Study finds “non-survivable” heat conditions have already occurred during recent global heatwaves
Deadly heat can occur even below the widely accepted 35°C wet-bulb threshold
Dry heat found to be as lethal as humid conditions, challenging existing safety limits
Older people face the highest risk due to reduced ability to regulate body temperature
India and other densely populated regions may be underestimating heat-related deaths
Sustained periods of non-survivable heat stress have already occurred during recent heatwave events across several parts of the world, killing thousands of people, scientists have found.
A new study published in the journal Nature Communications shows that these extreme, unliveable conditions have repeatedly exceeded deadly thresholds, particularly for older people, across six major heatwaves between 2003 and 2024.
The events examined include the West Asia (2024), South East Asia (2024), the United States and Mexico (2023), Australia (2019), India and Pakistan (2015), and Europe (2003).
The scientists used the HEAT-Lim model, which incorporates physiological factors such as sweating limits, humidity and age, to better explain historical heat-related deaths. These thresholds take into account both temperature and the body’s ability to cool itself.
The study found that deadly conditions occurred even when temperatures remained below the widely cited wet-bulb threshold of 35 degrees Celsius (°C).
Heat stress limits for human survivability have traditionally been defined by a six-hour exposure to a wet-bulb temperature of 35°C. However, the study notes that “extremely hot yet dry conditions are found to be just as deadly as hot and humid conditions”.
This suggests that the commonly used 35°C wet-bulb limit does not accurately capture the full range of dangerous conditions, particularly in dry environments.
The study highlights that these conditions are especially dangerous for people aged over 65. “In HEAT-Lim, the human heat stress limits for survival due to heat stroke are much lower than Tw 35°C, which can be further categorised by age and access to shade,” the authors said.
They added that the gap between traditional thresholds and actual risk becomes larger at high temperatures with low humidity, and when people are exposed to direct sunlight. This disparity is even more pronounced among adults aged over 65 due to reduced sweating capacity.
Supporting evidence has also emerged from India. In 2025, scientists in Pune found that April temperatures exceeded 40°C and crossed critical environmental limits for elderly people multiple times.
The study points out that extremely hot but dry conditions can be just as lethal as humid heat. During the 2023 Phoenix heatwave in the US, for example, several non-survivable periods occurred when relative humidity was at or below 20 per cent.
In South Asia, including India, deadly heat stress can occur at lower temperatures when humidity is high, or at very high temperatures even when humidity is low.
During the June 2015 heatwave in India and Pakistan, temperatures reached 46.68°C in Larkana, Sindhwith humidity at 18.26 per cent, and resulted in an estimated 2,500 deaths in India.
The study also notes that densely populated regions such as India face compounded risks. “The density of people over 65 years is consistently over 10 people per sq km, with large regions of 50-100 people per square kilometre. Coupled with high percentages of non-survivable days, these events were extremely deadly for older people in direct sun,” the authors said.
During the 2015 heatwave, between 75 per cent and 100 per cent of days reached non-survivable levels for older people exposed to direct sunlight.
The authors suggest that heat-related mortality in densely populated, low-latitude and developing regions such as India is likely to be significantly underreported. They attribute this to limited monitoring resources and the failure of traditional wet-bulb thresholds to capture many deadly conditions identified by the HEAT-Lim model.
The study says simple steps like staying in the shade can greatly reduce the risks, especially in India. It found that for heat events in Asia, including India and Pakistan, being in the shade could cut the number of dangerously hot days for older people by up to half. In many cases, this risk drops to about 25 per cent of days, compared with 75 per cent to 100 per cent if they are fully exposed to the heat.
Humans do have physiological mechanisms to cope with heat, but these come with limits, said Roy Monteiro, assistant professor at the Department of Earth and Climate Science at Indian Institute of Science Education and Research, Pune.
“At a physiological level, all warm-blooded organisms exhibit strategies to cope with increased heat. Humans respond to prolonged exposure by lowering core body temperature and resting heart rate, sweating more readily, and increasing blood flow to the skin,” he said.
“However, all these strategies come at a cost. Increased sweating requires maintaining hydration, while increased blood flow means the heart has to work harder.”
He added that these physiological responses are only effective when combined with behavioural changes. “These include seeking shade or cooler spaces, adjusting clothing and food habits, drinking more water, or reducing physical activity. These practices that have long been part of cultural traditions in many societies.”
Monteiro said modern lifestyles are eroding these traditional coping mechanisms. “Rural migrants to cities often lose access to social support systems and cultural knowledge that once helped communities adapt to heat, from seasonal diets to changes in work patterns and clothing.”
“At the same time, the demands of the modern economy leave little room for such adaptation. Gig workers, for instance, may not be able to rest when needed or increase water intake due to lack of facilities,” he said, warning that this weakens what he described as the body’s “frontline defence” against heat.
“Our natural responses — thirst, fatigue, the need for rest — are increasingly overridden by economic pressures,” Monteiro added. While climate change and urban heat amplify risks, the erosion of behavioural adaptation is itself a major concern.
“In the long run, the most effective responses will be those that rebuild this frontline defence — whether through stronger social systems, legal protections or a revival of traditional practices,” he said.
Non-survivable heat conditions have already been observed in India at current global warming levels of around 1.1°C — much earlier than previous models had predicted. These models relied on the higher 35 degrees Celsius wet-bulb limit.
The authors concluded that, “Our results underscore the vulnerability of humans to extreme conditions that will become all the more frequent as global warming continues.”