Introduction to the Study

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The Impact of Extreme Heat on Aging

Two white men in their 60s, one living in Arizona and the other in Washington state, have identical socioeconomic backgrounds and similar habits. However, the man in Arizona is aging more quickly than the man in Washington — 14 months faster, to be exact. Neither man smokes or drinks, and both exercise regularly. So, why is the man living in the desert Southwest more than a year older at the cellular level than his counterpart in the Pacific Northwest?

The Study’s Findings

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.

Health Consequences of Extreme Heat

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.

Cellular Aging and Heat Exposure

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 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. 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.

Results and Implications

“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.

Urban Heat Islands and Heat Exposure

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 nonwhite 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.

Conclusion and Future Research

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.”

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This article originally appeared in Grist at https://grist.org/health/extreme-heat-cellular-age-aging-smoking/. Grist is a nonprofit, independent media organization dedicated to telling stories of climate solutions and a just future. Learn more at Grist.org.