Extreme heat can place enormous pressure on hospitals, yet public health officials often wait months for complete data showing who was affected and how severely.

New research suggests that electronic health records could dramatically shorten that delay, allowing health systems to assess the impact of heat waves within days rather than months.

The study, published in Nature Health, was led by researchers from Harvard T.H. Chan School of Public Health and Boston University School of Public Health in collaboration with Truveta. The team analysed 8.1 million emergency department visits recorded during the warm seasons of 2023 to 2025 across 19 eastern US states.

Heat Drove Thousands of Visits

Researchers linked daily maximum temperatures with state-level emergency department visits for all causes, heat-related illness, kidney disease and psychiatric conditions.

During the June 2025 heat wave, around 9.2% of all emergency department visits were attributed to high temperatures, representing approximately 6,986 additional visits. During the August heat wave, the figure was about 8.3%, or roughly 6,351 extra visits. Together, the two events were associated with more than 13,000 excess emergency department visits.

Across both heat waves, roughly one in ten all-cause emergency visits on extreme-heat days was linked to high temperatures.

Kidney-related illness showed an especially strong association. Nearly 30% of emergency department visits involving kidney disease during the heat-wave periods were attributed to heat.

Risks Extended Beyond Heat Illness

The findings suggest that extreme temperatures can affect a much broader range of health problems than heat exhaustion or heatstroke alone.

Across the warm seasons from April to August between 2023 and 2025, higher temperatures were associated with a 6.3% increase in all-cause emergency department use. Visits related to kidney disease rose by 19.5%, while visits involving psychiatric conditions increased by 8.8%.

Researchers also found only a modest decline in heat-related emergency visits between the June and August 2025 events. This is important because it suggests that any seasonal adaptation to heat may be limited and that protective measures still need to be evaluated throughout the summer.

Faster Data Could Change Responses

Traditional studies often depend on insurance claims data, which may not become available until six to 18 months after an extreme weather event. By contrast, the electronic health record data used in this study were available within one week of an emergency department visit.

That speed could allow hospitals and public health agencies to respond while a heat season is still underway. If the data show a sudden rise in cases among older adults or people with kidney disease, health systems could strengthen outreach, improve access to dialysis services or increase staffing before the next heat event.

Near-real-time surveillance could also help officials assess whether interventions such as cooling centres, early-warning systems, worker protections and public health messaging are actually reducing harm.

Who Needs the Most Protection?

The researchers emphasised the importance of focusing on groups that may face greater risks, particularly older adults and people living with kidney disease.

Electronic health records could also help identify where the sharpest increases in emergency visits are occurring, allowing cities to target measures such as urban greening, cooling infrastructure and community outreach more precisely.

The study does not eliminate the need for longer-term epidemiological research, but it shows how faster access to health data could improve decision-making during climate emergencies. As extreme heat becomes more frequent and severe, the ability to track its health effects almost as they occur may become an increasingly important tool for protecting vulnerable patients.