Living in an area that experiences extreme heat can age a person’s genetic material as much as smoking can, according to a new study carried out in the US.
People living in places where temperatures regularly exceeded thresholds set by the US National Weather Service (about 40°C) had an average genetic age that was more than a year older than that of people in cooler areas. The study’s first author, Dr Eunyoung Choi, a post-doctoral associate at the Leonard Davis School of Gerontology at the University of Southern California (USC), told The National that the results were “striking”.
She said that while a link between extreme heat and genetic ageing was not unexpected, the effect was greater than predicted. “Just by the fact they live in areas with more heat days, they may experience additional biological ageing by 14 months,” she said. “This amount, it’s equal to the effect of smoking. It’s even higher than the effect of drinking [alcohol] … We were kind of surprised to see this much effect from heat.”
How can heat affect the body?
There is already evidence linking heat exposure to harmful health effects such as kidney problems, but it is not fully understood “what’s happening at the biological level before these clinical health issues appear”, Dr Choi said. “It might not show up as health conditions, but it might take a silent role at the cellular level and you find years later it leads to disability, disease and even death,” she said.
Published this week in Science Advances, the research analysed blood samples of 3,686 adults aged 56 or older, living across the US. The paper, which has the title "Ambient outdoor heat and accelerated epigenetic ageing among older adults in the US”, is co-authored by Prof Jennifer Ailshire, also of USC’s Leonard Davis School of Gerontology.
To look for a link between extreme heat and genetic ageing, the study employed heat index (HI) categories of average temperature and humidity from the US National Weather Service. The “caution” HI range was between 80° to 90°F (26.7° to 32.2°C), “extreme caution” was between 90° and 103°F (32.2° and 39.4°C) and “danger” was defined as between 103° and 124°F (39.4° and 51.1°C).
HI values where participants lived were compared to their genetic age, determined by the presence or absence on their DNA of chemical signatures called methyl groups, each made of one carbon atom and three hydrogen atoms. This pattern of methylation, affected by factors such as the environment, can determine when genes are turned off and on.
Can the environment change our DNA?
“DNA methylation acts like a light switch,” Dr Choi said. “Environmental exposure can turn it off or on, and that can change gene expression, which changes things in your body.”
Genetic ageing falls into the bracket of what's known as an epigenetic effect, as it relates to alterations to genetic material that do not involve a change to the sequence of a person's DNA. Epigenetic effects have attracted increased interest in recent years.
Rebecca Oakey, professor of epigenetics at King’s College London, said that while hundreds of thousands of people’s genomes had been sequenced, this information “still has not been telling us everything we need to know about health and disease”. The genome is, she said, “a linear sequence” and for genes what is most important is the instructions that tell them when and where to be “expressed”, or turned on.
She said that in recent years there has been a focus among researchers on understanding “the impact of environmental factors on the genome”.
“Everybody wants to know what not to do, like don't breathe asbestos or go into the sun,” she said. “People want to find an explanation for the instructions of the genome from environmental factors.” However, although they can be harmful to the person or other organism that experiences them, Prof Oakey said environmental effects were not always bad.
Epigenetic changes may have long-term consequences, with a 2023 study by two researchers at Tohoku University in Sendai, Japan, finding “growing evidence” that epigenetic changes altered the phenotype – the physical or chemical characteristics of an organism – in later generations. The researchers noted that environmental factors such as stress, abnormal diet and exposure to chemicals “affect the epigenome” in parental reproductive cells, including through their effects on DNA methylation.
When it came to limiting the potentially harmful effects on the epigenome from extreme heat, Dr Choi said that using air conditioning was one approach. However, she added that societies should not rely solely on this, as not everyone can afford to run air conditioning constantly during hot weather. She recommended “multilayer solutions”.
This may involve subsidies for people unable to afford to use air conditioning, plus changes to the design of urban environments, such as the introduction of shading at bus stops and expansion of green areas, which these can lower temperatures. Many such measures have already been introduced in the UAE.