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LOS ANGELES — As climate change intensifies heat waves across America, scientists have uncovered a disturbing connection between outdoor heat and how quickly our bodies age at the biological level. Research shows that exposure to ambient heat doesn’t just make us uncomfortable—it may actually accelerate our aging process at the cellular level.

Scientists from the University of Southern California have discovered that older adults living in areas with more frequent hot days show signs of accelerated biological aging, as measured through specialized “epigenetic clocks” that track molecular changes throughout the body. These findings provide crucial insight into how climate change might affect public health beyond the already well-documented risks of heat-related illness and death.

“Participants living in areas where heat days, as defined as Extreme Caution or higher levels (≥90°F), occur half the year, such as Phoenix, Arizona, experienced up to 14 months of additional biological aging compared to those living in areas with fewer than 10 heat days per year,” said study lead author Eun Young Choi. “Even after controlling for several factors, we found this association. Just because you live in an area with more heat days, you’re aging faster biologically.”

Heat and Humidity: The Cruel Combo

Published in Science Advances, the research captures how heat affects our bodies even before we develop clinical conditions. Rather than waiting for disease to manifest, the researchers measured epigenetic changes—alterations in how our genes are expressed without changing the underlying DNA sequence—that serve as early indicators of accelerated aging.

The study examined data from 3,686 adults aged 56 and older across the United States, representing the first nationally representative analysis of heat’s effects on biological aging. The researchers classified heat days according to the National Weather Service heat index scale, which accounts for both temperature and humidity: “caution” level (80°F-90°F), “extreme caution” (90°F-103°F), and “danger” (103°F-124°F).

“It’s really about the combination of heat and humidity, particularly for older adults, because older adults don’t sweat the same way. We start to lose our ability to have the skin-cooling effect that comes from that evaporation of sweat,” explained Jennifer Ailshire, senior author of the study. “If you’re in a high humidity place, you don’t get as much of that cooling effect. You have to look at your area’s temperature and your humidity to really understand what your risk might be.”

While southern regions of the U.S. showed the highest concentration of heat days, the researchers found that heat is truly a nationwide concern—every single census tract in their sample experienced at least some “caution+” level heat days.

It’s Getting Hot(ter) In Here

Results showed that exposure to more heat days was consistently associated with accelerated epigenetic aging, though different aging markers responded differently depending on the timeframe. For short-term heat exposure—the day of blood collection and the preceding week—greater heat exposure was linked to increased biological aging. For example, experiencing a caution+ level heat day on the day of blood collection was associated with a 1.07-year increase in biological age acceleration.

Long-term heat exposure showed even more concerning effects. When examining heat exposure over one-year and six-year periods, the researchers found significant associations with accelerated epigenetic aging across all three epigenetic clocks they measured. This suggests that while our bodies might recover from occasional hot days, persistent exposure to high temperatures may lead to more permanent changes in how our genes are expressed, ultimately causing our biological systems to age more rapidly.

The findings are particularly alarming when considered alongside climate projections. The frequency, intensity, and duration of extreme heat events are expected to increase dramatically in the coming decades, potentially affecting more than 100 million Americans by 2050.

‘We Need To Get A Lot Smarter’

This study contributes to a growing body of evidence suggesting that epigenetic changes represent a critical pathway through which environmental exposures affect our health. Previous animal studies have shown that heat stress can induce a “maladaptive epigenetic memory,” creating lasting changes in gene expression patterns across various species. The current research confirms that similar processes likely occur in humans as well.

The next steps for the researchers will be to determine what other factors might make someone more vulnerable to heat-related biological aging and how it might connect to clinical outcomes. In the meantime, the study results could prompt policymakers and urban planners to prioritize heat mitigation strategies.

“If everywhere is getting warmer and the population is aging, and these people are vulnerable, then we need to get really a lot smarter about these mitigation strategies,” said Ailshire. She highlighted the importance of placing sidewalks and building bus stops with shade in mind, planting more trees, and increasing urban green space.

While standing in front of a fan or cranking up the air conditioning might provide immediate relief from the heat, this research suggests that our cells may be keeping score of every sweltering day—and that the biological clock is ticking a little faster with each one.