A new way to quickly identify who is most at-risk during heat waves offers an opportunity to implement protective measures during ongoing climate events.
BOSTON, MA — As summer heat waves grow longer and more dangerous, public health officials often must wait months to learn who and how many people were harmed. A new study published on August 31, 2026 in Nature Health suggests that delays are no longer necessary, offering the opportunity to assess whose health is most at risk, take protective measures, and test whether interventions are working while the heat season is still underway.
The study, led by researchers at the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health (Harvard Chan C-CHANGE) and the Department of Epidemiology at Boston University School of Public Health (BUSPH) in collaboration with Truveta, offers a novel approach to extreme heat preparedness.
The research team analyzed 8.1 million emergency department (ED) visits from the summers of 2023–2025, using de-identified electronic health records (EHR) from Truveta member health systems across 19 Eastern U.S. states. They linked daily maximum temperatures to state-level counts of ED visits for all causes, heat-related illnesses, kidney diseases, and psychiatric conditions, and then estimated the increase in ED visits on hotter days and the number of visits during the June and August 2025 heat waves that were attributable to extreme temperatures.
“This is a proof of concept for how near-real time EHR data can be turned into actionable intelligence during an ongoing climate event,” says Amruta Nori-Sarma, lead author and deputy director of Harvard Chan C-CHANGE. “If we can see, for example, that older adults with kidney disease are driving a spike in emergency department visits during a heat wave, health systems can respond by increasing outreach, ensuring dialysis access, and staffing up for the next event, well before traditional data sources would typically be available.
“Additionally, while heat related emergency visits might be expected to ease as summer progresses and people adapt, our data show only a modest decline between the June and August heat waves. This has important implications for real-time evaluation of the success of adaptation programs implemented locally. ”
“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,” says Mary Willis, senior author and assistant professor of epidemiology and environmental health at BUSPH. “These data underscore the importance of investment in timely population health surveillance and corresponding intervention strategies in the era of increasing extreme heat.”
“As climate-related health threats become more frequent and severe, timely evidence is increasingly essential for effective public health response,” says Brianna Cartwright, director and lead of Truveta Research at Truveta and co-author on the paper. “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.”
Key findings
- During the June 2025 heat wave, about 9.2% of all-cause ED visits were attributable to heat, corresponding to approximately 6,986 additional visits across 19 states.
- During the August 2025 heat wave, about 8.3% of all-cause ED visits were attributable to heat, corresponding to approximately 6,351 additional visits.
- Across both events, roughly 1 in 10 all-cause ED visits on extreme heat days were due to high temperatures.
- Nearly 30% of kidney disease–related ED visits during heat wave periods were attributable to heat.
- Warmer temperatures were associated with higher ED visit rates for all-cause utilization (6.3% across the entire warm season, April-August 2023-2025), kidney diseases (19.5% across the warm season for all years), and psychiatric conditions (8.8% across the warm season for all years).
Why rapid-response heat surveillance is critical
Traditionally, studies of heat and health have relied on insurance claims data that arrive with long delays, often 6 to 18 months after an extreme heat event. This lag makes it difficult to adjust public health messaging, emergency staffing, or protective programs in time to help the most impacted communities during the same summer.
In contrast, the research team used EHR data available within one week of the ED encounter.
The researchers envision several ways this kind of rapid assessment could strengthen climate and health preparedness:
- Heat action plans and early warning systems: Near-real-time ED data can validate whether warnings and interventions (such as opening cooling centers) are reducing health burdens.
- Urban greening and cool infrastructure: Rapid analyses can help target investments to areas and populations showing the strongest ED spikes during heat.
- Worker protections and public health messaging: EHR-based surveillance can highlight which populations, such as outdoor workers, older adults, or people with chronic kidney disease, need tailored messages and protections during upcoming heat events.
- Evaluating interventions within the same season: By comparing ED patterns before and after new measures are implemented, health agencies can see whether programs are protecting patients and adjust quickly if they are not.
Implications for policy and practice
The study underscores the need for:
- Investment in data systems that allow timely, secure use of EHRs for public health situational awareness.
- Integration of near-real-time ED surveillance into heat action plans, emergency preparedness, and climate resilience efforts.
- Focused attention on populations who are most at risk, especially older adults and people with kidney disease.