Older adults can reach uncompensable heat stress at conditions younger people may still tolerate.
At 1.5°C warming, about 290 million people over 60 could face at least 180 hours of uncompensable heat annually.
At 3°C warming, more than 80% of India’s population aged 60 and above could face at least 180 hours of such heat.
Older adults can reach dangerous levels of heat stress under conditions that younger people may still tolerate, new research suggests. The study 'Exceeding human heat tolerance in a warming, ageing world: a global projection modelling study' published in The Lancet Planetary Health found that the heat conditions under which people aged 60 and above can no longer maintain a stable core temperature are roughly 4.7°C to 7.5°C lower than those for young adults.
Led by Stanford researcher Qinqin Kong, the study found that older adults face uncompensable heat — when the body can no longer shed enough heat to keep its core temperature stable — more frequently and across wider areas as global temperatures rise. The researchers say assessments based on young-adult heat-tolerance limits may therefore underestimate the exposure faced by older populations.
What is uncompensable heat stress?
Uncompensable heat stress occurs when the body cannot lose heat quickly enough to maintain thermal balance. As heat accumulates, core body temperature can continue rising, increasing the risk of heat-related illness.
This is not the same as saying that there is one fixed air temperature at which every older person becomes unsafe. Heat tolerance also depends on humidity, air movement, physical activity, clothing and hydration.
The researchers incorporated age-specific physiological heat-tolerance limits into climate and population projections, rather than applying a threshold derived from young adults across all age groups. The study compared heat tolerance across younger, middle-aged and older adults to estimate how exposure changes with age.
They found that older adults could encounter uncompensable heat far more often as global warming increases. At 1.5°C of warming above pre-industrial levels, about 22% of people aged over 60 — roughly 290 million people — could experience at least 180 hours of such heat exposure annually, equivalent to roughly six hours a day for a month. At higher warming levels, the exposed population rises substantially.
The measure is a physiological threshold, not a forecast of heatstroke or death. It indicates conditions in which the body is unable to maintain heat balance, and therefore provides a way of identifying potentially dangerous exposure that conventional age-neutral assessments may miss.
Why ageing makes heat harder to handle
The body mainly relies on sweating and increased blood flow to the skin to release excess heat.
As people age, both mechanisms can become less effective. Blood vessels near the skin normally widen in response to heat, allowing more blood to reach the body's surface. The body then transfers heat from the blood to the environment. Sweating provides another route for cooling as sweat evaporates from the skin.
Older adults may have a weaker cardiovascular response and reduced sweating capacity. This can limit how quickly heat is moved to the skin and lost to the environment.
Thirst and the perception of temperature can also change with age. An older person may therefore become dehydrated or overheated without recognising the problem as quickly as a younger person.
The World Health Organisation identifies older adults as a group particularly vulnerable to heat because ageing can reduce the body's ability to regulate temperature. Chronic diseases, medication, mobility limitations and living conditions can further increase exposure or reduce the ability to escape heat.
Heat can worsen existing illness
The danger does not begin only when someone develops heatstroke.
Heat places additional strain on the cardiovascular system and can worsen existing cardiovascular, respiratory, kidney and diabetes-related conditions. Dehydration can reduce circulating blood volume and place further stress on the heart and kidneys.
Some medicines can also affect sweating, fluid balance, blood pressure or kidney function. This does not mean older people should stop prescribed medicines during hot weather. The WHO and other health authorities advise people to seek medical advice before making changes to medication.
The broader evidence shows why this matters. The 2025 Lancet Countdown reported that the global rate of heat-related mortality had risen 23% since the 1990s, with an average of 546,000 heat-related deaths each year during 2012–2021. It also found that adults older than 65 and infants younger than one experienced more than 20 heatwave days per person in 2024, around four times the level recorded two decades earlier.
While the new study measures physiological heat tolerance rather than deaths, separate research published in The Lancet Planetary Health in 2026 found that each additional heatwave was associated with 8.83 additional deaths per 10,000 person-years among US adults aged 65 and older. Across the contiguous United States, researchers estimated that 8,307 observed heatwaves between 2000 and 2018 were associated with about 17,600 excess deaths.
Why India faces a particular risk
India's exposure is shaped by more than high daytime temperatures. Humidity can reduce the effectiveness of sweating, while dense urban areas can remain hotter after sunset. Poorly ventilated homes may retain heat overnight, reducing the body's opportunity to recover.
The 2025 Lancet Countdown India data sheet found that people in India experienced an average of 19.8 heatwave days in 2024. Of those, 6.6 days would not have occurred without human-caused climate change.
The new research suggests that age could substantially alter the scale of that exposure. Its projections identify South Asia among the regions facing the greatest future exposure. At 3°C of global warming, the study projects that more than 80% of India's population aged 60 and above could experience at least 180 hours of uncompensable heat annually, on average. In Delhi and the wider Indo-Gangetic Plain, older adults are projected to experience close to or more than 2,000 hours of such heat a year at 3°C of warming.
That does not mean more than 80% of older Indians would necessarily suffer heatstroke. It means the environmental conditions could exceed the body's capacity to maintain thermal balance for substantial periods. The distinction matters when interpreting what the study does — and does not — show.
The risks can also be greater for older people living alone. Confusion, unusual weakness, dizziness or difficulty moving may go unnoticed if nobody is nearby. The researchers also caution that people living in hotter climates may be more acclimatised to heat; they tested higher heat-tolerance limits for such populations, but said the main conclusions remained unchanged.
What needs to change?
The central finding challenges the use of a single, age-neutral measure of heat danger. A heat warning designed around the physiological limits of a healthy younger adult may not capture all the conditions that place an older person under dangerous heat strain. Individual health, humidity, duration of exposure, activity and access to cooling also matter.
The World Health Organisation says older adults should be protected from excessive heat by staying cool, avoiding strenuous activity during the hottest periods, drinking regularly and being checked on during heatwaves.
For public-health systems, the new evidence strengthens the case for treating older adults as a priority group in heat-health action plans, including targeted warnings, cooling facilities and medical preparedness.
The question is therefore not simply how hot the weather becomes, but whether the human body can still keep up with it. For older adults, the latest evidence suggests that limit can arrive earlier than conventional, age-neutral assessments have assumed.
