the researchers envision extend across the preparedness landscape. Near-real-time emergency department data can validate whether heat action plans and early warning systems are actually reducing health burdens, for example by confirming whether opening cooling centers coincides with fewer heat-related visits. Rapid analyses can help target urban greening and cool infrastructure investments toward the neighborhoods and populations showing the strongest spikes during heat. Electronic health record surveillance can also highlight which groups, such as outdoor workers, older adults, or people with chronic kidney disease, need tailored public health messaging and worker protections as upcoming heat events approach. By comparing emergency department patterns before and after new measures are introduced, health agencies can see within a single summer whether programs are protecting patients.

Underlying the study is a technological shift in how clinical data can be mobilized for public health. The records used came from Truveta, a platform that aggregates de-identified electronic health records from member health systems, and were available within one week of each emergency department encounter. That turnaround, compared with the months-long lag of insurance claims, changes the fundamental character of heat surveillance: it moves from forensic reconstruction to situational awareness. As Brianna Cartwright, director and lead of Truveta Research and a co-author on the paper, noted, 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 and the potential to transform how climate-related health emergencies are monitored, understood, and answered.

The policy implications outlined by the authors are direct. They call for investment in data systems that permit timely, secure use of electronic health records for public health situational awareness, and for the integration of near-real-time emergency department surveillance into heat action plans, emergency preparedness frameworks, and broader climate resilience efforts. They also urge focused attention on the populations the data identify as most at risk, especially older adults and people with kidney disease. As Mary Willis, senior author and assistant professor of epidemiology and environmental health at Boston University School of Public Health, emphasized, the surge of more than 13,000 excess visits from just two heat waves underscores the importance of investing in timely population health surveillance and corresponding intervention strategies in an era of increasing extreme heat.

As climate change lengthens and intensifies heat seasons across the United States and beyond, the gap between harm and response has become one of the most consequential variables in public health. This study suggests that the gap can be closed not by new medical treatments but by faster information: a shift in how existing clinical data are collected, linked, and interpreted. If the approach demonstrated across 19 states can be scaled and sustained, the summers ahead may be the first in which health systems can watch a heat wave’s impact unfold in real time, redirect resources toward the patients and communities being hit hardest, and learn within weeks rather than years which protective measures actually save lives.

Subject of Research: Near-real-time surveillance of heat wave health impacts using electronic health records

Article Title: Rapid response to heat waves: Using electronic health records to protect patients in near-real time

Article References: Rapid response to heat waves: Using electronic health records to protect patients in near-real time. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: heat waves, electronic health records, emergency department visits, public health surveillance, extreme heat, kidney disease, climate change, Nature Health, Harvard Chan C-CHANGE, Boston University School of Public Health, Truveta, heat action plans