The year 2024 marked the first time that the global average temperature exceeded the 1.5 C threshold set by the Paris Agreement. One direct consequence is more intense heat extremes, which already pose a significant threat to human health.
By 2080, the city of Montréal, for example, could experience between 37 and 54 days per year with temperatures exceeding 30 C compared to only 11 between 1976 and 2005.
Older adults are considered most vulnerable to heat. During the exceptionally hot European summers of 2022, 2023 and 2024, approximately 84 per cent of the 181,446 deaths attributed to heat occurred among adults aged 75 and older. The pattern was similar during the 2021 heat dome in western Canada.
It’s urgent to thoroughly investigate the reasons why seniors are more susceptible to extreme heat, especially considering population aging, like in provinces like Québec where one in four people will be over 65 by 2031. We also need to discover whether this vulnerability can be alleviated.
This article is part of our ongoing series The Grey Revolution. The Conversation Canada and La Conversation are exploring the impact of the aging boomer generation on Canadian society, including housing, working, culture, nutrition, travelling and health care. The series explores the upheavals already underway and those looming ahead.
Why older bodies struggle with heat
The health effects of heat stem primarily from excessive heat buildup in the body. Compared to young adults, older adults generally experience a more pronounced increase in internal body temperature when they are exposed to heat. This change doesn’t begin suddenly at age 65, but develops gradually starting in adulthood.
In our research, we have observed that, for the same level of heat exposure, internal body temperature tends to increase by approximately 0.1°C per decade of life. In other words, a 90-year-old person could reach an internal body temperature nearly 0.7°C higher than that of a 20-year-old under the same conditions. This partly explains why older adults are more likely to develop cardiovascular complications or kidney complications due to heat.
To prevent overheating, the body activates several mechanisms to shed heat. The blood vessels in the skin widen to exchange more heat with the environment — a physiological response known as “cutaneous vasodilation.” The sweat glands, meanwhile, produce sweat, the evaporation of which helps cool the body.
Several studies have shown that these responses become less effective with advancing age. Starting at age 40, people generally sweat less, while skin vasodilation is reduced during heat exposure. Until recently, most research suggested that these limitations stemmed primarily from the organs that cause these responses, specifically skin blood vessels and sweat glands.
However, one question remained largely unresolved: Does aging also affect the nerve signals the brain sends to trigger these protective mechanisms? In other words, is the neural control of thermoregulation also impaired with age? This is the question we sought to answer in our latest research.
How to better tolerate heat
Repeated exposure to heat can trigger a series of physiological adaptations that allow the human body to better tolerate hot environments. This phenomenon, known as heat acclimation, is widely used by athletes before competitions held in high heat, as well as by military personnel before deployment to hot regions.
Acclimation generally involves exposing oneself to heat for 60 to 90 minutes a day over a period of seven to 14 days, either through exercise in a hot environment or through passive methods such as hot baths or saunas.
There are numerous benefits: sweating begins more quickly and becomes more profuse, the blood vessels in the skin dilate sooner, the resting internal body temperature decreases, and the heart doesn’t have to work as hard during heat exposure. The result is that, for the same level of heat exposure, the body experiences fewer adverse effects.
In fact, many of these adaptations occur as early as the first week. This phenomenon has been known since the late 1700s and appears to occur in everyone.
This strategy could be particularly useful in early summer in northern climates, when the first heat waves strike before the body has naturally adapted to high temperatures. Until recently, the scientific community had very little data on how older adults maintain this adaptive capacity.
Hot baths
To address this question, we compared the physiological responses to heat of 15 people aged 60 to 78 with those of 16 young adults aged 21 to 40, before and after seven days of heat acclimation involving daily 90-minute hot baths at 40°C.
Before and after these hot baths, we measured the nerve signals responsible for sweating and the dilation of blood vessels in the skin. After seven days of acclimation, these signals were activated more quickly, meaning that the body triggered its heat-loss mechanisms while having accumulated less heat. This response was associated with earlier sweating and cutaneous vasodilation.
Daniel Gagnon is a Professeur agrégé, Université de Montréal. Thomas Deshayes is a Professeur adjoint, Faculté des sciences de l’activité physique, Université de Sherbrooke.
This article was first published by The Conversation and is republished under a Creative Commons licence. Read the original article.
We also observed a gradual decrease in resting internal body temperature as well as a lower heart rate during the hot baths, while maintaining identical conditions throughout the experiment.
Contrary to our initial hypothesis, these adaptations were comparable in young adults and older adults. These results, based on approximately 30 individuals, suggest that aging does not necessarily impair the nervous system’s ability to adapt to heat.
Although this finding is encouraging, other questions remain. In particular, it remains to be determined what minimum dose of heat (in terms of intensity, duration, and frequency of exposure) is necessary to induce beneficial physiological adaptations. In our study, participants took seven daily 90-minute baths at 40°C, which was a demanding protocol, but was well tolerated. However, the feasibility of such an approach on a large scale remains uncertain.
Future research will need to identify simpler and more accessible heat acclimation strategies to help older adults adapt to increasingly frequent episodes of extreme heat.