Phoenix heat is notorious for the harm it causes in the short-term, like heat stroke and heart problems. Just last year, temperatures there topped 90 degrees Fahrenheit for a record-breaking 188 days. More than 140 of those days surpassed 100 degrees.
A new study published in Science Advances shows that prolonged heat exposure of that magnitude can even modify how people's genes behave, speeding up aging at the molecular level and potentially impacting people's long-term health.
The impact is "similar to the effect of smoking and drinking," says Eunyoung Choi, a gerontologist at the University of Southern California's Leonard Davis School of Gerontology and the study's lead researcher.
The study looked at people over 56 living across the country with very different heat experiences. People from extremely hot parts of the country like Phoenix — where the heat index, a combination of temperature and humidity — topped 90 degrees for more than half the year, looked biologically about 14 months older, epigenetically, than similar people living in cooler places like Seattle, where fewer than 10 days of each year exceed that threshold.
That may not sound like an enormous difference, says Boston University gerontologist Deborah Carr, who wasn't involved in the study, but for the person affected, "it's just a tremendous strain not only on their own lives and the lives of their families and caregivers, but also has a larger societal impact."
Research links premature aging with an earlier onset of health issues like dementia, diabetes and cardiovascular disease. The sooner those symptoms begin, the more of a toll they take on people's well-being and productivity, Carr says.
"If that's something that takes you out of the workforce for a year, that makes you go to the doctors every day for a year, that increases your medication budget — it really can have tremendous impacts," she explains.
Epigenetic aging
The researchers tracked the biological age of people in their study by looking for subtle changes in their DNA after different periods of exposure to heat, from a few days of extreme temperatures to several years.
"We know that some people seem to age faster than others, and that's because biological aging doesn't always match chronological aging," Choi says.
Biological age is often correlated with chemical tags that accumulate and are shed from people's DNA over time, a process called methylation; it is often referred to as "epigenetic aging." "DNA methylation acts like a light switch for genes — so it can turn them on and off," Choi says.
Methylation doesn't change the genes themselves, but modifies the way those genes work — sometimes for the better, but oftentimes not. Many studies in both animals and humans have shown that DNA methylation patterns change over time and can be used as a sort of molecular clock, ticking along as people age.
Epigenetic aging can accelerate because people don't eat well or don't get enough exercise. But it is also associated with emotional or physical stress, as well as exposure to environmental harms like air pollution "and, in this case, heat," Choi says.
The researchers looked at blood samples from more than 3,600 older adults across the U.S. and assessed people's epigenetic age. They could figure out how much heat those people experienced over a few days, a few months, one year and six years prior to that sample collection using climate and weather models looking into the past, taking into account both temperature and humidity — factors that influence the danger of different heat conditions.
The outcomes were clear. People who experienced more heat over the long-term aged faster, biologically, than those living in cooler places. That's even after taking into account people's financial status, education, physical activity and whether they smoked.
Researchers have known for years that heat exposure correlates with long-term issues like worsened risk of cardiac problems. But "the mechanisms for how that happens haven't been clear," says Tom Clanton, a physiologist and heat expert at the University of Florida's College of Health and Human Performance, who wasn't involved in the research. This work, he says, begins to explain how those delayed effects might happen. At the genome level, heat exposure makes "you sort of accelerate your way towards an old heart, and an old vulnerable heart," he says.
Climate pressures
Heat causes plenty of shorter-term health problems for people of all ages. The number of emergency department visits rises during heat waves, as does the number of cardiovascular issues, kidney problems and even deaths not directly linked to the heat.
The risks rise as human-driven climate change accelerates, says Choi. The number of extreme heat days in the U.S. could rise by 20 to 30 days across much of the country by the middle of the century, according to the country's National Climate Assessment, a comprehensive accounting of past and potential future changes to the country because of climate change.
At the same time, as heat risks grow because of climate change, Carr says, the U.S. population is aging. Older people's bodies generally deal with heat less adeptly than younger people. Sweating capacity decreases. Less blood flows to the skin where it can be cooled. Some medications interfere with people's ability to manage heat.
But heat makes life harder for everybody, not just older people, says Robert Meade, a heat physiology expert at Harvard University's T.H. Chan School of Public Health. "It's basically like if you're trying to tread water and someone hands you a brick," he says. "It's extra strain on all your physiological systems. It's just an extra weight that needs to be responded to in order to maintain homeostasis."
The next step to connect heat exposure to specific health problems, Meade thinks, is to further personalize the analyses, because even within the hottest parts of the country, people experience very different heat exposures. "What people actually experience in their homes, whether they're in an overheated mobile unit or they have air conditioning available, can be wildly different temperatures," he says.
Researchers indicate that the next frontier of this work should look at how heat exposure within people's homes shapes health outcomes. That kind of specific, personalized risk assessment could help scientists further pinpoint exactly how heat hurts people, Meade says — and maybe even how to prevent the damage or reverse it.
Copyright 2025 NPR
