A new study from USC’s Leonard Davis School of Gerontology suggests that extreme heat may speed up biological aging in older adults. This raises concerns about how climate change and heat waves could impact long-term health at the molecular level.
People in hotter neighborhoods tend to show greater biological aging than those in cooler areas. Unlike chronological age, which is based on birthdate, biological age reflects how well the body functions. A higher biological age than chronological age is linked to greater disease risk and shorter life expectancy. While heat exposure has long been known to harm health, its effect on biological aging was unclear until now.
Rate of ageing
The researchers studied more than 3,600 adults aged 56 and older across the U.S. Blood samples collected over six years were analyzed for DNA methylation changes, which affect how genes are turned “on” or “off.” Using epigenetic clocks—mathematical tools that estimate biological age—they compared changes in biological age to local heat index histories from 2010 to 2016.
The National Weather Service classifies heat index values into three levels of risk: “Caution” (80°F–90°F), “Extreme Caution” (90°F–103°F), and “Danger” (103°F–124°F). The study counted all days in these categories as “heat days.”
The analysis found that people in areas with more extreme heat days experienced faster biological aging, even after accounting for socioeconomic background, lifestyle, and demographics.
“People in places where extreme heat occurs half the year—like Phoenix, Arizona—aged biologically up to 14 months more than those in cooler areas with fewer than 10 heat days per year,” the researchers report. “Even when we controlled for several factors, the association remained: living in a hotter area means aging faster biologically.”
Strong link
All three epigenetic clocks used in the study—PCPhenoAge, PCGrimAge, and DunedinPACE—showed this link over periods ranging from one to six years. PCPhenoAge also detected changes in as little as a week, suggesting that heat-related aging effects can occur quickly and accumulate over time.
Older adults are especially vulnerable to heat because they sweat less efficiently, reducing their ability to cool down. The study used heat index rather than air temperature alone, as humidity plays a key role in heat stress.
“Heat and humidity together make a big difference, especially for older adults,” the researchers explain. “If you live in a humid area, sweating doesn’t cool you as effectively. You need to consider both temperature and humidity to understand your risk.”
Next, the team will investigate other factors that increase vulnerability to heat-related aging and how it connects to health outcomes. In the meantime, the findings highlight the need for heat-mitigation strategies in urban planning. Policymakers, architects, and city planners could incorporate age-friendly features such as shaded sidewalks, covered bus stops, tree planting, and more green spaces.
“With rising temperatures and an aging population, we need smarter strategies to protect vulnerable groups,” the authors conclude.
