Today I Learned: Living in Extremely Hot Cities May Be Speeding Up Your Aging Process

Ever think the extreme heat—and the icky, sweaty feeling that comes with it—is aging you by a decade? Well, you may not just be imagining it. While living in a tropical country means we Filipinos have learned to embrace the hotter climate, a recent study suggests that greater exposure to higher temperatures may, in fact, be accelerating aging.

Researchers from the University of Southern California (USC) examined how biological age changed in more than 3,600 participants aged 56 and older throughout the United States. Unlike chronological age, which simply counts the number of years you’ve been alive, biological age measures how well the body functions at the molecular, cellular, and system levels.

Throughout the six-year study, blood samples were collected at different intervals and analyzed for epigenetic changes, or the changes that happen to individual genes switched “on” or “off” by a process called DNA methylation. Using mathematical tools called epigenetic clocks, researchers analyzed methylation patterns and estimated biological ages at particular time points. They then compared changes in the participants’ biological ages to their location’s heat index history and the number of heat days reported by the US National Weather Service from 2010 to 2016.

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Living in Extreme Heat May Be Aging You Faster

The analysis showed a strong link between neighborhoods experiencing more days of extreme heat and individuals experiencing greater increases in biological age. Participants living in areas where heat days occur for around 180 days or half the year and reach more than 32°C or higher, such as Phoenix, Arizona, experienced up to 14 months of additional biological aging, compared to residents in regions with fewer than 10 heat days per year.

The correlation remained true even after accounting for socioeconomic and other demographic differences. “Even after looking at other factors—age, sex, ethnicity, education—this magnitude we found is comparable to the effects of smoking,” shared study co-author and USC Leonard Davis PhD in Gerontology alumna Eun Young Choi.
In addition, heat-related epigenetic changes could happen relatively quickly, with some accumulating over time. “Just because you live in an area with more heat days, you’re aging faster biologically,” added Choi.

Why Heat is Hard on the Body

Extreme heat poses various health risks, and prolonged exposure to hot weather can impact the body in different ways, including raising the risk of heart disease and kidney dysfunction. According to Jennifer Ailshire, senior author of the study and professor of gerontology and sociology at the USC Leonard Davis School, older adults are particularly vulnerable to the effects of high heat.

This research wanted to examine the physical toll of heat before disease or organ dysfunction appeared. After all, a higher biological age compared to one’s actual age is associated with a greater risk of disease and early death.

“The effects of this extreme heat might not show up right away as diagnosable health conditions, but they could be taking a silent toll at the cellular or molecular level. So, over time, this biological deterioration could accumulate and eventually, years later, lead to disability and disease,” said Choi.

Furthermore, the effects of extreme heat are not equal across a population. “It will disproportionately affect really vulnerable people, like those in prison and those who don’t have socio-economic resources to avoid outdoor heat,” added Choi.

A Word of Caution

The study used heat index to take relative humidity into account as researchers analyzed the results. The heat index measures how the human body perceives heat, which may differ from the actual temperature.

“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 Ailshire.

“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,” she added.

In the Philippines, a heat index between 42°C and 51°C falls in the “danger” category. This poses health risks like heat cramps, heat exhaustion, and heat stroke. During days when temperatures are at an all-time high, it’s crucial to stay hydrated and limit outdoor activities, especially between 10 a.m. and 4 p.m. Sun protection is a must, from using sunscreen, umbrellas, and hats. Wearing loose and lightweight clothing can also help, and you can also use tools for staying cool, like portable fans and cooling towels.

While this recent study shows how extreme heat can impact biological aging, some experts have said the study should not be overly interpreted as conclusive proof of increased biological age. In an interview with the Australian Broadcasting Corporation, Dr. Christian Nefzger, an epigenetic expert from the University of Queensland, said, “These [epigenetic] changes may represent stress or environmental adaptations that could be reversible. More research is needed before drawing firm conclusions about the long-term consequences of true biological aging in response to higher temperatures.”