Extreme heat drives thousands of additional emergency visits as researchers warn health systems must prepare

Extreme heat can place substantial additional pressure on emergency departments, with new research estimating that thousands of hospital visits across the US during two major heatwaves were attributable to high temperatures.

Researchers analysing more than eight million emergency department visits across 19 states found that heat was responsible for an estimated 9.2% of all emergency visits during a four-day heatwave in June 2025. During a second major heatwave in August, an estimated 8.3% of visits were attributable to the heat.

Published in Nature Health, the study provides one of the clearest indications yet of how periods of extreme temperature can affect demand across emergency healthcare rather than simply increasing the number of patients diagnosed specifically with heatstroke or heat exhaustion.

The researchers analysed 8,169,637 emergency department visits recorded between April and August during 2023, 2024 and 2025. Electronic health records from 19 US states were combined with daily temperature data to examine how emergency demand changed as temperatures increased.

During the June 2025 heatwave, which affected large parts of the eastern United States between 22 and 25 June, researchers estimated that high temperatures were responsible for approximately 6,986 additional emergency department visits.

A second heatwave between 10 and 13 August was associated with an estimated 6,351 additional visits. Taken together, the two relatively short periods of extreme heat generated more than 13,000 additional emergency visits across the states included in the study.

The effects were not limited to conditions that would ordinarily be described as heat-related illness. Researchers found associations between higher temperatures and several different categories of emergency care, including kidney disease and mental health conditions.

Kidney problems were particularly strongly associated with extreme temperatures. During the heatwave periods, researchers estimated that almost 30% of emergency department visits relating to kidney disease were attributable to the heat.

That relationship has a clear physiological explanation. Hot weather increases fluid loss through sweating, raising the risk of dehydration and placing additional strain on the kidneys. People with existing kidney disease can be particularly vulnerable, while severe dehydration can also contribute to acute kidney injury in otherwise healthy people.

Mental health was another area in which emergency demand increased with temperature. Across the warm seasons studied between 2023 and 2025, warmer temperatures were associated with an 8.8% increase in emergency department visits for psychiatric conditions.

The relationship between heat and mental health is increasingly recognised within public health research. High temperatures can disrupt sleep, affect the body’s ability to regulate stress and exacerbate some existing psychiatric conditions, while certain medications can also interfere with temperature regulation or increase vulnerability to dehydration.

The latest findings demonstrate why measuring the health effects of heat solely through diagnoses such as heatstroke can substantially underestimate its impact.

Extreme temperatures can aggravate existing cardiovascular, respiratory and renal disease, interact with medications and contribute to dehydration, while people with mental health conditions, older adults, children, pregnant women, people experiencing homelessness and those working outdoors can face additional risks.

For emergency departments, that means a heatwave can produce a surge involving many different types of patients rather than a clearly defined group arriving with recognisable heat-related illness.

Researchers found that the relationship between temperature and emergency department use was not restricted to the most extreme days. Across the entire April to August period during the three years studied, warmer temperatures were associated with a 6.3% increase in all-cause emergency department visits.

The study is particularly significant because of the way researchers obtained the information. Instead of relying on datasets that can take months or years to become available, the team used continuously updated electronic health records.

Researchers say this could eventually allow public health authorities and healthcare systems to monitor the health consequences of extreme temperatures much closer to real time.

That could make heat alerts more clinically useful. If health systems can identify rapidly when emergency visits are increasing alongside temperatures, hospitals could potentially adjust staffing and capacity, while public health teams could intensify interventions aimed at populations known to be particularly vulnerable.

Lead author Amruta Nori-Sarma, assistant professor at Boston University School of Public Health, said timely real-world health data could provide a clearer understanding of which populations and geographical areas were being affected during extreme heat and help authorities respond more effectively.

The research has limitations. The 19 states included in the analysis do not represent the entire United States, and the study estimates the proportion of emergency visits attributable to temperature using statistical modelling rather than identifying individual patients whose attendance was definitively caused by heat.

Researchers also examined associations over several days because the health effects of extreme temperatures do not necessarily occur immediately. Some conditions may worsen on the day temperatures rise, while others can result in an emergency visit several days later.

Nevertheless, the scale of the estimated effect has significant implications for healthcare planning as periods of extreme heat become more frequent and intense.

A separate body of research has already warned that healthcare systems themselves can struggle during extreme heat. Hospitals and care facilities may face increased patient demand at the same time as staff experience difficult working conditions, buildings overheat and electricity systems come under additional strain from increased cooling requirements.

The issue is not confined to the United States. Health leaders in England are also beginning to question whether summer preparedness needs to become more systematic. England’s chief nursing officer Duncan Burton recently suggested the NHS may require increased planning for future hot summers, similar to the extensive preparations traditionally undertaken ahead of winter.

The US findings help demonstrate why that conversation matters. If extreme temperatures can contribute to close to one in every 10 emergency visits during the most intense heatwaves, hot weather becomes more than a public health warning about staying hydrated and avoiding the midday sun.

It becomes a capacity-planning issue for the entire health system.

For decades, healthcare systems in countries with temperate climates have organised substantial parts of their operational planning around winter, anticipating increases in respiratory infections, hospital admissions and pressure on emergency services.

Increasingly extreme summers may require a similar approach. Hospitals may need to anticipate additional emergency demand, identify vulnerable patients before heatwaves arrive and ensure their buildings, staffing and community services can continue operating safely during prolonged periods of high temperatures.

The latest research suggests that the impact of heat is already visible inside emergency departments. The question for health services is increasingly whether they can prepare for that pressure before the temperature starts to rise.

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