About the study

For the study, researchers collected and analyzed health data from nearly 25,000 adults in Taiwan between 2008 and 2022. They compared participants' chronological age with their biological age. Twelve health measurements, known as biomarkers, were used to determine how heat affected participants' rate of aging. The researchers also accounted for individual factors that could impact aging markers, including exercise, smoking, and preexisting disease.

The research team defined heat waves as periods of at least two consecutive days of abnormally high temperatures, as well as any time officials issued heat warnings. The team also calculated total heat exposure, considering the number of heat wave days, their duration, and cumulative temperatures.

The findings showed that two years of exposure to heat waves accelerated biological aging by an average of eight to 12 days. "This small number actually matters," said lead author Cui Guo, an assistant professor at the University of Hong Kong. "This was a study of a two-year exposure, but we know heat waves have actually been occurring for decades."

Even modest increases in heat exposure added up. Biological age rose by about nine days for every four additional heat wave days over two years. Manual workers, who tend to spend time outdoors, were strongly affected — their biological age accelerated by 33 days. Rural residents and those without access to air conditioning also saw greater impacts.

"Most countries are facing an aging population," Guo said. Because biological aging is closely associated to many diseases and death, speeding it up could be an indicator of serious health issues.

While the mechanisms remain unclear, the researchers suggested that extreme heat may shorten telomeres, disrupt cellular balance, and cause DNA and mitochondrial damage — processes tied to aging. Importantly, Guo emphasized that the study does not suggest people are literally losing days off their lifespan; instead, the findings reflect measurable changes in biological markers linked to disease risk.

Commentary

The study's findings come amid record-breaking heat events worldwide. In 2024 — the hottest year on record — climate change caused 41 extra days of extreme heat globally, according to World Weather Attribution. The World Meteorological Organization also reported that at least 10 countries experienced temperatures above 50 degrees Celsius (122 degrees Fahrenheit) in 2023 and 2024.

According to a 2023 study, heat is one of the deadliest forms of extreme weather and is responsible for more than 800 deaths in the United States each year. Exposure to extreme heat could also exacerbate chronic diseases such as cardiovascular conditions, diabetes, asthma, and mental health disorders.

How to stay safe in extreme heat

As extreme heat becomes more common, here are eight ways to stay safe:

1. Check the heat index. Factor in humidity, wind, and time of day — afternoons between 3–5 p.m. are often most dangerous.
2. Acclimate gradually. Ease into outdoor time on hot days instead of sudden, prolonged exposure.
3. Dress smart. Choose lightweight, breathable, loose-fitting fabrics, and add sunscreen and a wide-brimmed hat.
4. Seek shade. Natural, tree-covered areas are cooler than asphalt or concrete.
5. Know the symptoms. Watch for heat exhaustion signs like dizziness, nausea, or excessive sweating, and move to a cool place immediately if they appear.
6. Stay hydrated. Drink water often, avoid alcohol and sugary drinks, and eat water-rich fruits.
7. Use cooling tools. Apply cold towels, ice packs, or take quick dips in water to lower body temperature.
8. Take breaks. Rest in shade or indoors to give your body time to cool down.