Deadly Heatwaves Exceed Human Tolerance Limits

New research finds extreme heat and humidity already posing lethal risks, especially for older adults.

Apr. 9, 2026 at 6:55pm

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A vast, atmospheric landscape painting in muted earth tones, depicting a desolate desert scene shrouded in a hazy, oppressive heat haze, conveying the overwhelming scale and danger of extreme heat conditions.As climate change drives longer and more intense heatwaves, new research reveals the grave, underestimated risks posed by extreme heat and humidity.Phoenix Today

A new study revisited six major global heatwave events over the past two decades, including the 2023 Phoenix heatwave, and found that deadly heat exposure has already exceeded previously assumed human tolerance limits. The research calculated not just temperature and humidity, but how the human body functions under those conditions, discovering 'non-survivable' periods even without reaching the assumed 95°F (35°C) wet-bulb temperature ceiling. The findings show conventional heatwave metrics have understated the danger as climate change drives longer and more intense heat events.

Why it matters

As the climate continues to warm, the study warns that deadly heat conditions have already placed hundreds of millions of people at grave risk, especially older adults whose bodies are less able to cope with extreme heat and humidity. This research highlights the urgent need to reevaluate heatwave preparedness and adapt to the escalating public health threat of climate change.

The details

Researchers revisited six major heat events across the world over two decades, including the 2023 heatwave in Phoenix, Arizona. They calculated not just temperature and humidity, but how the human body functions under those conditions. The study found 'non-survivable' six-hour stretches for people over 65 outdoors in full sun in all six locations, even though none reached the previously assumed human tolerance limit of 95°F (35°C) wet-bulb temperature. Wet-bulb temperature represents the lowest temperature achievable through evaporative cooling, but sweating and evaporation decline with age, causing the body's core temperature to keep rising even in the shade with water.

  • The 2023 heatwave event in Phoenix, Arizona was one of the six major heat events analyzed in the study.
  • The 2015 heatwave in Larkana, Pakistan was another event included in the research.

The players

Sarah Perkins-Kirkpatrick

Lead author of the study published in Nature Communications and researcher at Australian National University.

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What they’re saying

“If it's already happening now, then what does a future that is two or three degrees warmer hold?”

— Sarah Perkins-Kirkpatrick, Researcher

The takeaway

This research shows that deadly heat conditions have already exceeded previously assumed human tolerance limits, posing grave risks to public health, especially for vulnerable populations like older adults. As the climate continues to warm, urgent action is needed to reevaluate heatwave preparedness and adapt to the escalating threat of extreme heat driven by climate change.