HEAT STRESS limits for human survivability may be cooler and drier than thought, a new analysis of six historical heatwaves has found.1
The deadly heat stress threshold has been previously defined by a six-hour exposure to a wet bulb temperature (a measure of humidity characterised by the lowest temperature that can be achieved through evaporative cooling)2 of 35oC.1
However, the recently developed physiology-based HEAT-Lim model says otherwise.
The study precedes yesterday’s Red Extreme Heat Warning for Wednesday and Thursday this week across parts of central and southern England and Wales, issued by the Met Office, with the current UK highest temperature on record for June (35.6 oC) forecast to be broken.
Heat Stress
Heat stress is a leading cause of weather-related deaths.3
It can amplify underlying illnesses including cardiovascular disease, diabetes, mental health, and asthma.
Further, it can increase the risk of accidents and transmission of infectious disease.
Deadly Conditions Currently Occurring
The study used HEAT-Lim to determine whether deadly thresholds were surpassed during six events where conditions were climatologically extreme, high heat-related mortality was reported, or both.
Analysis exposed that non-survivable conditions are already occurring during present-day heat events, all of which are below the 35 oC wet bulb temperature.
Researchers are particularly concerned that temperatures regularly surpass deadly thresholds for older people directly exposed across all events.
Further, they noted, extremely hot but dry conditions are found to be just as deadly as hot and humid ones.
The authors emphasised the importance of adjusting and using increasingly accurate physiology-derived methods to assess risk in relation to potentially deadly heat stress.
References
1 Perkins-Kirkpatrick SE et al. Deadly heat stress conditions are already occurring. Nat Commun. 2026;DOI:10.1038/s41467-026-70485-1.
2 Met Office. What are wet bulb temperatures and what are they used for?. Available at: https://www.metoffice.gov.uk/blog/2025/what-wet-bulb-temperatures-are-and-what-they-are-used-for. Last accessed: 23 June 2026.
3 World Health Organization. Heat and health. Available at: https://www.who.int/news-room/fact-sheets/detail/climate-change-heat-and-health. Last accessed: 23 June 2026.
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