Extreme heat may be accelerating aging in older adults: Study

Increased exposure to extreme heat may be speeding up the aging process in older adults, a new study has found.

The bodies of Americans who reside in neighborhoods with more frequent hot days are breaking down faster than those of their cooler-dwelling peers, according to the study, published this week in Science Advances.

These findings are raising concerns among scientists about how climate change and heat waves might be deteriorating bodily processes at the molecular level and impacting long-term health.

"The frequency, intensity, and duration of extreme heat events are expected to grow rapidly in the coming decades, affecting more than 100 million Americans in 2050," warned the authors, from the University of Southern California's Lenoard Davis School of Gerontology. 

Although scientists have long been aware that exposure to extreme temperatures can wreak havoc on health, the dynamics between heat and biological aging — how well the body functions at the molecular, cellular and system levels — were previously unclear.

If a biological age surpasses "chronological," or birthdate-based age, the body becomes increasingly vulnerable to the development of disease and death, the authors explained.

To unravel this link, the researchers combed through data of more than 3,600 participants aged 56 and older in the University of Michigan's Health and Retirement Study, which surveys the health of a representative sample more than 20,000 older Americans.

The USC scientists analyzed blood samples taken at various points in the six-year study period, looking for epigenetic changes: shifts in the way individual genes are turned on and off by a process called DNA methylation, which impacts gene activity but does not alter DNA sequence.

The authors used mathematical tools called "epigenetic clocks" to understand these on-and-off patterns and estimate biological ages at each point in time the blood samples were taken. They then compared shifts in biological age to heat records from the participant's place of residence, harnessing National Weather Service data.

The federal agency’s heat index, the researchers noted, groups potential health risks into three categories: “caution,” which includes heat index values from 80 to 90 degrees Fahrenheit; “extreme caution,” from 90 to 103 degrees Fahrenheit; and “danger,” from 103 to 124 degrees Fahrenheit.

The heat index accounts for both air temperature and relative humidity, which the researchers described as particularly important due to the differences older adults experience when sweating.

"We start to lose our ability to have the skin-cooling effect that comes from that evaporation of sweat,” senior author Jennifer Ailshire, a USC professor of gerontology and sociology, said in a statement.

“If you’re in a high humidity place, you don’t get as much of that cooling effect," Ailshire added. "You have to look at your area’s temperature and your humidity to really understand what your risk might be.”

Ultimately, the analysis identified a signification correlation between extreme heat days and surges in biological age, according to lead author Eunyoung Choi, a USC postdoctoral scholar in gerontology.

That correlation persisted even after the scientists controlled the data for socioeconomic and demographic differences, as well as lifestyle habits, she explained.

Those individuals who lived in areas of "extreme caution," such as Phoenix, Ariz., endure up to 14 months additional biological aging in comparison to peers who lived in areas with fewer than 10 hot days per year, Choi noted.

"Just because you live in an area with more heat days, you’re aging faster biologically," she said in a statement.

Going forward, the researchers said they will be focusing on determining what other factors could make people more vulnerable to heat-linked biological aging.

They also expressed hope that their study would prompt policymakers and architects to prioritize heat mitigation. Updates of city infrastructure, for example, should involve positioning bus stops and sidewalks in the shade, planting more trees and expanding green space, according to Ailshire.

“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,” she added.