The woman in her sixties stood at the edge of the silver tub and told herself she could not do it. The water was 40 degrees Celsius, hot enough to feel punishing, and the room was dim and unfamiliar, part of a human physiology lab at the University of Ottawa. She lowered herself in slowly, grimacing as the heat wrapped around her skin, and she stayed there, shoulder-deep, for two hours. Then she came back the next day and did it again. And again. For seven straight days. Caroline Li-Maloney, the Ph.D. candidate running the study, watched it all unfold with a mixture of sympathy and scientific fascination. “I’m going to be straight up: People hated doing this,” she says. But the women endured it because they understood what was at stake. Li-Maloney’s research is focused on a question that has become increasingly urgent: can women in their sixties, seventies and eighties — a group that is particularly vulnerable to dying during heat waves — train their bodies to handle extreme heat? If the answer is yes, it could mean that some of the most fragile members of society are not entirely at the mercy of the dangerously hot summers that climate change is making more frequent and more severe.
Extreme heat is not just uncomfortable; it kills. When the human body’s core temperature climbs above roughly 37 degrees and stays there, heat exhaustion can set in, and with prolonged exposure the situation can spiral into delirium, seizures, organ failure and death. During a single European heat wave in June, more than 10,000 people are estimated to have died from heat-related causes. The numbers are staggering, and they highlight a cruel reality: heat waves do not affect everyone equally. Older adults, especially women, are among the most likely to die when temperatures spike. The reasons include age-related changes in sweating, circulation and heart function, as well as the fact that older people are more likely to have chronic health conditions that make heat more dangerous. But researchers have long known that young, healthy people can develop a kind of heat immunity through repeated exposure. At the University of Ottawa’s Human and Environmental Physiology Research Unit, scientists are now exploring whether older bodies, too, can be trained to cope better — whether a week of uncomfortable soakings in hot water can leave a lasting mark on the physiology of a person in her late seventies, lowering her risk during a real heat emergency.
The science of heat acclimation is well established in athletes, soldiers and outdoor workers, who have been studied for decades as models of human performance under stress. When someone is exposed to heat repeatedly, their body begins to make protective adjustments. Sweating starts earlier and becomes more intense, because moisture evaporating from the skin is one of the most effective ways the body sheds heat. Tiny blood vessels near the surface of the skin dilate more quickly and more fully, allowing warm blood to move from the core of the body out toward the skin, where heat radiates away into the surrounding air. At first, these changes force the heart to work harder, pumping faster to maintain blood pressure as blood is shunted toward the skin. But over time, the body adapts by increasing blood volume, so the heart can pump the same amount of blood with fewer beats. The result is a body that is simply better at cooling itself in hot conditions. Studies suggest that a structure in the brain called the hypothalamus acts like a thermostat, fine-tuning these responses. After repeated heat exposures, nerve cells in the hypothalamus become more sensitive to body temperature and send signals more quickly to the sweat glands and blood vessels, telling them to do their jobs earlier and more effectively. There is also evidence that changes happen at the cellular level. Jeremy McCormick, a physiologist at the University of Ottawa, and his colleagues immersed a small group of healthy men in their sixties and seventies in hot water for a week and then analyzed the immune cells in their blood. They found increased concentrations of heat shock proteins, a type of molecule that stabilizes and repairs other proteins damaged by heat stress. Those internal repairs, along with other molecular changes, appeared to ease some of the cellular strain caused by heat. It is a kind of conditioning, rehearsing the body for the worst conditions it might face.
Intriguingly, the idea of heat adaptation also shows up when scientists look at whole populations. Every community has a temperature at which people are least likely to die from natural causes, a concept called the minimum mortality temperature, and that number tends to reflect the climate people live in. In cooler places like the United Kingdom, the safest temperatures hover in the high teens or low twenties. In hot countries, the minimum mortality temperature sits far higher, a sign that residents have adapted, at least partly, to their environment. Abderrezak Bouchama, a critical care physician at King Abdullah International Medical Research Center in Saudi Arabia, has studied this in an especially elegant way. He and his colleagues calculated the minimum mortality temperature for residents of Mecca, Saudi Arabia, and for the hundreds of thousands of international pilgrims who travel there each year for the Hajj. Looking at natural deaths during the five-day pilgrimage from 2006 to 2014, they found that the minimum mortality temperature for locals was 26 degrees Celsius, while for pilgrims it was 23.5 degrees. Even Mecca’s residents, however, have limits: their mortality rates began to climb sharply when temperatures rose above 38 degrees. Some of these population differences may be explained by air conditioning, lifestyle, or even genetics, but Bouchama believes physiological acclimation plays a role. There is also evidence that people who live in places with distinct seasons become slightly more heat-tolerant over the summer, with higher sweat rates and lower skin temperatures by late summer than in early summer. The opposite is true as well: adaptations fade during the cold season. That may help explain why early-summer heat waves are often deadlier than those later in the season, before people have had a chance to reacclimate to the heat.
For a long time, scientists were not sure whether this kind of adaptation was possible for older adults, whose bodies are not always as flexible as those of the young. But recent research suggests that the ability to adapt does not disappear with age. Daniel Gagnon, a heat physiologist at the Montreal Heart Institute and the Université de Montréal, led a study in which 15 adults in their twenties and thirties and 16 adults in their sixties and seventies followed a weeklong heat acclimation protocol. Each day, they immersed themselves in 40-degree water for an hour, then alternated between shoulder-high and waist-high water for another hour, ensuring their core body temperatures stayed at or above 38.5 degrees. It was grueling. But by the end of the seven days, both groups had improved in remarkably similar ways. They began to sweat and vasodilate earlier when exposed to heat, their average body temperatures were 0.4 degrees lower than at the start, and their resting heart rates were about five beats per minute lower. “The upshot,” Gagnon says, “is that even if we’re older, our body still has the ability to adapt to heat.” The message is hopeful: aging does not necessarily mean losing the capacity to learn from heat. It may mean needing more time, more care, and more support, but the fundamental physiological machinery of adaptation remains alive.
Li-Maloney’s study takes that idea one step further, focusing specifically on older women and testing whether the benefits of hot water immersion translate into a better experience during an artificial heat wave. Before beginning their week of daily hot water soaks, the women sat for an hour in a room set to 36 degrees Celsius with 45 percent humidity, a simulation of a brutal summer day. It was, Li-Maloney says plainly, “sucks, I’m not going to lie.” Then the women went through the acclimation protocol, enduring the same 40-degree tubs and the same long, uncomfortable immersions. Afterward, they faced the artificial heat wave again. The differences were significant. Their core temperatures stayed about 0.2 degrees lower than before the training, their heart rates were a few beats per minute lower, and they reached dangerous body temperatures much later than they had the first time, if they reached them at all. Perhaps just as important, the women themselves felt the difference. They understood that the research was not only about filling in a gap in the scientific literature; it was also about protecting themselves and others like them. “They love the idea that a study like this might make them at least feel better when summer starts,” Li-Maloney says. Many had lived through heat waves and knew the worry of watching temperatures climb on summer forecasts. Participating in the study gave them a sense of agency in the face of a threat that often feels impossible to control.
There are still many questions to answer before hot water immersion could become a practical strategy for protecting vulnerable populations. Scientists need to figure out how long the adaptations last, how often people need to repeat the training to stay protected, and whether such intense heat exposures are safe for people with heart disease, mobility problems, or cognitive impairment. Jessica Mee, a physiologist at the University of Worcester in England, points out a critical concern: “If they were ever left to do it by themselves outside of a laboratory environment, are they potentially at risk?” A frail older person soaking in a hot tub alone could easily become overheated or faint. So researchers are cautious. But the potential payoff is substantial. If a simple, low-cost intervention like weekly hot water baths could help older adults prepare for summer before the first heat wave arrives, it might reduce illness and death during dangerous weather events. It could also help people who cannot tolerate vigorous exercise in the heat, since sitting in warm water is far less physically demanding than hot yoga, running, or other methods that have been shown to produce acclimation. Li-Maloney points out that by the time the first heat wave of the season hits, a person who has trained in advance should be more protected. The women in her study seem to agree. One of them eventually traveled to Thailand, where she explored the sweltering capital in temperatures that regularly exceeded 30 degrees Celsius. While other members of her tour group wilted in the heat, she felt wonderful. “Everyone [else in] my tour group was suffering,” Li-Maloney recalls the woman telling her, “and I felt great.” It is a small story, but an illuminating one. It suggests that the body, even in old age, can be taught to cope with the world as it is becoming.
