Two heat waves in the summer of 2025 were associated with more than 13,000 additional emergency department visits across 19 Eastern states, according to research published Monday in Nature Health. The more consequential finding may be how quickly researchers were able to see it.
The team analyzed 8.1 million emergency department visits from the summers of 2023 through 2025 using de-identified electronic health records, and reported that the records were available within one week of each visit. Heat and health studies have traditionally relied on insurance claims that arrive 6 to 18 months later, long after the season in which officials might have acted.
That lag is the practical problem the study targets. A cooling center that opens too late, an outreach call that never goes out, a dialysis clinic that does not adjust staffing, and a warning system nobody evaluated until the following spring all stem from the same gap between when harm happens and when anyone can measure it.
The research was led by the Center for Climate, Health, and the Global Environment at the 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.
The Numbers Behind the Headline Figure
During the June 2025 heat wave, about 9.2 percent of all-cause emergency department visits were attributable to heat, corresponding to roughly 6,986 additional visits across the 19 states. During the August 2025 heat wave, about 8.3 percent were attributable to heat, corresponding to roughly 6,351 additional visits. Across both events, close to one in 10 emergency visits on extreme heat days was linked to high temperatures.
The condition-specific figures are where the picture sharpens. Nearly 30 percent of kidney disease related emergency visits during the heat wave periods were attributable to heat. Across the full warm season from April through August in all three study years, warmer temperatures were associated with a 19.5 percent higher rate of kidney disease visits, an 8.8 percent higher rate of psychiatric visits, and a 6.3 percent increase in overall emergency department use.
One finding cuts against a common assumption. Emergency visits might be expected to decline as summer progresses and people adapt, but the researchers reported only a modest decline between the June and August events.
Lead author Amruta Nori-Sarma, deputy director of Harvard Chan C-CHANGE, called the work a proof of concept for how "near-real-time EHR data can be turned into actionable intelligence" in a statement released with the study. She said that seeing older adults with kidney disease driving a spike in real time would let health systems increase outreach, ensure dialysis access and staff up before the next event. Senior author Mary Willis of Boston University said the findings underscore the need for investment in timely population health surveillance.
Reading the Study Honestly
This is an observational analysis linking daily maximum temperatures to state-level counts of emergency visits. It establishes association, not causation, and the excess visit figures are model-based estimates rather than counted cases.
Three limitations belong in the foreground. The data come from Truveta member health systems, which do not cover every hospital in the 19 states, so the counts represent the systems included rather than the full regional burden. The analysis is limited to Eastern states and does not describe the arid Southwest, where heat exposure patterns differ. Attribution was calculated at the state level rather than the neighborhood level, which limits how precisely the findings can guide local targeting.
The study is peer-reviewed, and the published paper carries a Truveta co-author who works for the company that supplied the data, a relationship readers should weigh when evaluating claims about the value of the data platform itself.
Federal surveillance has documented related patterns. A CDC analysis of heat-related illness emergency visits during the 2023 warm season found rates rose sharply across several regions compared with prior years, especially among males and adults aged 18 to 64. Separate research has suggested that humidity may amplify heat risk in ways that temperature readings alone understate.
Who Actually Carries the Risk
The findings point to specific groups rather than the general public. Older adults with chronic kidney disease appear repeatedly in the data, and the mechanism is straightforward. Impaired kidneys handle fluid shifts poorly, and dehydration during extreme heat can push a stable patient into acute injury.
People on dialysis face a compounding problem, since fluid restriction between sessions collides directly with heat-driven fluid loss. Anyone in that situation should be following a nephrology team's specific instructions rather than general hydration advice.
The psychiatric finding deserves careful handling. An 8.8 percent warm-season increase in psychiatric emergency visits is meaningful for planning crisis staffing, but it does not establish that heat causes psychiatric illness. Some medications used in psychiatry impair the body's ability to regulate temperature, and people experiencing homelessness face both higher psychiatric burden and higher heat exposure.
Outdoor workers, infants, people without reliable air conditioning, and people taking diuretics or blood pressure medications round out the higher-risk groups. Households should not conclude that everyone faces the same danger.
Turning the Data into Local Action
Practical steps remain unglamorous and effective. Check on older relatives and neighbors during heat events rather than assuming they will call. Know where the nearest cooling center is before a heat wave, since many cities publish locations only when an advisory activates. Ask a clinician whether any current medication increases heat risk, without stopping anything independently.
Anyone with confusion, fainting, a body temperature that will not come down, hot dry skin, or a sharp drop in urination needs emergency evaluation. Those are signs of heat stroke or acute kidney injury, not ordinary discomfort.
Whether health agencies adopt this kind of real-time surveillance is now a budget and data-governance question rather than a technical one. The researchers called for investment in systems allowing timely, secure use of electronic health records for public health situational awareness. MedicalDaily will report which jurisdictions take it up before the 2027 heat season.
Key Questions Answered
What did the study find? Two 2025 heat waves were associated with more than 13,000 excess emergency department visits across 19 Eastern states, based on an analysis of 8.1 million visits from the summers of 2023 through 2025.
Which conditions drove the increase? Kidney disease stood out. Nearly 30 percent of kidney disease-related emergency visits during the heat wave periods were attributable to heat. Psychiatric visits and all-cause visits also rose across the warm season.
Does this prove heat caused the visits? No. This is an observational analysis showing association. The excess visit counts are model-based estimates, not directly counted cases.
Who faces the highest risk during extreme heat? Older adults, people with chronic kidney disease or on dialysis, outdoor workers, infants, people without reliable air conditioning, and people taking medications that affect temperature regulation or fluid balance.
Why does the speed of the data matter? Traditional claims-based studies arrive 6 to 18 months later. Records available within a week would let agencies adjust cooling center operations, outreach, and hospital staffing during the same season.
What are the study's main limitations? The data cover participating health systems rather than all hospitals, the analysis is limited to Eastern states, attribution was calculated at the state level, and the data provider co-authored the paper.
What symptoms require emergency care during a heat wave? Confusion, fainting, a body temperature that will not come down, hot, dry skin, or a sharp reduction in urination. These can indicate heat stroke or acute kidney injury.
