Every summer, the same mystery plays out: an older relative suffers in the heat while younger people nearby seem fine. Now, researchers at McGill University in Montreal think they have found one reason why—and more importantly, a way to fix it.
In their study, the scientists discovered that aging doesn't just weaken the body's cooling parts like sweat glands and skin blood vessels. It also dulls the brain's ability to send out the early warning signals that tell the body to start cooling down in the first place. "The neural control of thermoregulation is also impaired with age," the researchers explained.
This finding matters because heat is becoming more dangerous everywhere. In 2024, global temperatures crossed a worrying threshold for the first time: the 1.5 degrees Celsius limit set by the Paris Agreement. The consequences are already visible. During the exceptionally hot summers of 2022, 2023, and 2024 in Europe, roughly 84 percent of the 181,446 deaths blamed on heat struck people aged 75 and older. In Quebec, where one in four people will be over 65 by 2031, Montreal alone could see 37 to 54 days a year above 30 degrees Celsius by 2080—compared to just 11 such days between 1976 and 2005.
The McGill team found something striking about how internal body temperature changes with age. For every decade of life, internal temperature rises about 0.1 degrees Celsius more than a young adult's would under the same conditions. That means a 90-year-old could run nearly 0.7 degrees Celsius hotter than a 20-year-old doing the exact same thing. That extra heat puts pressure on the heart, blood vessels, and kidneys.
But here is where the hope comes in. The researchers say heat acclimation—deliberately exposing the body to heat for 60 to 90 minutes a day over one to two weeks—can reverse much of this vulnerability. Sweating starts sooner, skin blood vessels dilate faster, resting body temperature drops, and the heart doesn't work as hard. Many of these benefits appear within just the first week. The phenomenon has been observed since at least the 1700s and works for nearly everyone.
The strategy could be especially useful in early summer, before the body has naturally adjusted to warmer weather. For older adults who might struggle with traditional exercise in heat, passive methods like warm baths or saunas also count. "It is urgent to thoroughly investigate why older adults are more susceptible to extreme heat," the researchers noted—and this discovery brings that investigation closer to a practical solution.
In a warming world, understanding why older bodies struggle with heat is not just interesting science. It is a roadmap for keeping grandparents, neighbors, and future generations healthier when temperatures climb.