For a long time, the danger of heat waves has been assessed based on the physiology of healthy young adults, and today the risk to seniors is widely underestimated. By incorporating for the first time their true physiological limits into global climate projections, researchers show that hundreds of millions of older people could be exposed to heat levels their bodies would no longer be able to withstand, much earlier than anticipated.
Heat waves: older adults reach their limits sooner
According to a new global model published in The Lancet Planetary Health, the health impacts of climate warming on the elderly should be approached differently. Up to now, many studies estimated risks using the cooling capacities of healthy young adults as a reference. Yet the body does not respond the same way at 25 years old and after 60. This difference becomes crucial during a heat wave, especially when high temperatures and high humidity combine. In this study, researchers Qinqin Kong from Stanford University, and Daniel J Vecellio from the University of Nebraska Omaha, focused on “uncompensable heat stress,” an extreme situation in which the body can no longer expel enough heat. Sweat and blood circulation are then no longer sufficient to maintain thermal balance: the internal temperature keeps rising.
There is no single outdoor temperature at which the human body would abruptly stop functioning. The danger depends on several parameters, including air temperature and humidity, but also age, health status, and the individual’s physiological capacities. For example, air that is cooler but highly humid can make sweat evaporation much harder.
That is precisely where the new study makes a major shift. The researchers used experimentally established tolerance limits for several age groups, then cross-referenced them with global climate and demographic projections. People aged 60 and older were distinguished from middle-aged adults and younger people. This approach avoids assuming that everyone has the cooling capacity of a healthy young adult.
The gap is substantial. The conditions under which people aged 60 and older can no longer maintain a stable body temperature correspond to heat exposure roughly 4.7 to 7.5 °C lower than that of young adults, according to the experiments used by the researchers. This difference should not be interpreted as a simple air-temperature threshold: it depends on the heat-humidity combination.
With aging, several protective mechanisms lose effectiveness. Older adults tend to sweat less, blood vessels respond more slowly, and the cardiovascular system must work harder under heat. The risk is not limited to heat stroke. High temperatures can worsen cardiovascular, kidney, or respiratory diseases before the body reaches the extreme stage of uncompensable heat stress, the study authors explain.
Laboratory experiments show how concerning crossing this threshold can become. In conditions just above the compensable threshold, a young adult’s core temperature can reach levels associated with heat stroke in about 3 to 10 hours. For an aging population with lower tolerance, the question is no longer merely about the discomfort of a very hot day, but about the physical ability to stay in this environment for an extended period.
At 1.5°C of warming, older adults are already more exposed than younger adults at 4°C
The most striking finding concerns the effect of age compared with warming itself. “One of our most striking results is that older people face uncompensable heat more frequently and over a broader range starting at 1.5°C of global warming, compared with young adults even at 4°C,” explains Qinqin Kong to ABC News Australia.
At only 1.5°C of global warming above pre-industrial levels, 22% of people aged 60 and older would face dangerous heat conditions for at least one month per year, about 290 million people worldwide. Young adults would reach a comparable level of risk only under substantially higher warming scenarios.
In other words, looking only at changes in global mean temperatures is not enough. Demographic aging itself becomes a multiplier of climate risk. Stanford estimates that when accounting for the decline in thermal tolerance with age, the number of people repeatedly exposed to uncompensable heat becomes between 2 and 8 times higher than estimates obtained by applying the thresholds of young adults to the entire population.
The scale of the phenomenon then increases very rapidly with warming. Each additional degree would expose about 1 billion more people to at least 180 hours of uncompensable heat per year. The human body can tolerate only a limited range of ambient heat, as the study notes. The challenge, then, is to determine where and when this physiological boundary will be crossed, and for which population groups.
Heat waves and extreme temperatures: South Asia on the front line
The global map drawn by the researchers is deeply uneven. South Asia, parts of China, and several Gulf countries concentrate a disproportionately large share of the danger. These regions combine high temperatures, sometimes high humidity, large populations, and, in many cases, limited adaptation capacities.
With a global warming of 3°C, India, China, Pakistan, and Bangladesh would alone account for about 1.5 billion people repeatedly subjected to uncompensable heat. This would represent almost three-quarters of the world’s exposed population in this scenario. In Pakistan, nearly 295 million people would be affected, i.e., more than 80% of the population.
In some small states, the phenomenon could become nearly universal. At 3°C of warming, 100% of the population of Bahrain and Qatar would be exposed, while the share would approach 95% in the United Arab Emirates. For seniors, the situation appears even more severe: in seven countries, including Bangladesh and Cambodia and several Gulf states, the entire elderly population could face this level of risk.
The challenge also lies in the duration. A heat wave is not only dangerous when the thermometer peaks in the afternoon. The absence of nighttime cooling prevents the body from recovering. In the most affected areas, around 3°C of warming or more, older people could endure dangerous heat day and night for up to about three consecutive months. The sequence of hot hours becomes as important as the record on the thermometer.
Dangerous temperatures that call for protecting older people in new ways
These results do not mean that every exposed older person will necessarily develop heat stroke. They describe environmental conditions in which the physiological capacity to maintain thermal balance can be exceeded. The study therefore focuses on one mechanism by which extreme temperatures threaten health, and not on the total number of deaths or hospitalizations attributable to heat.
This distinction is essential. A person can suffer dehydration, have heart disease worsen, or experience severe kidney strain well before crossing the physiological limit studied. Conversely, access to a cool home, air conditioning, or a cooling center can greatly reduce actual exposure, even when external conditions become theoretically uncompensable.
The authors emphasize the need to design action plans that are tailored to age and local realities. Alerts based solely on general temperatures may miss part of the vulnerability. Health services must also consider hot nights, duration of exposure, chronic illnesses, social isolation, and real access to cooling resources. For countries with fragile power grids or where air conditioning remains out of reach for a large portion of the population, adaptation becomes even more challenging.
“Our results indicate that intolerable heat will affect many more people, across wider regions and for longer periods than previously recognized,” the researchers write. This study thus does not merely reveal a hotter world. It describes a world in which the combination of warming and an aging population shifts the boundaries of health risk, with older people likely to reach their physiological threshold far sooner than others.
