Extreme heat may accelerate aging in older adults

Increased exposure to extreme heat may accelerate biological aging in older people, raising a new scenario for how climate change and heat waves could affect long-term health and aging at the molecular level.

The University of Southern California (USC) study found that people living in neighborhoods that experience more days of intense heat show greater biological aging than residents of colder regions.

Biological age, explains Jennifer Ailshire , senior author of the study, is a measure of how the body functions at the molecular, cellular, and systemic levels, as opposed to chronological age based on date of birth; having a biological age older than chronological age is associated with increased risk of disease and mortality. While exposure to extreme heat has long been associated with negative health consequences, including an increased risk of death, heat’s link to biological aging has been unclear.

The research, published in Science Advances , looked at how biological age changed in more than 3,600 Health and Retirement Study (HRS) participants aged 56 and older from across the United States. Blood samples taken at various times over the six-year study period were analysed for epigenetic changes, or changes in the way individual genes are turned 'on ' or ' off ' through a process called DNA methylation.

The researchers used mathematical tools called epigenetic clocks to analyze methylation patterns and estimate biological ages at each point in time. They then compared changes in participants' biological ages to their location's heat index history and the number of hot days between 2010 and 2016.

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

“Participants living in areas where hot days, defined as extreme caution or higher levels (≥32°C), 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 hot days per year,” Choi said. “Even after controlling for several factors, we found this association. Living in an area with more hot days means you’re biologically aging faster.”

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

Older people are more vulnerable to the effects of intense heat.

The study used the heat index, rather than just air temperature, to take relative humidity into account when analysing the results. “ It’s really the combination of heat and humidity , particularly for older people, because they don’t sweat in the same way. We start to lose the ability to have the cooling effect on the skin that comes from evaporation of sweat,” Ailshire explains. “If you’re somewhere with a lot of humidity, you don’t get as much of a cooling effect. You have to look at the temperature and humidity of your area to really understand what your risk might be.”

The researchers’ next steps will be to determine what other factors might make a person more vulnerable to heat-related biological aging and how that might relate to clinical outcomes. In the meantime, the study’s results could also prompt policymakers, architects and others to consider heat mitigation and age-friendly features as they upgrade cities’ infrastructure, from laying down sidewalks and building bus stops with shade in mind to planting more trees and increasing urban green space, Ailshire says.

"If the whole world is warming and the population is ageing, and these people are vulnerable, then we need to be much smarter about these mitigation strategies," he concludes.