Health: Heatwaves raise health burden in USA (Nature Health)

Nearly one in ten emergency department visits during major US heatwaves in 2025 were attributable to heat, while household air conditioning may avert more than 5,000 heat-related deaths in the US annually, according to two papers published in Nature Health.

Extreme heat is linked to health conditions ranging from heat rash and muscle cramps to severe heat stroke, as well as cardiovascular, respiratory, and kidney diseases. As heat exposure and ageing populations are expected to increase future health burdens, rapid and accurate monitoring of heat-related illness and increased access to cooling are important public health responses. Although air-conditioning ownership in the USA is estimated to be around 90%, access to cooling is much lower in many other parts of the world, including Europe, where household air conditioning remains relatively uncommon.

In one paper, Amruta Nori-Sarma and colleagues analysed 8,169,637 emergency department visits from electronic health records across 19 eastern US states between April and August in 2023, 2024, and 2025. They estimate that 9.2% of all-cause emergency department visits during the June 2025 heatwave and 8.3% during the August 2025 heatwave were attributable to heat over the day of exposure and the following four days. This is equivalent to 6,986 and 6,351 visits, respectively.

In a separate study, Kai Chen and colleagues analysed 29,991,000 deaths in the contiguous USA from 2010 to 2020. They found that household air conditioning use averted an estimated 5,267 heat-related deaths annually, reducing heat-attributable mortality by more than half. They also observed that the health benefits plateaued above the median level of household air conditioning usage, suggesting that over-use of air conditioning provides no additional protection against heat-related mortality.

Together, the findings suggest that near-real-time health data may help to guide responses during heat events. While air conditioning remains an important protective measure against heat-related mortality, its benefits and the harms of over-use must be balanced. Amruta Nori-Sarma and colleagues note limitations of the research, including the use of state-level health data and diagnostic coding, which may not always capture heat impacts. Similarly, Kai Chen and colleagues note the use of county-level air conditioning estimates, use of 2015 cooling data across the full study period, and the lack of assessment of the indirect harms of air conditioning as limitations of their findings. 

• Article
• Published: 31 August 2026

Nori-Sarma, A., Cartwright, B.M., Zanobetti, A. et al. Impact of the summer 2025 heatwaves on emergency department visits in the USA. Nat. Health (2026). https://doi.org/10.1038/s44360-026-00194-y

• Article
• Published: 31 August 2026

Chu, L., Akkiraju, K., Rao, N.D. et al. Impact of household air conditioning usage on heat-related mortality in the USA. Nat. Health (2026). https://doi.org/10.1038/s44360-026-00184-0

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