A much higher number of elderly people than previously expected are at high risk of illness or death from extreme heat as temperatures climb under the effects of climate change, scientists have discovered.
A new study by researchers at Stanford University in the United States factored in the way that age affects the body’s tolerance for heat, shining light on a new level of risk from heatwaves on the global population.
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As the number of people aged 60 years and older is projected to more than double from one billion in 2020 to 2.1 billion in 2050, representing 21 percent of the world’s population, the Stanford study warns that rising temperatures could become far more deadly than expected.
“Previous assessments often assumed that everyone could tolerate heat as well as a healthy young adult, which substantially underestimated the risk to older people,” Chris Mullington, honorary clinical senior lecturer at Imperial College London in the United Kingdom, told Al Jazeera. The new study “reinforces that dangerous heat cannot be defined by one universal temperature threshold. It depends partly on who is being exposed”.
Why are elderly people so vulnerable to extreme heat?
As we age, our bodies lose the natural ability to cool themselves – we sweat less and our hearts have to work harder as blood vessels do not expand as much. As temperatures rise, the body eventually reaches a point at which it cannot cool quickly enough to compensate. This point – known as “uncompensable heat stress” – is reached much sooner and at lower temperatures by elderly people than younger adults.
People aged 65 and older face more exposure to uncompensable heat at 1.5C of global warming above preindustrial levels than younger adults would at 4C of warming, the researchers found.
Long-term global warming is currently estimated at about 1.4C above preindustrial levels. The annual global average exceeded 1.5C for the first time in 2024, according to the European Union’s Copernicus agency, but long-term, multidecade warming has not yet crossed this threshold.
However, the Stanford researchers estimate that the number of people who experience at least 180 hours a year beyond their age group’s tolerable heat limit is already more than double what it was previously thought to be. This is a key threshold identified by the report.
“Those 180 hours are accumulated across the year and are not necessarily continuous,” Mullington said. “Importantly, this describes exposure to potentially uncompensable conditions; it does not mean that everyone exposed will inevitably become ill.”
It does mean, however, that older adults are at risk of heat-related death and illness at much lower temperatures than previously thought.
“Conditions that a healthy young adult might still tolerate could already cause an older person to store heat, experience a rising core temperature and suffer serious illness including heatstroke,” Mullington said.
What effect does extreme heat have on the human body?
Extreme “ambient” heat is unusually severe heat, or heat at a level that crosses a climatic or health-related threshold. This can lead to severe illness, such as heatstroke or heart attack, or even death.
It is tricky to predict health consequences from global averages as air temperature alone is not a reliable marker of potential physical danger. “Humidity, sunlight, air movement, physical activity, clothing and an individual’s capacity to lose heat all matter,” Mullington said.
“A relatively small rise in the global average shifts the entire temperature distribution, producing a much larger increase in the frequency, severity and duration of conditions at the dangerous extreme.”
This means that conditions which a healthy young adult might be able to tolerate could cause an older person to store heat, experience a rising core temperature and suffer serious illness such as heatstroke, Mullington said.
Furthermore, only a proportion of deaths during heatwaves are directly caused by hyperthermia – overheating – according to the study, which attributed most cases to comorbidities, particularly cardiovascular, renal and respiratory conditions that are exacerbated by elevated body temperature.
Where will elderly people be worst affected?
Warming around the world is happening at a speed not seen in the past 10,000 years, according to NASA data cited in the Stanford study. At the same time, the world’s population is rapidly ageing due to longer life expectancies and falling fertility rates, meaning a greater proportion of people are vulnerable to severe heat.
Elderly people in low-income countries such as China, India and Pakistan are likely to be at highest risk, even at global warming just 1.5C above preindustrial levels. As that temperature rises, the geographical areas affected will expand.
According to the study, each additional degree of global warming above preindustrial levels will bring about one billion more people into the bracket of experiencing 180 hours of uncompensable heat per year. Most of these people are likely to be elderly, as they reach this threshold much sooner than younger adults.
Predictions vary according to the projected temperature rise above preindustrial levels.
At 3C of warming above preindustrial levels, the study’s authors predicted, India, China, Pakistan and Bangladesh will account for 1.5 billion people facing uncompensable heat exposure, nearly three-quarters of the total population affected worldwide.
What can be done to mitigate the effects of extreme heat?
Stanford researchers concluded that understanding where, when, and to what extent human heat tolerance limits will be breached is increasingly important for the development of mitigating strategies.
Suggested adaptations included revising the urban landscape to expand green infrastructure, enhance ventilation, reduce anthropogenic heat emissions, establish cooling centres and improve heat warning and response systems.
These efforts could prioritise clinically vulnerable communities, especially older adults who might be socially isolated, economically disadvantaged or have chronic health conditions.
Since most heat-related deaths among older adults occur indoors, ensuring broader access to air-conditioning or other cooling facilities supported by a resilient energy infrastructure was also among the recommendations put forward by the study.
In the long term, however, reducing emissions remains the key to global warming control.
