For more than a decade, climate science operated with a number that offered a certain comfort: 35 degrees Celsius on the wet-bulb scale, sustained for six hours, was the theoretical limit beyond which the human body cannot cool itself. No major heatwave on record had crossed it. The danger, it seemed, was still some distance off.

According to research published Wednesday in Nature Communications, that distance was an illusion. Scientists reanalyzed six of the deadliest heatwaves in modern history and found that every one of them produced conditions that were physiologically unsurvivable for vulnerable people. None approached the 35°C wet-bulb threshold.

The gap between model and mortality comes down to the human body itself. The old benchmark, established in a seminal 2010 paper by climate scientists Steven Sherwood and Matthew Huber, treated humans as simplified heat-dissipation engines. The new model, called HEAT-Lim and co-developed by researchers at the University of Sydney and Arizona State University, incorporates something the old framework largely ignored: sweating declines with age, and direct sunlight makes everything worse.

When those physiological factors were layered in, the picture darkened considerably.

Six heatwaves, six breaches

The study examined events in Mecca (2024), Bangkok (2024), Phoenix (2023), Mount Isa, Australia (2019), Larkana, Pakistan (2015), and Seville, Spain (2003). With the exception of Australia’s sparsely populated Mount Isa, each was associated with at least 1,000 deaths. The 2003 European heatwave alone was linked to more than 70,000 excess deaths.

All six included periods where an older adult — someone over 65 — standing in full sun for six hours could not have survived. In Larkana and Phoenix, the heat was lethal for older adults even in shade. In Larkana, conditions crossed into unsurvivable territory for healthy young adults aged 18 to 35 in direct sun.

The wet-bulb temperatures recorded during these events ranged from 24.3°C in Phoenix to 30.9°C in Larkana — well below the 35°C line that was supposed to mark the boundary of human endurance.

The wrong threshold

Prof. Sarah Perkins-Kirkpatrick, the study’s lead author at the Australian National University, described the findings as a shock. “My first thought was ‘Oh shit’ — I really didn’t expect to see that, especially when you zoom in to individual cities,” she told The Guardian. “If it’s already happening now, then what does a future that is two or three degrees warmer hold?”

The 35°C wet-bulb limit had become a fixture in climate communication, often cited as the point where adaptation reaches its ceiling. But HEAT-Lim shows that the true ceiling is lower, more fragmented, and more dependent on individual vulnerability than anyone assumed.

Crucially, the study found that extremely hot and dry conditions — like those in Phoenix, where dry-bulb temperatures hit 46.7°C — can be just as deadly as the hot, humid conditions that dominate most discussions of wet-bulb risk. The body’s primary cooling mechanism, sweating, fails when evaporation cannot keep pace, whether that failure is driven by ambient humidity or by sheer thermal load.

Who dies, and who counts

The geography of risk is stark. Over the Middle East and Australia, non-survivable conditions overlapped with areas where population density of people over 65 fell below five per square kilometre. Over southern Asia and the India-Pakistan corridor, that same age group was concentrated at densities of 50 to 100 per square kilometre.

The 2024 Mecca event was particularly pointed. The Hajj pilgrimage draws large numbers of older adults who walk considerable distances in open air. Despite what the researchers described as meticulous planning to mitigate heat impacts, a relatively high number of pilgrims still died.

The study’s authors warn that heat mortality in densely populated, low-latitude, and developing regions is “undoubtedly and seriously underreported.” The HEAT-Lim model defines lethality as conditions that cause heatstroke — a core body temperature of 43°C — but the 2003 European event demonstrated that heat kills through cardiovascular and respiratory failures at temperatures well below that threshold.

Prof. Ollie Jay, a co-author and director of the University of Sydney’s Heat and Health Research Centre, put the implication plainly: “Conditions that threaten human life are already here and the risk moving forward is almost certainly much greater than we previously thought.”

Even Sherwood, whose own 2010 work established the 35°C benchmark, endorsed the revision. “The fact we are so close to physiological limits means that mitigating higher temperatures is essential to humans still being able to live and thrive in the hottest and most humid places,” he said, citing northern Australia, much of the tropics, and especially India and the Middle East.

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