Extreme Heat Can Accelerate the Aging Process in Older Adults

A new study from the USC Leonard Davis School of Gerontology suggests that greater exposure to extreme heat can 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.

How Heat and Biological Aging are Linked

People in areas that experience more days of intense heat events show, on average, more biological aging than residents of cooler regions, said Jennifer Ailshire, lead author of the study and a professor of gerontology and sociology at the USC Leonard Davis School. Biological age is a measure of how well the body is functioning at the molecular, cellular and systemic levels, as opposed to chronological age, which is based on date of birth. Biological age that is higher than chronological age is associated with a higher risk of disease and death. While exposure to extreme heat itself has long been associated with negative health consequences, including an increased risk of mortality, the link between heat and biological aging was unclear.

Measuring Epigenetic Changes

Ailshire and co-author Eunyoung Choi, a doctoral student in gerontology at USC Leonard Davis and a postdoctoral fellow, examined how biological age changed in more than 3,600 participants in the Health and Retirement Study (HRS) who were 56 and older from across the United States. Blood samples collected at various time points during the six-year study period were analyzed for epigenetic changes, or alterations in the way individual genes are “switched off” or “switched on” through a process called DNA methylation. The researchers used mathematical tools called epigenetic clocks to analyze methylation patterns and estimate biological age at each point in time. They then compared changes in the participants’ biological age to heat mapping of their location and the number of heat days reported by the National Weather Service from 2010 to 2016.

The National Weather Service’s heat mapping categorizes heat index values into three levels based on the potential risk of adverse health effects. The “caution” level includes heat index values between 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 in the study as hot days. The analysis revealed a significant correlation between neighborhoods with more days of extreme heat and individuals who showed greater increases in biological age. This correlation remained even after controlling for socioeconomic and other 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 (≥90°F), occur for half the year experienced up to 14 months of additional biological aging compared to those living in areas with fewer than 10 hot days per year. The researchers found this association even after adjusting for multiple factors. Just living in an area with more hot days makes you age biologically faster. All three epigenetic clocks used in the study – PCPhenoAge, PCGrimAge and DunedinPACE – uncovered 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, suggesting that heat-related epigenetic changes may occur relatively quickly and that some of them may accumulate over time.

Climate Impacts on Communities

Older adults are especially vulnerable to the effects of high heat. Ailshire pointed out that when analyzing the results, the heat index was used, and not just air temperature, to account for relative humidity. “It’s really the combination of heat and humidity that’s important, especially for older adults, because older adults don’t sweat the same way. We gradually lose the ability to get the skin-cooling effect that comes from the evaporation of sweat,” she explained. If you are in a place with high humidity, that cooling effect is not as strong. You have to look at the temperature and humidity in your area to really understand what the risk might be.

The next steps for researchers will be to determine what other factors might make someone more susceptible to heat-related biological aging and how this might relate to clinical outcomes. Meanwhile, Ailshire said the study findings could also prompt decision makers, architects and others to consider aspects such as heat protection and age-friendly features when modernizing urban infrastructure, from placing sidewalks and building bus stops to take into account shade, to planting more trees and increasing urban green spaces.