People in neighborhoods that experience more days of high heat show faster aging at the molecular level than residents of cooler regions, say USC researchers.

A new USC Leonard Davis School of Gerontology study suggests greater exposure to extreme heat may accelerate biological aging in older adults, raising new concerns about how climate change and heat waves could affect long-term health and aging at the molecular level.

People in neighborhoods that experience more days of high heat show greater biological aging on average than residents of cooler regions, said Jennifer Ailshire, senior author of the study and professor of gerontology and sociology at the USC Leonard Davis School.

Biological age is a measure of how well the body functions at the molecular, cellular, and system levels, as opposed to chronological age based on one’s birthdate; having a biological age greater than one’s chronological age is associated with higher risk for disease and mortality. While exposure to extreme heat has itself long been associated with negative health outcomes, including increased risk of death, heat’s link to biological aging has been unclear.

Measuring epigenetic changes
Ailshire and her coauthor Eunyoung Choi, USC Leonard Davis PhD in Gerontology alumna and postdoctoral scholar, examined how biological age changed in more than 3,600 Health and Retirement Study (HRS) participants aged 56 and older from throughout the U.S. Blood samples taken at various time points during the six-year study period were analyzed for epigenetic changes, or changes in the way individual genes are turned “off” or “on” by a process called DNA methylation.

The researchers used mathematical tools called epigenetic clocks to analyze methylation patterns and estimate biological ages at each time point. They then compared participants’ changes in biological age to their location’s heat index history and number of heat days reported by the National Weather Service from 2010 to 2016.

The National Weather Service Heat Index Chart categorizes heat index values into three levels based on the potential risk of adverse health effects. The “Caution” level includes heat index values ranging from 80°F to 90°F, the “Extreme Caution” level includes values between 90°F and 103°F, and the “Danger” level includes values between 103°F and 124°F. Days in all three levels were included as heat days in the study.

The analysis revealed a significant correlation between neighborhoods with more days of extreme heat and individuals experiencing greater increases in biological age, Choi said. This correlation persisted even after controlling for socioeconomic and other demographic differences, as well as lifestyle factors such as physical activity, alcohol consumption and smoking, she added.

“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,” she said. “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.”

All three epigenetic clocks employed in the study – PCPhenoAge, PCGrimAge, and DunedinPACE – revealed this association when analyzing epigenetic aging over a 1- to 6-year period. PCPhenoAge also showed the association after short (7 days) and medium (30-60 days) periods of time, indicating that heat-related epigenetic changes could happen relatively quickly, and some of them may accumulate over time.

Source: https://gero.usc.edu/2025/02/26/study-extreme-heat-may-speed-up-aging-in-older-adults/