Prolonged exposure to extreme heat could make you biologically older, a new study has found.
The study, published in Science Advances on Wednesday, found that older adults living in hotter regions of the US showed signs of accelerated ageing at a cellular level.
Scientists at the University of Southern California analysed blood samples from nearly 3,700 people aged 56 and above and measured their biological age using an epigenetic clock, a tool for analysing DNA changes over time. Biological age is determined by a set of DNA markers.
They compared the biological aging rates of the participants with local heat index data from 2010 to 2016. The heat index accounts for both temperature and humidity, which together determine how extreme the heat feels to the human body.
The results showed a clear link between extreme heat exposure and faster ageing.
Prolonged exposure to heat was linked to an increase in biological age by up to 2.48 years, an effect comparable to smoking, the study said.
People experiencing more extreme heat days – defined as temperatures above 32.2C – tended to age faster at the molecular level, putting them at greater risk of age-related diseases and earlier death, the researchers said.
Study participants living in areas where heat days accounted for half of the year like 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, said Eunyoung Choi, a postdoctoral scholar at USC and co-author of the study.
“Just because you live in an area with more heat days, you’re aging faster biologically,” Dr Choi said.
The link remained even after the researchers accounted for factors like socioeconomic status and lifestyle habits. “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.
Though extreme heat has been known to increase the risk of immediate health issues such as heatstroke, its impact on long-term ageing has been less clear.
Biological ageing is different from chronological aging. While your actual age is determined by the years you have lived, biological age reflects how quickly your cells and tissues deteriorate.
Studies show that people with higher biological age are more vulnerable to conditions like heart disease, cognitive decline and frailty.
The new study found that sustained heat exposure could speed up these processes, worsening health outcomes for older adults. Heat stress could trigger inflammation and damage to cells, leading to accelerated ageing.
“It’s really about the combination of heat and humidity, particularly for older adults, because older adults don’t sweat the same way,” said Jennifer Ailshire, senior author of the study and professor of gerontology and sociology at USC.
“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.”
The findings come as the climate crisis makes extreme heat more frequent and intense across the world. The number of extremely hot days has increased in recent decades, and heatwaves now last longer and cover larger areas.
The year 2024 was the hottest on record and cities across the world shattered temperature records. Heatwaves in Europe, North America and Asia have been linked to tens of thousands of deaths in recent years.
Scientists warn that without drastic cuts to greenhouse gas emissions, extreme heat will become an even bigger public health threat.
Older adults are particularly vulnerable because their bodies struggle to regulate temperature efficiently. As more people age in a warming world, scientists say cities must rethink how to protect their populations from chronic heat exposure.
“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,” Dr Ailshire said.
The researchers suggest policymakers and urban planners take extreme heat into account when designing cities. Strategies like increasing green spaces, providing shaded bus stops, and ensuring older adults have access to cooling centres could help reduce long-term health risks.
