In 1976, a British Army physician named H. Foster wrote a letter to the British Medical Journal about a pressing issue in European public health: the epidemic of fatal heart attacks among middle-aged Finnish men. In it, he speculated that the reason Finland had one of the highest rates of coronary heart disease in the world was the nation’s sauna.
On the face of it, the hypothesis made sense. A sauna is a stressful environment for the body, with an ambient temperature of 80°C (176°F) and above. The pulses of steam place repeated strain on users’ cardiovascular system. Stay in for too long, and you will surely perish.
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But more recent research suggests Foster may have got it entirely wrong, and that frequently partaking in Finnish sauna – as well as other kinds of heated bathing – may significantly reduce the risk of heart attacks, while improving health across the board.
Having grown up mostly in swampy, humid Washington DC, I have never really enjoyed feeling hot and sweaty. But after nearly two decades of writing about health and longevity, watching as data piled up suggesting that limited heat exposure might benefit our bodies and minds, I decided to give sauna a go. I found the research so compelling that I ended up spending three years digging into it and writing a book about the myriad benefits of heat. What’s more, along the way, I discovered why we are so well-primed to reap all these benefits.
We humans have a complicated relationship with getting hot. For starters, it can be uncomfortable – and we are hardwired to know that extreme heat can injure and potentially kill us, via a temperature-sensing network of pathways in our skin, gut and brain. But on the other hand, humans intuitively know heat can be good for us – that much seems clear, given that so many cultures around the world participate in heat-bathing rituals, including Native American sweat lodges, the Mexican temazcal, the Russian banya, the Middle Eastern hammam, the Korean jjimjilbang, the  and the Japanese onsen. Two thousand years ago, of course, most European cities had their own lavish Roman baths. Now, we have the modern hot tub, invented by Roy Jacuzzi in 1968.
All of these represent variations on the same basic recipe: heat an enclosed space to a very high temperature and then add people. Steam may be optional, but sweating is mandatory.
“There are sweat baths all over the world; it’s as common as the baking of bread or fermenting the grape into wine,” says Mikkel Aaland, author of Sweat, a 1978 study of global heat-bathing traditions.
It took until the mid-1980s for scientists to start to catch on to the profound benefits of getting sweaty. Back then, researchers at what was then the University of Kuopio in central Finland enrolled about 2600 middle-aged men in a long-term study of cardiovascular disease, intended to identify potential risk factors driving the Finnish fatal heart attack epidemic that so intrigued Foster. Participants had extensive blood tests and physical examinations and filled out a lengthy questionnaire about their daily habits, family health history, and social and working lives. As the subjects aged, researchers looked for correlations between various lifestyle factors and outcomes such as heart attacks, strokes and cardiovascular disease.
Over the years, this  has yielded key insights into possible relationships between heart disease and alcohol consumption, cholesterol levels,  and other potential risk factors. About 1000 women were added to the study cohort in 2001, with research finding . Then, around 2015, a Finnish cardiologist named Jari Laukkanen noticed that the subjects’ intake questionnaire had also asked about their sauna habits – how often they used the sauna, for how long and even at what approximate temperature.
Laukkanen compared the men’s stated sauna habits with their health outcomes. The correlation was startling: , and powerfully so. Men who said they used sauna two to three times per week had a 22 per cent lower rate of fatal heart attacks compared with those who bathed only once per week. The most frequent sauna bathers, men who went four to seven times per week, had a 63 per cent reduction. According to the researchers, the frequent sauna bathers also experienced lower all-cause mortality over 20 years: 24 per cent lower for the twice-or-thrice-weekly bathers and 40 per cent lower for the most frequent bathers.
The more that the participants used sauna, it seemed, the longer and healthier their lives were. There was also a plausible mechanism: heat stress causes the heart rate to accelerate, increasing blood flow and dilating blood vessels, similar to what happens during light-to-moderate exercise.
Saunas, it turned out, are good for you – and the deaths that so concerned Foster are now to have been related to poor diet, rural poverty and easy access to vodka and cigarettes from the neighbouring Soviet Union.
Laukkanen’s study, along with more than three dozen follow-up papers, has helped drive the boom in sauna use in the UK, Ireland, North America, and Japan over the past decade or so. In the world of physiology, the Finnish sauna studies changed the conversation around heat and its effects on human health.
“Prior to those studies, everyone was focused on prevention of heatstroke, reduction of heat risk and how you regulate that,” says , a physiologist at the University of Windsor in Ontario, Canada. “Since those data came out, I think the focus has shifted to the idea of heat therapy.”
Instead of looking at heat as something that is always dangerous, even potentially deadly, the Laukkanen sauna studies raised a revolutionary question: could heat be harnessed for healing?
Our evolutionary history suggests it can. While heat, especially extreme heat, can be highly dangerous to humans, as seen in the recent European heatwave, we are also uniquely equipped to handle hot conditions – more so than any other mammal, with the possible exception of the camel – thanks to our ability to sweat. This isn’t an accident. Humans’ ability to sweat to cool ourselves may, in fact, be one of our evolutionary superpowers, a trait that enabled our long-ago ancestors to climb from the middle to the top of the food chain. The secret lies in special structures called eccrine sweat glands, which secrete a mixture of water and salt onto our skin, where it evaporates, carrying away body heat. These are distinct from the sweat glands in our armpits and other hairy regions, which produce the more smelly type of sweat.
A little more than a decade ago, a University of Pennsylvania geneticist named Yana Kamberov began digging into the origin of our sweatiness as a species. She  that, at some point, more than a million years ago, a rapid-fire sequence of genetic mutations caused eccrine sweat glands to proliferate all over the bodies of our prehuman ancestors. At the same time, these primates lost much of their body hair. Humans have 10 times as many eccrine sweat glands as one of our closest relatives, the chimpanzee, and our relative hairlessness allows that sweat to evaporate more easily.
“That’s the key step,” says , a physiologist at the University of Oregon who has studied heat and athletic performance extensively. The change of state from liquid water to vapour transports an enormous amount of heat energy from our bodies. “That sweat has to evaporate. Dripping sweat doesn’t do anything.”
Their newfound ability to sweat allowed these ancient ancestors to forage for food during the middle of the day, when their predators were forced to stay in the shade. It also enabled them to pursue prey animals that tired more easily in the heat. “Humans were born to run,” declared the evolutionary biologist Dennis Bramble and anthropologist Daniel Lieberman in their describing our evolution into the ultimate endurance athletes.
Although we humans are now more likely to run marathons in big cities than to chase down antelope, our ability to sweat still helps us in a number of ways. This is because it enables us to tolerate heat stress, which unlocks myriad therapeutic benefits, independent of exercise. When we put our innate cooling system to the test by sitting in a sauna or similar hot space – even a hot tub or an ordinary hot bath – we reap benefits. For starters, our skin temperature heats up quickly. This activates our cardiovascular system, as our heart rate accelerates and our brain diverts up to 70 per cent of our blood flow to the skin, in an effort to cool us down.
Shortly after, sweating ensues, as our eccrine sweat glands draw liquid from our cells, intracellular spaces and blood plasma and pull it to the skin. In a sauna, we might sweat more than a litre of fluid per hour. To replace that liquid, our body seeks to expand the volume of blood plasma, which is one reason why it is important to hydrate during and after sauna use.
This is similar to the physiological response to gentle or moderate exercise (minus the beneficial effects of muscle activation). So, it isn’t surprising that Laukkanen and his colleagues went on to discover that the Kuopio study participants’ regular sauna bathing delivered a , including a reduction in the incidence of strokes, as well as a lowered rate of respiratory illnesses.
One of the most striking findings from the Kuopio study of middle-aged Finnish men was that the most frequent sauna bathers, who bathed four to seven times a week, had a 63 per cent lower incidence of Alzheimer’s disease compared with once-weekly bathers. Even better, this finding was echoed in a similar  on a different set of men and women. This retrospective study looked at nearly 14,000 people using Finnish mobile health clinics from the 1970s, aged between 30 and 69 at the time of treatment. It found that men and women who had said they used sauna nine to 12 times per month had less than half the rate of Alzheimer’s disease diagnoses over 20 years versus those who used a sauna less than four times a month or not at all, and a 20 per cent reduction over 39 years. It suggests that frequent sauna use may at least delay dementia, if not prevent it outright.
This picture is made even more vivid by the curious story of a man named Doug Whitney, who was born with a genetic mutation that virtually guaranteed he would develop Alzheimer’s disease in his late 40s or early 50s. But he didn’t, remaining cognitively normal through his 60s and into his late 70s. It turned out that Whitney had worked for decades as an engine-room mechanic aboard naval ships, at temperatures of 50°C (122°F) or more. In this sauna-like work environment, he often became so hot that he needed to be hosed off to cool down. While that may not seem healthy or even safe, it may have saved him from dementia.
When Jorge Llibre-Guerra at Washington University in St Louis, Missouri, and his colleagues analysed Whitney’s cellular proteins, they discovered he had extremely high levels of different heat shock proteins, a stress-response mechanism that kicks in as our core temperature rises. These proteins help maintain cell membranes, other cell proteins and the structure of our DNA, while speeding up the removal of cellular junk such as misfolded proteins. The researchers speculated that the heat shock proteins may have helped protect him from accumulating the .
There are even hints that heat treatment may  mental health. One of the more striking findings of the was that frequent sauna users had a lower risk of experiencing psychosis over their lifetimes than the less-frequent users.
More recently, researchers have begun investigating the possible use of “whole-body hyperthermia”, in which a person’s core body temperature is intentionally elevated, as a potential treatment for depression. One study, for instance, found that when people with severe depression had their core body temperature warmed to 38.5°C (101°F), then allowed to cool, over 45 to 90 minutes, they of poor mental health, with the effect lasting up to six weeks.
The exact mechanism by which heat therapy might relieve depression is unclear. One idea is that it reduces brain inflammation. Another is that warming people up may have a similar effect to antidepressant medications: studies in rats have found that heating up their brains stimulates a brain region known as the dorsal raphe nucleus to produce more of the brain-signalling molecule serotonin. Larger studies are ongoing to determine whether heat therapy – perhaps paired with a cold plunge or talk therapy or both – might be a useful adjunct to antidepressants and psychotherapy.
Indeed, there is so much positive correlational data on heat therapy and a multitude of health benefits that it seems fair to ask: is there anything sauna and heat therapy can’t do? The more pertinent question, though, is: can we go beyond correlations and prove that heat therapy really causes all these beneficial effects?
point out that their sauna-use data lacked a true control group of people who never used sauna. They were comparing people who used sauna almost every day with people who said they used it once a week. But this wasn’t a deliberate omission. In Finland, almost everyone goes to sauna, it is part of daily life, and out of the entire 2327-person original Kuopio study cohort that Laukkanen examined, only 12 participants said they never used sauna at all. So, there effectively was no control group.
Another critique is that perhaps the frequent sauna users were healthier to begin with? After all, it takes a certain degree of grit to handle a 90°C (194°F) Finnish sauna and its recurring blasts of ±ôö˛â±ô˛â,Ěýor steam. They could also have been wealthier or less stressed, since they had time to visit sauna every day or two. But Laukkanen insists that his team controlled for factors including age, smoking, alcohol consumption, physical activity, body mass index, cardiovascular risk factors and socioeconomic status. Indeed, , Laukkanen and his colleagues found that frequent sauna bathing appears to effectively compensate for the deleterious health effects of low socioeconomic status.
Also, shorter-term have suggested that sauna use may indeed improve cardiovascular health in a mechanistic way. In one such study, Laukkanen and his colleagues found that a single Finnish sauna session of 30 minutes at 73°C (163°F) slightly. In another , the group found that sauna use activated the parasympathetic (or “rest-and-digest”) nervous system.
Given all the evidence supporting the case for heat’s healing power, the next question is: what is the most effective dose of sauna or other heat therapy?
Sadly, the data is patchy and, right now, there is no single prescription. “The jury is still out on this,” says , who did his PhD work in Laukkanen’s lab and now works at the Montreal Heart Institute.
Personally, I sauna like the Finns, who simply stay in for as long as they feel like. I also try to make it to a session every week. If you don’t happen to be in Finland or don’t have a sauna nearby, other studies suggest that enjoying a hot tub or even a hot bath may convey similar health benefits. A large  for example, found that people who took regular hot baths at home had a significantly reduced risk of death from cardiovascular conditions – to the tune of over 25 per cent for those who took hot baths close to every day, compared with those who bathed two or fewer times a week.
Good things happen when we get really hot, it seems. And ongoing clinical trials may help us understand whether heat-based therapies might prove effective in treating  or staving off at scale. As for me, after decades of drilling into the science of ageing and understanding the vital role of diet, quality sleep, regular exercise and strong social connections in maintaining our long-term health, I am sure that targeted heat therapy deserves its own spot in our longevity toolkit. Apologies to H. Foster.
