When summer temperatures climb and the air feels heavy, most people think about sweaty commutes, restless nights and the electric bill. A new study suggests something else is happening in the background. For older adults, that heat may be nudging their bodies to age faster at the cellular level.
Researchers from the University of Southern California analyzed data from 3,686 adults aged 56 and older across the United States. They found that people who lived for years in hotter neighborhoods showed signs of accelerated biological aging compared with peers in cooler areas, even when income, race, smoking, alcohol-use and pollution were taken into account.
Heat and the body’s internal clock
The team did not look only at birthdays. They focused on epigenetic age, a measure based on chemical tags on DNA that help switch genes on or off.
These tiny marks, known as DNA methylation, change in response to the environment and can reveal whether someone’s body is biologically older or younger than the number on their ID. Faster epigenetic aging is linked to higher risks of heart disease, disability and earlier death.
In this study, scientists used three established epigenetic clocks that estimate how quickly the body is wearing down. They compared those clocks with local heat records at the census tract level, using the heat index, which combines temperature and humidity to reflect how hot it actually feels to the human body.
What the scientists measured
The researchers counted how many days in each neighborhood reached what the National Weather Service classifies as at least “caution” for heat stress. That category starts when the heat index reaches roughly 80 degrees Fahrenheit and goes up from there.
They also looked at more intense “extreme caution” days, when conditions are more likely to trigger heat illness, especially in vulnerable groups.
They examined several time spans. Some were short, like the week and month before a participant’s blood draw. Others stretched across one year and then six years. For the most part, more hot days in every window were linked to a body that looked older according to at least one of the epigenetic clocks. The strongest effects appeared with long-term exposure.
In the hottest regions, where residents endured at least about 140 days a year with temperatures above 90 degrees, older adults showed up to roughly 14 months of extra biological aging compared with people in places that saw fewer than 10 such days a year. That difference is similar in size to what scientists see with heavy drinking or smoking.
Who feels the heat the most
The study did not find strong evidence that any one demographic group was biologically more sensitive to heat once exposure was equal.
What it did show is that exposure is far from equal. Older adults who were non‑Hispanic Black or Hispanic, who had lower education or lower wealth, and those living in socially vulnerable or suburban areas were more likely to reside in hotter neighborhoods with more heat days across all time windows.
In practical terms, that means many people who already face higher risks for poor health are also the ones living with more days of punishing heat. Add in common issues like limited green space, busy roads and older housing, and the burden of heat starts to look less like an unlucky streak and more like a pattern.
How heat might speed up aging
Scientists are still piecing together the biology. Laboratory studies in animals suggest that severe heat can leave a kind of “epigenetic memory” in cells, altering DNA methylation in ways that affect immune function, heart tissue and other systems long after the heat wave ends.
In people, the new findings hint that repeated exposure to high heat may gradually push these molecular systems out of balance. The physical strain might not show up right away as a diagnosis, yet the body’s internal clock appears to move ahead a little faster. Over years, those small steps can add up.
What this means for everyday life
Climate change is already driving more frequent and intense heat events across the United States, and public health agencies warn that older adults are among the groups most at risk.
That does not only matter during headline‑grabbing heat waves. It also matters on the long, sticky stretches of hot days when an older person decides to skip a walk, sit by a fan instead of a cooling center, or hesitate to switch on the air conditioner because of the monthly bill.
Experts suggest that simple changes can help. Cities can add shade trees and shelters at bus stops, open cooling centers and design outreach programs that check on older residents at home. Community services that already visit seniors, such as meal-delivery programs, can fold heat checks into their routine.
Families and neighbors can play a role too. That might mean helping an older person find a cooler place to spend the afternoon, making sure medications are reviewed for heat‑related side effects, or just calling during hot spells to see how they are coping.
Small actions can reduce the need for emergency care and, to a large extent, may slow some of the hidden biological toll of chronic heat.
At the end of the day, this research adds a new layer to how we think about a warming world. Extreme heat is not only a short-term danger for older adults. It may also be quietly reshaping how fast they age from the inside out.
The study was published in Science Advances.
