Scientists discover extreme heat can accelerate aging

By Stephen Beech

Extreme heat may speed up the aging process in older people, warns a new study.

People living in areas that experience more hot weather days show faster aging at the molecular level than residents of cooler regions, say American scientists.

Their findings suggest greater exposure to extreme heat may accelerate biological aging in older people, raising new concerns about how climate change and heatwaves could affect long-term health.

Study senior author Professor Jennifer Ailshire, of the University of Southern California USC Leonard Davis School of Gerontology, explained that 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 birthdate.

Having a biological age greater than one’s chronological age is associated with a higher risk for disease and death.

While exposure to extreme heat - categorized as 90 Fahrenheit (32.2 Celsius) or higher - has itself long been associated with health issues, including increased risk of death, Ailshire says heat’s link to biological aging has been unclear.

Ailshire and her co-author Dr. Eunyoung Choi examined 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 time points during the six-year study period were analysed for epigenetic changes, or changes in the way individual genes are turned “off” or “on” by a process called DNA methylation.

The research team 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 a 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 80F (26.6C) to 90F, the “Extreme Caution” level includes values between 90F and 103F (39.4C), and the “Danger” level includes values between 103F and 124F (51.1C). Days in all three levels were included as heat days in the study.

The analysis, published in the journal Science Advances, revealed a "significant correlation" between areas with more days of extreme heat and individuals experiencing greater increases in biological age.

The correlation persisted even after taking into account socio-economic and other demographic differences, as well as lifestyle factors such as physical activity, alcohol consumption and smoking.

Dr. Choi said: “Participants living in areas where heat days, as defined as Extreme Caution or higher levels 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.

“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.”

She said all three epigenetic clocks employed in the study revealed this association when analyzing epigenetic aging over a one- to six-year period.

One also showed the association after short (seven days) and medium (30 to 60 days) periods of time, indicating that heat-related epigenetic changes could happen relatively quickly, and some of them may accumulate over time.

Ailshire says older people are "particularly vulnerable" to the effects of high heat.

She noted that the study used heat index, rather than just air temperature, to take relative humidity into account as they analyzed results.

Ailshire said: “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.

“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 said the next steps for the research team will be to determine what other factors might make someone more vulnerable to heat-related biological aging and how it might connect to health.

Ailshire says that, in the meantime, the findings could also prompt policymakers, architects, and others to keep heat mitigation and age-friendly features in mind as they update urban infrastructure, such as building bus stops with shade in mind or planting more trees and increasing urban green spaces.

She added: “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."