Extreme heat can age you as fast as a smoking habit todayheadline

Two white men in their 60s live hundreds of miles away from each other, one in Arizona and the other in Washington state. They are the same age and have identical socioeconomic backgrounds. They also have similar habits and are in roughly the same physical shape. But the man in Arizona is aging more quickly than the man in Washington — 14 months faster, to be exact. Neither man smokes or drinks. Both exercise regularly. So why is the subject living in the desert Southwest more than a year older at the cellular level than his counterpart in the Pacific Northwest?

A study published this week in the journal Science Advances makes the case that extreme heat is aging millions of Americans more quickly than their counterparts in cooler climates. The impact of chronic exposure to high temperatures, researchers found, is equivalent to the effect of habitual smoking on cellular aging. 

As global average temperatures continue to rise due to the greenhouse gas effect caused by burning fossil fuels, wider swaths of the global population are being exposed to extreme heat, which has killed more than 21,000 Americans since 1999. In 2023, Phoenix, Arizona, where some of the people analyzed in the study live, saw 31 days straight of temperatures above 110 degrees Fahrenheit. That year was the warmest year on record globally — a record that was quickly surpassed by 2024. 

Exposure to above-average heat has serious short-and long-term health repercussions. People may experience heat-related illness, such as dehydration and fainting, or sustain heat stroke — the most serious form of heat-related illness that can lead to death. Older adults and young children are particularly vulnerable to these impacts because they have trouble thermoregulating, or maintaining a steady internal body temperature. Over months and years, heat exposure can exacerbate existing chronic conditions like kidney and cardiovascular disease, and raise a person’s risk of mental health issues and dementia. 

Eun Young Choi, a postdoctoral gerontological researcher at the University of Southern California’s Leonard Davis School of Gerontology and the lead author of the study, wanted to find out what might be driving the long-term health consequences of exposure to extreme heat on a cellular level, particularly in people approaching their 60s. She was particularly interested in “non-clinical manifestations” of heat exposure, meaning she hoped to capture how heat was affecting people who weren’t showing up in emergency rooms with heat-related illness or heat stroke. Her hypothesis was that heat was chipping away at overall health, whether or not someone could feel it acutely. 

In order to test that theory, Choi analyzed blood samples from more than 3,600 people over the age of 56 who had participated in a large national health and retirement study. Those participants had taken a blood test in 2016 or 2017. Choi and her coauthor, Jennifer Ailshire, then used weather and climate data to estimate how many “heat days,” as defined by the National Weather Service, each participant had been exposed to in the years, months, and days leading up to the date of the blood test. They sorted the participants into demographic groups based on race, socioeconomic status, exercise habits, and other factors, and then compared the people in those groups to each other using a series of biological tests that determine how quickly a person’s cells are aging. 

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“With longer-term heat exposure — one year and six years — we see a consistent association between heat and [cellular] age” across different biological tests, Choi said. People living in places where temperatures are at or above 90 degrees F for half of the year have experienced up to 14 months more biological aging compared to people living in areas with fewer than 10 days of temperatures at or above 90 degrees. 

“This study is one of the first empirical assessments suggesting that longer-term exposure to heat is directly associated with an acceleration of the aging process,” said Vivek Shandas, a professor at Portland State University who studies the effects of climate change on cities and was not involved in the study.  It “adds to the existing work by suggesting that near-term mortality may be the result of older adults having longer-term and periodic exposures to heat.”

Two previous studies found that people exposed to heat age more quickly, and studies in mice consistently show that heat ages cells, but Choi’s study is the first nationally representative research to draw the connection. The size and diversity of her pool of subjects helped drown out many of the factors that usually sully this type of data. Choi didn’t find any major differences between demographics — an indication that heat damages cells across the board in older individuals. 

What Choi didn’t account for, however, are all the ways people adapt to protect themselves from heat. Some people, particularly wealthier Americans, might stay inside with the air conditioning blasting all day and night. 

Previous research has shown that above-average temperatures don’t affect all populations equally. Extreme heat is particularly dangerous for people who live in urban areas with patchy tree cover and lots of concrete. These zones, in places like New York City and Chicago, are called urban heat islands, and they can get up to 7 degrees F hotter than surrounding rural areas. Urban heat islands tend to coincide with neighborhoods where non-white communities were historically confined by racist zoning practices, which is one reason that the average person of color is exposed to more severe heat in urban areas than the average non-Hispanic white person. These populations are also less likely to be able to afford air conditioning. 

“We know that some demographics, such as those working outside, unhoused populations, people living in urban heat islands, incarcerated populations, and lower-income residents generally have longer periods of exposure to extreme heat (over decades),” Shandas said. “Accordingly, we might draw on these findings to suggest that some certain populations will need greater attention and care as we see forecasts for heat waves.”

Choi hopes future studies will continue to tease out these differences, particularly because by 2040, 1 in 5 Americans will be 65 or older — up from 1 in 8 in the year 2000. The results of Choi’s study also have implications for all age groups, not just people in their 50s and older. “I don’t think the underlying biology is significantly different,” she said. “We would expect to see some significant effects of heat in younger adults. And we really need to track people from their birth to older ages to see whether any of these effects can be reversible.”