The study accounted for the effects of ageing on the body’s ability to tolerate heat, revealing a higher level of heatwave risk for older people worldwide.

The findings come as the global population ages, with the number of people aged 60 and older projected to more than double from one billion in 2020 to 2.1 billion by 2050, representing 21 percent of the world’s population.

“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 more vulnerable to extreme heat?

As people age, their bodies become less effective at cooling themselves, with reduced sweating and greater strain on the heart as blood vessels become less able to expand.

“They (elderly people)  are also less sensitive to temperature, so they may be exposed to heat that is harmful to their bodies without realising, which may also delay adaptation strategies to reduce heat exposure,” Peter KaHung Chan, an epidemiologist at Oxford University, said.

Older people “are more likely to live in older buildings with poorer heat resilience”, Chan added.

As temperatures rise, the body eventually reaches a point at which it cannot cool itself quickly enough to compensate.

This condition, known as “uncompensable heat stress”, occurs sooner and at lower temperatures in elderly people than in younger adults.

People aged 65 and older face more exposure to uncompensable heat at 1.5C (2.7F) of global warming above pre-industrial levels than younger adults would at 4C (7.2F) of warming, according to researchers behind a Lancet study.

Long-term global warming is currently estimated at 1.4C above pre-industrial levels.

The annual global average exceeded 1.5C for the first time in 2024, according to the European Union’s Copernicus agency, although long-term, multidecade warming has not yet crossed that threshold.

Meanwhile, Stanford researchers estimate that the number of people experiencing at least 180 hours a year beyond their age group’s tolerable heat limit is already more than double previous estimates.

The 180-hour threshold identified by the report represents accumulated exposure over a year rather than necessarily continuous exposure.

“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.”

The findings nevertheless indicate that older adults can face risks of heat-related illness and death at temperatures lower 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 refers to unusually severe heat that crosses a climatic or health-related threshold.

Such conditions can cause severe illness, including heatstroke and heart attack, and can also result in death.

However, air temperature alone is not a reliable measure of the potential danger to human health.

“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.”

As a result, conditions that a healthy young adult may tolerate can cause an older person to accumulate heat, experience a rising core temperature and develop serious illnesses such as heatstroke.

Furthermore, only a proportion of deaths during heatwaves are directly caused by hyperthermia, or overheating.

According to the study, most cases are attributed to comorbidities, particularly cardiovascular, renal and respiratory conditions that can be exacerbated by elevated body temperature.

Where will elderly people be worst affected?

Global warming is occurring at a rate not seen during the past 10,000 years, according to NASA data cited in the Stanford study.

At the same time, the global population is ageing because of longer life expectancies and declining fertility rates, increasing the number of people vulnerable to severe heat.

Elderly people in low- and middle-income countries such as China, India and Pakistan are expected to face some of the highest risks, even at 1.5C of global warming above pre-industrial levels.

As temperatures rise further, the geographical areas exposed to dangerous heat are expected to expand.

According to the study, each additional degree of global warming above pre-industrial levels could bring about one billion more people into the category experiencing 180 hours of uncompensable heat each year.

Most of those affected are expected to be elderly because they reach the heat-stress threshold sooner than younger adults.

The projected impact varies depending on the amount of warming above pre-industrial levels.

At 3C (5.4F) of warming, the study’s authors predicted that India, China, Pakistan and Bangladesh would account for 1.5 billion people facing uncompensable heat exposure, nearly three-quarters of the total population affected worldwide.

What can be done to reduce the effects of extreme heat?

The Stanford researchers concluded that identifying where, when and to what extent human heat-tolerance limits will be exceeded is increasingly important for developing measures to reduce the health impacts of extreme heat.

Proposed adaptations include expanding green infrastructure in cities, improving ventilation, reducing anthropogenic heat emissions, establishing cooling centres and strengthening heat-warning and response systems.

Such measures could prioritise clinically vulnerable communities, particularly older adults who may be socially isolated, economically disadvantaged or living with chronic health conditions.

Since most heat-related deaths among older adults occur indoors, the study also recommended broader access to air conditioning and other cooling facilities supported by resilient energy infrastructure.

In the longer term, however, reducing emissions remains central to limiting global warming.