As the planet gets hotter, the human body can only do so much to keep itself cool. For older adults — defined in the study as people 60 and older — the limit may arrive much sooner than previously understood.
A new study from Stanford University found older adults are far more vulnerable to extreme heat than younger people. Even if global warming is limited to 2.7 degrees above preindustrial levels, older adults are projected to be exposed to heat their bodies cannot tolerate more often than young adults would in a world warmed nearly three times as much.
Previous global projections of heat beyond what the human body can manage mostly used healthy adults in their 20s to set the baseline.
The study, published in The Lancet Planetary Health, is the first to project global exposure to “uncompensable heat stress,” a condition when the body is exposed to heat that exceeds its ability to maintain a stable core temperature, for different age groups.
“It doesn’t mean immediate death once exposed,” said Qinqin Kong, the study’s lead author and a postdoctoral scholar at Stanford University.
Instead, once the heat threshold is crossed, core body temperature will continue to rise. If the exposure to high temperatures lasts long enough, it can eventually lead to serious or lethal injury.
That distinction makes this level of heat different from familiar measures such as air temperature or the heat index because it focuses on how weather affects the body, said Eran Bendavid, a Stanford professor of medicine and study co-author.
Researchers combined climate projections and population estimates with heat-tolerance limits established through controlled laboratory experiments. In those experiments, participants are placed in environmental chambers where heat and humidity are progressively increased. At the same time, researchers monitor participants’ core temperatures to determine when they can no longer maintain thermal balance.
The research indicates the temperature at which adults 60 and older can no longer maintain a stable core temperature is roughly 8.5 to 13.5 degrees lower than it is for younger adults, depending on humidity.
There are several reasons.
Sweat glands become less effective with age, reducing one of the body’s primary mechanisms for shedding heat. Blood vessels also become less effective at dilating to move warm blood toward the skin, where that heat can escape, Kong said.
“As we age, multiple parts of our bodies’ cooling mechanisms become less effective,” Kong said.
The thirst response can also weaken with age, making dehydration more likely.
Chronic health conditions and medications common among older adults can further interfere with sweating, circulation and other mechanisms the body relies on to withstand extreme heat.
When researchers incorporated those age-specific differences into their projections, the scale of older adults’ potential exposure to heat that could damage their bodies increased dramatically.
At 2.7 degrees of global warming above preindustrial temperatures, about 22% of people 60 and older worldwide — roughly 290 million people — are projected to experience at least 180 hours per year in which environmental conditions exceed their age-specific ability to withstand heat. Younger adults would not experience a comparable level of exposure until warming approaches 7.2 degrees.
The researchers also found that every additional degree of warming could expose roughly another billion people ages 15 and older worldwide to at least 180 hours of such heat each year. At 5.4 degrees of warming, older people in some of the hardest-hit parts of South Asia and the Persian Gulf could face such conditions day and night for three consecutive months annually.
But Bendavid said the study’s central contribution isn’t necessarily the billion-person figure. Scientists already knew that more warming would expose more people to dangerous heat. What they didn’t know is how much of this burden would be borne by older people.
“The world is getting older, and the world is getting warmer. There’s going to be some confluence of these demographics and these climate conditions. It’s a perfect storm,” Bendavid said.
The effects won’t be distributed evenly. South Asia and the Arabian Peninsula are among the regions where the researchers project the most intense convergence of aging populations and uncompensable heat.
Lara Cushing, an associate professor of environmental health sciences at UCLA who was not involved with the research, called the analysis significant. She said the study illustrates a fundamental limit to society’s ability to respond to rising temperatures naturally.
“You can’t adapt your way out of every problem,” Cushing said.
Air conditioning, cooling centers and other interventions can protect people, but they do not eliminate the underlying physical limits, she said. Unless people can spend their lives in air-conditioned environments, continued warming will increasingly put people into conditions their bodies cannot tolerate.
Cushing said socioeconomic inequality is an important part of the picture. Low-income households, unhoused people and outdoor workers may have fewer resources to escape dangerous temperatures. Access to air conditioning, transportation and safe housing can determine whether two people experiencing the same weather face very different levels of actual risk.
“So much of this is about resources you have to mitigate your risk,” Cushing said.
That means older adults shouldering some of the greatest climate-related health burdens may also be among those with the fewest resources to protect themselves.
The researchers also acknowledge a limitation: The thresholds used in the model come largely from controlled experiments conducted in the United States. People who routinely live in hotter climates may be somewhat more acclimatized to heat. Researchers tested scenarios allowing higher heat-tolerance limits in warmer climates and found that doing so reduced the estimated exposure but did not change the study’s overall conclusions.
Heat can be particularly dangerous because people don’t always recognize that they are developing heat exhaustion before their condition progresses.
“It’s a silent killer,” Cushing said.
Kong said protecting an aging population will also require changes beyond individual behavior. Heat action plans should recognize that risk isn’t one-size-fits-all, he said. Communities may need earlier warnings for older adults, accessible cooling spaces and systems to reach people who may otherwise be isolated during extreme heat.
Cushing said those adaptations remain essential, but the findings also reinforce the importance of addressing the underlying cause of increasing heat exposure: continued global warming.
In the Bay Area, a growing older population is increasingly confronting rising temperatures. While coastal communities are somewhat buffered by the Pacific Ocean, inland parts of Alameda, Contra Costa and Santa Clara counties routinely experience much higher temperatures during heat waves.
She added that the age-related vulnerability identified by the study isn’t only a concern for people who are old today. Children born now will become older adults toward the end of the century, when they could experience some of the most severe effects of a climate that has continued warming throughout their lives.
“We have to think to ourselves, what kind of world are we leaving them?” she said.