Do heatwaves make us age faster?
Researchers fear that climate change could contribute to increased rates of biological ageing, with implications for people’s long-term health.
- 28 February 2025
- 3 min read
- by Linda Geddes
Heatwaves have long been associated with a bump in hospital admissions and deaths, but a new study suggests they may also speed up the rate of biological ageing in older people’s organs and tissues.
With the frequency, intensity and duration of heatwaves expected to grow rapidly in the coming decades, the findings raise new questions about the impact that climate change could have on people’s health.
Unlike chronological age based on birthdate, biological age is a measure of how well the body functions at the molecular, cellular and system level, regardless of the number of years on your clock.
Having a biological age greater than your chronological age is thought to heighten your risk of disease and disability, because your cells are functioning less efficiently.
DNA changes
Animal studies have previously suggested that even a single episode of extreme heat stress can trigger lasting chemical modifications to their DNA through a process called methylation.
Methylation affects signalling in various cell types, including immune and heart muscle cells. However very few studies have looked into whether extreme heat can cause similar alterations to human DNA.
To investigate, Dr Eunyoung Choi and Prof Jennifer Ailshire at the University of Southern California in Los Angeles, USA, examined blood samples from more than 3,600 people aged 56 and older from across the US at various time points between 2010 and 2016.
They looked for changes in the way individual genes were turned on or off through methylation. They then used a mathematical tool to estimate their biological or “epigenetic” age based on these changes, and compared this to the number of heat days each person had experienced during the same period.
Heat days were defined as days on which the daily maximum heat index – a measure of how hot it feels when the effects of humidity are added to temperature – met the thresholds provided by the US National Weather Service for potential risks of adverse health effects.
Hot and humid days
The research, published in Science Advances, found a significant link between repeated exposure to heat index values of 32°C (90°F) or above and accelerated biological ageing, even after controlling for socioeconomic and demographic differences, or lifestyle factors such as physical activity, alcohol consumption and smoking.
At these heat index values, sunstroke, muscle cramps and/or heat exhaustion are possible or likely – particularly with prolonged exposure and/or physical activity.
“Participants living in areas where heat days – 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 ageing compared to those living in areas with fewer than 10 heat days per year,” Choi said. “Just because you live in an area with more heat days, you’re ageing faster biologically.”
Ailshire added that older adults are particularly susceptible to the combined effects of heat and humidity. “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,” she said. “If you’re in a high humidity place, you don’t get as much of that cooling effect.”
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Mitigation strategies
The next step will be to study how such heat-related changes might connect to clinical outcomes, such as the longer-term risk of heart attacks or stroke.
Choi and Ailshire hope their results may also push architects and policymakers to consider heat mitigation strategies, such as planting more trees, increasing urban green space and placing bus stops in the shade, as they update cities’ infrastructure. “If everywhere is getting warmer, the population is ageing and these people are vulnerable, then we need to get really a lot smarter about these mitigation strategies,” Ailshire said.
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