Is it time to rethink heat?

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Bill Gifford is a science journalist whose latest book is Hotwired: How the Hidden Power of Heat Makes Us Stronger.

It’s common, as climate change takes hold, to lament the demise of winter, which has grown shorter, warmer and rainier in many places. But the real unsung casualty of climate change may in fact be summer. Many of us used to look forward to the warm months as a season of freedom and fun; now, they come wrapped in existential dread.

Last summer, the high temperature in Toronto hit 32 C or above on 14 different days, twice the five-year average. It was another long, hot, steamy season. And although it paled beside the North American heat wave of 1936, which brought three days of 40 C temperatures to the city and killed hundreds of people, it still felt a little like a harbinger of doom. It’s not just Toronto: Where I live, in Salt Lake City, Utah, summer high temperatures routinely crest 8 to 10 degrees Fahrenheit above their historical averages.

The media and political rhetoric around weather and climate only heighten the anxiety. Forecasters now hype any warm week in midsummer as a “deadly heat wave,” brought on by climate change, and caution us not to venture outside in “sauna-like” weather. As then-U.S. Secretary of Commerce Gina Raimondo warned, at the July, 2022, launch of a public-health website called heat.gov, “This summer – with its oppressive and widespread heat waves – is likely to be one of the coolest summers of the rest of our lives.”

Which is a depressing thought. Warm weather is no longer our well-earned respite from frigid winters; it’s now framed as a deadly threat. Public-health officials warn, rightly, of the risks of heat exhaustion and heat stroke, especially to the elderly, athletes and workers. But over the past decade or so, some scientists have been exploring a different narrative: What if heat, in certain circumstances, can also heal us?

It sounds a little crazy, but an emerging current of research has found that targeted, limited heat exposure may improve overall health and athletic performance – and potentially even mental health.

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This paradigm shift began about a decade ago, with the publication of some extraordinary studies out of Finland. In 2015, a team led by cardiologist Jari Laukkanen revealed that men who used sauna frequently – every day, or every other day – had about a 60 per cent reduced risk of dying from a sudden heart attack, and a 40 per cent lower rate of all-cause mortality, over 20 years.

The underlying data came from a long-running study of cardiovascular disease in middle-aged Finnish men, dating back to the 1980s. The researchers compared the men’s long-term health outcomes with their self-reported sauna use, and the link practically jumped off the page. Frequent sauna users also cut their risk of stroke in half, and Alzheimer’s disease by nearly two-thirds.

This was a huge deal, a magnitude of benefit rivalled only by exercise or quitting smoking. And it was wholly unexpected (outside Scandinavia, that is). In one blow, the Finnish sauna studies completely changed the conversation around heat and health.

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“Prior to those studies, everyone was focused on prevention of heatstroke, reduction of heat risk, and how you regulate that,” says Anthony Bain, an assistant professor of physiology at the University of Windsor. “Since those data came out, the focus has shifted to the idea of heat therapy.”

It makes sense: When we step into a hot space, like (say) an 80 C sauna, our heart rate accelerates, sending more blood flowing to our skin in an effort to cool us down. This causes our blood vessels to expand in order to accommodate the increased blood flow. Soon, signals from our sympathetic nervous system activate our sweat glands – between two and four million of them, all over our bodies. Those sweat glands secrete small amounts of water, which then evaporates to cool our skin. Think of stepping out of a hot shower, and how chilly you feel; that’s the immense cooling power of evaporation.

If this sounds a bit like what happens when we exercise – accelerated heart rate, increased blood flow, and copious sweating – you’re not far off. All that’s missing is the activation of muscle, unless we happen to be doing push-ups or riding a stationary bike in the sauna. (Please don’t.)

The Finnish studies were observational, meaning they could not establish for certain that sauna use was the direct cause of these folks’ apparent better health. It was merely a strong correlation. Perhaps some subjects visited the sauna more frequently because they had more free time than those who could manage it only once per week. Or perhaps they were healthier to begin with.

“If you’re going to the sauna four to seven times a week, that tells me you are less stressed, or more affluent,” acknowledges Earric Lee, a scientist at the Montreal Heart Institute who did his PhD research in the same Finnish lab that produced the sauna studies. This is called “healthy user bias,” and it confounds all kinds of epidemiological research, especially in areas like exercise and alcohol consumption: Subjects who are healthier to begin with tend to have better outcomes from whatever they do.

Dr. Laukkanen and his colleagues argued that they had already controlled for variables including socioeconomic status, alcohol use and smoking – and yet the effect remained robust. A later analysis did find that the more fit study subjects reduced their risk of dying the most (by more than 50 per cent) but the less-fit men who used saunas frequently still lowered their mortality risk by a solid 28 per cent. So sauna use appeared to confer at least some benefit on its own.

Meanwhile, other studies from around the world confirm that limited heat exposure – in many different forms – can be good for us. In the U.S., studies done at the University of Oregon have found that hot-water immersion (basically, soaking in a hot tub) confers similar cardiovascular benefits to using a sauna. In Japan, physicians have used a type of infrared sauna therapy called “WAON” to treat serious heart ailments and other conditions. Another large Japanese study found that people who simply took hot baths frequently reduced their risk of cardiovascular disease and stroke by more than 25 per cent.

Directionally, it seems safe to say that heat exposure – no matter the source – can improve cardiovascular health, at the very least.

In sports, the narrative was the same: Heat was considered a threat, a danger to the athletes’ health. Which it can be. But some athletes have also come to find that heat stress can actually improve their performance. Tour de France cyclists have been experimenting with heat training for years, deliberately conditioning themselves to endure the ever-hotter days of July in France. Other sports are catching on, especially after the 2020 Tokyo Summer Olympic Games, which were the hottest and most humid Olympics ever held.

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The full extent of the toll that the heat would take was not fully clear until the triathlon “test event,” a World Cup race held on the Olympic course in central Tokyo in August 2019. Many athletes were unprepared for the steamy weather, recalls Olav Aleksander Bu, head performance coach for the Norwegian team. “Some people basically came as late as possible into the test event from Europe, and their strategy was to not stay outdoors at all before the race start,” he told me. “They were just delaying everything, absolutely to the latest point. And one of the reasons that I heard was that they wanted to avoid sweating.”

Bad plan.

As the race unfolded, the favourites crumbled. But Mr. Bu had been preparing his athletes for hot conditions. Because Norway is not very warm, he had his athletes ride bikes and run wearing multiple excess layers of clothing, making themselves as hot and sweaty as possible, while monitoring their core temperature via special, experimental devices. It was miserable, but it worked. Despite coming from a cold climate, the Scandinavians dominated, taking two of the top four spots. (A Canadian, Tyler Mislawchuk, won.)

How to stay safe when exercising in the heat

Heat training for performance had been something of a fringe idea until then. Now it was mandatory at the elite level. By the time the COVID-delayed Tokyo Games were held in August, 2021, many athletes across a wide range of endurance sports – not only cyclists and triathletes, but distance runners, race walkers and even dressage horses – had incorporated some kind of heat-adaptation work into their training.

What does heat adaptation mean?

Basically, it means that we feel more comfortable in warm conditions. Heat adaptation explains why a 25-degree day in August feels so much nicer than the same temperature in May. After two or three months of warm weather, our innate cooling system simply works better: We sweat more, and our sweat becomes less salty, so it evaporates more easily from our skin surface. Heat adaptation also means that our heart rate stays lower, at a given level of temperature and effort, so any activity feels easier. Our resting core body temperature also drops slightly. Athletes who heat train notice a performance boost, similar to that derived from altitude training. And anyone can do it, not just elite athletes: I used basic heat training methods, riding my bike slowly during the hottest part of the day, to prepare for a bike ride in Texas called the Hotter’n Hell Hundred – 100 miles in 100 Fahrenheit heat. Without heat training, I never could have done it; with heat adaptation, it was a piece of cake.

The principles of heat adaptation have also been used to protect athletes, notably American football players, whose helmets and heavy pads make them uniquely vulnerable to heat illnesses. The state of Georgia implemented minor changes to the way high-school football practices are run, including requiring a weeklong heat-acclimation period at the beginning of the season in late July/early August, when players are required to practise wearing only shorts and T-shirts, no pads. Every team is required to keep a tub of ice water by the field, to cool players who get overheated, and if the heat and humidity exceed certain levels, practice must be curtailed or cancelled. Before the changes, Georgia had the highest fatality rate of any state. Since the rules were put in place, in 2012, no high school players have died there.

It wasn’t that much of a surprise to learn that deliberate heat exposure could improve athletic performance and heat tolerance, and even save lives. What was surprising was finding that heat therapy, also known as “whole-body hyperthermia,” can potentially improve mental health.

In 2013, a small study conducted in Switzerland found that when severely depressed patients were warmed up to a core body temperature of about 38.5 C, the equivalent of a mild fever, their depression symptoms almost completely disappeared. The scientific literature is full of intriguing studies like this, but often they turn out to be one-offs, with findings that are never repeated. But a follow-up study done in Arizona not only repeated the original findings, but the researchers were dumbfounded to discover that their subjects were still less depressed six weeks later. Since then, a handful of other studies have also found that severely depressed patients respond well to heat treatment, delivered not only via infrared sauna but hot-water immersion (i.e., a hot tub) and even hot yoga practice. All of which suggests that there might be something to it.

In June of 2024, I travelled to Colorado to participate in a similar study, this time combining infrared sauna plus a dip in an icy cold plunge, followed by talk therapy. I had a personal interest in this, because I have struggled with depression for much of my adult life. Often, depression has gained the upper hand, affecting my work, my relationships and my physical well-being. The treatment wasn’t enjoyable – like being slow-roasted in a sweaty oven, then dunked in an ice bath – but after a night of sleep, I felt alive in a way that I hadn’t for a long time.

From 2019: Could an ice-cold swim be an antidote to depression and anxiety?

Multiple ongoing studies are now testing the possible use of whole-body hyperthermia as an adjunct treatment for depression, including one combining heat treatment with talk therapy. Researchers noticed that study subjects became more talkative during and after their infrared sauna treatment, perhaps because heat does something to break down inhibitions. This, too, may explain the rise of “social sauna” in cities like Toronto and New York, as people flock to hot spaces where they feel comfortable letting their guard down.

A few years ago, U.S. military scientists did an intriguing experiment: They exposed mice to repeated bouts of heat stress, each session more severe than the last. Over time, the animals developed a resistance to the heat, down to the cellular level: Their cells activated what are called heat shock proteins, specialized molecules that help make cells more stress-resistant. (One scientist I spoke to called them “mommy proteins,” that take care of other proteins and structures in the cell.) Those heat shock proteins, in turn, enabled the mice to withstand levels of heat exposure that otherwise would have killed them.

Which brings us to a paradox: The best protection against extreme heat is … exposure to heat. Heat is both complaint and cure.

Heat stress is an emerging public-health threat in much of the world, particularly in places like Toronto that are relatively unaccustomed to extreme hot weather. Yet research on athletes and sauna users and patients with depression has found that, in the right dose and the right context, heat exposure can also be healing.

What are we supposed to do with this knowledge?

As our world gets hotter, we must obviously respect the heat and the dangers it poses, especially to vulnerable populations such as workers and the elderly. Yet at the same time, I found it calming and even empowering to know that we can harness this powerful stressor to help make ourselves stronger, both physically and mentally.

But perhaps the most important lesson I learned, in my three-year exploration of the interplay between heat and health, is that sometimes the best way to deal with discomfort or fear is not to try and “conquer” it, but to lean in and get to know it a bit better.