Extreme heat can be particularly dangerous for older people, putting them at increased risk for heat stroke and death. But could it also affect how their DNA functions, and accelerate the ageing process itself?

A new study, published in Science Advances, suggests it could. The analysis of more than 3,600 older adults in the US found that those living in neighbourhoods prone to extreme heat — 32°Celsius or above — showed more accelerated ageing at a molecular level compared with those in areas less prone to extreme heat.

The findings suggest that heat waves and rising temperatures from climate change could be chemically modifying people’s DNA and speeding up their biological ageing. The authors estimated that a person living in an area that reached 32°C or above for 140 days or more in a year could age up to 14 months faster than someone in an area with fewer than 10 extreme heat days a year.

The researchers analysed three biomarker ageing estimates, known as epigenetic clocks, which were derived from blood samples of people age 56 and older as part of a separate national population study. They then looked at these age estimates alongside six years of daily climate data, comparing them across geographies.

Epigenetic clocks measure biological changes that could predict the risk of disease or death associated with older age. They estimate “how well the body is functioning at the molecular and the cellular level”, said Eun Young Choi, a postdoctoral associate at the USC Leonard Davis School of Gerontology in the US and a co-author on the paper. While DNA is fixed at birth, external factors like stress or pollution can trigger molecular changes that turn genes on and off and affect how they operate. “DNA is like a blueprint,” Choi said, but these epigenetic changes are like the “switchboard that controls which part of the blueprint gets activated”.

This is the first population-level analysis to establish a connection between heat exposure and epigenetic ageing in humans, building on separate research finding such changes in fish, mice and guinea pigs.“It’s important, suggestive work” that could offer a biological explanation for geographic health trends, said Nancy Krieger, a professor of social epidemiology at Harvard T.H. Chan School of Public Health in the US, who was not involved in the study.

Many of the places in the US that the study found to have the most extremely hot days “are also some of the states that have the worst health profiles”, she said.

But, Krieger added, heat most likely isn’t the only factor. And the authors emphasised there’s not enough data to definitively conclude that heat exposure caused the acceleration in ageing — only that the two appear linked. In addition, the data they analysed didn’t contain details about individuals’ lifestyles, such as whether they had access to air conditioning or spent the majority of time indoors.

There’s also some debate among scientists about whether epigenetic clocks are the best measure for ageing, said Rachel Morello-Frosch, an environmental health scientist at the University of California, Berkeley, US. How to best measure biomarkers, and how to use them to predict future health, is an “evolving space in the scientific field”, she said.

Experts say there are still many questions left to answer: how might things like air conditioning affect epigenetic ageing? Can short visits to hotter areas cause you to age faster? Can moving away from hotter areas reverse it?

Because the analysis was based on a single blood sample from each subject, “we don’t really have that kind of longitudinal data”, said Jennifer Ailshire, a professor of gerontology and sociology at the USC Leonard Davis School of Gerontology and co-author of the study.

This study may open the door to research into how interventions, like air conditioning, could stave off the adverse effects of ageing, said Mariana Arcaya, a professor of urban planning and public health at the Massachusetts Institute of Technology, US.

“Now, the literature is focused on, ‘Can you survive in extreme heat without medical intervention?’” she said. This study, in contrast, suggests that even if people aren’t at immediate risk of health crises or death because of the heat, “there may still be an effect”.

NYTNS