What are children introduced to, on average, at the age of four and a half months? Your answer will vary depending on where you are from. In Finland, the answer is the sauna.
This centuries-old tradition is so deeply embedded in Finnish society that almost every family in Finland has a sauna, on top of the many public saunas available. Even newer student residences in the Nordic country have them on their rooftop!
A typical Finnish sauna is a log- or wood-panelled room containing a heater, which is usually topped by rocks that hold on to the heat. Nude bathers sweat on benches inside the room. The temperature is very high, routinely between 80 and 90ºC at the level of the bather’s face, and the relative humidity inside the sauna will vary between 10 and 30%. Water is thrown onto the rocks, which produces a steam called löyly which raises the humidity and condenses on the skin and in the airways to intensify the heat. Bathers stay in the high heat for 5 to 30 minutes, then cool off outside the sauna, and come back, going through as many hot-cold cycles as they can tolerate.
While many people go to the sauna to relax or to socialize, this cultural practice has acquired over the years a number of claimed benefits. Going to the sauna, we hear, can protect us from respiratory infections. It can alleviate pain, lower the blood pressure of people with hypertension, and even ward off serious illness.
Are any of these alleged health benefits backed up by good scientific evidence?
Sweating the small stuff
It should come as no surprise that sitting in a blisteringly hot room and sweating profusely will produce changes in the body. First, there’s the temperature of the skin, which rapidly goes up to 40ºC, with blood flow to the skin increasing dramatically. Typically, only 5 to 10% of what the heart pumps ends up reaching the skin. In a sauna, that number goes up to 50 to 70%, or seven liters of blood per minute.
Then, there’s the sweat. In a typical session, the average bather will secrete half a kilogram of sweat. That’s the weight of two Golden Delicious apples! This is also why rehydration after a session is essential.
Heart rate goes up to 100 beats per minute typically, and sometimes to 150 beats per minute during intense sessions or in people who are not accustomed to the sauna. Blood pressure may also change, although different studies have reported different results on that front. The levels of many hormones rise during the session, including cortisol (the primary stress hormone) and norepinephrine (which increases alertness), while testosterone levels remain stable.
These changes are normal: the high temperature is stressing the body, and the body is compensating. Blood flows more to the surface and sweat is lost in an attempt to lower the temperature of the body. In case you were worried about what these changes might mean in the long term, the body goes back to normal within a few hours of leaving the sauna.
Short-term changes take place, sure, but what about long-term changes? Are there clear health benefits to sweating it out in a sauna regularly?
If you take a casual look at the scientific literature on this question, you may be led to believe that we know for sure that this is the case. However, on closer inspection, very important caveats become apparent.
Slicing the salami
Studies have apparently shown that frequent users of Finnish saunas are less likely to die of a sudden cardiac arrest or to develop problems like respiratory diseases, body-wide inflammation, or blood clots in a vein. But when I looked at these studies, I noticed that they all involved the same core group of researchers: Jari Laukkanen, Tanjaniina Laukkanen, and Setor Kunutsor. Moreover, the speed at which they were publishing papers on the benefits of the sauna was rather impressive, with no fewer than twelve such papers published in 2018 alone.
This speed can be explained by the way in which their studies are done. They all use data from the same cohort of Finnish people who have been followed over time since the mid- to late-1980s. It is known as the Kuopio Ischemic Heart Disease Risk Factor study, after Kuopio, the most populous city in the Eastern Finland province where the participants were recruited. They were examined at the beginning of the study, as well as four and eleven years later, and asked to fill out questionnaires about what they ate and what their lifestyle habits were. National registries were also consulted to know who ended up developing which disease over time.
And in the 1980s, when the study began, participants were asked once via questionnaire how often they went to the sauna during a typical week. This self-report was then used by Laukkanen, Laukkanen, Kunutsor and their colleagues to separate people into those who go to the sauna once a week or less, two to three times a week, and four times or more, and for each group of sauna bathers, the researchers asked: how many ended up developing this or that disease? For each disease, a separate publication. This is colloquially known as “salami slicing,” or the splitting of one data set into multiple slices for publication.
There are many issues with using the Kuopio cohort to derive reliable information about the health benefits of sauna bathing, and these limitations are important since these papers represent the vast majority of the scientific evidence for the long-term benefits of the practice. The cohort study itself is observational, meaning that people choose how often to use the sauna; they are not randomly assigned a schedule, as they would be in a clinical trial. This means that whether the high sauna frequency causes the health benefit cannot be known with certainty, as other factors might influence how often one goes to the sauna. The cohort also largely consists of white men recruited from one area of Finland, so how generalizable any result will turn out to be is hard to know. When participants were separated into groups based on how often they go to the sauna, the groups were not always comparable. They differed according to average age, alcohol consumption, smoking habits, and degree of physical activity.
But even more importantly, sauna frequency was only evaluated once.
In the 1980s.
When Prince’s “When Doves Cry” was at the top of the charts.
How accurate is this information thirty years later?
Using that same cohort, these researchers would go on to report that bathing in a Finnish sauna regularly was strongly associated with a lower risk of developing a psychotic disorder, a serious, disabling mental illness. Given the extraordinary nature of this claim and the challenges of deriving useful information from the Kuopio cohort study when it comes to sauna habits, the alarm bells of skepticism should be ringing right about now.
In North America, however, the word “sauna” probably suggests something even more far-fetched: detox.
Detox? Not so much
The wellness world has embraced saunas—the Finnish sort, but also infrared saunas and sweat lodges—as a means to sweat out ill-defined toxins. With the exception of medical interventions following genuine intoxication, the idea of detoxing is not based in science. Interestingly enough, Canadian researchers wondered if you could sweat out persistent organic pollutants (POPs), molecules like polychlorinated biphenyls and organochlorine pesticides that tend to accumulate in the human body in fatty tissue. According to their calculations, the short answer is no. Given how little fat is excreted in the sweat (which consists mostly of water, sodium, chloride, potassium, urea, and amino acids), any pollutant that hides in our fat is unlikely to get released via a heavy sweat in the sauna. Trying to detox by sweating profusely is a fool’s errand, it turns out.
Another common idea is that taking frequent trips to the sauna can protect us from respiratory infections, like the common cold and the flu. The evidence, however, is far from convincing. Apart from another slice of the Kuopio salami, courtesy of Laukkanen, Laukkanen, and Kunutsor, I could only find data presented at a conference in 1991 and a German-language article from 1977 (both cited here). They both involved small groups of school children either bathing in a sauna weekly or not, with the number of sick days compared between groups. Hardly convincing evidence.
Meanwhile, some people report that the sauna helps relieve their aches and pains, and others with asthma or chronic bronchitis say they can breathe a little bit better inside the sauna. But just like with the increasing heart rate and hormonal surges, these reported improvements are short-lived. As for lowering blood pressure in people with hypertension, there is some evidence that the sauna may be up to the challenge, but the studies are not as rigorous or as convincing as they could be.
Bottom line: sweating in a Finnish sauna clearly affects our body temporarily, but whether or not the hot air and perspiration reinforce our body’s defences against illness remains to be convincingly established. And although sauna bathing is quite safe, there are risk factors to point out. From 1970 to 1986, 230 people died in Finland of hyperthermia, meaning a very high body temperature. Most of these deaths occurred inside a sauna and were linked to the recent consumption of alcohol, which can lead to a dangerously low blood pressure inside a sauna and thus fainting. This is also why going to the sauna alone is not recommended. Meanwhile, people with unstable cardiovascular conditions should abstain. Consult your physician if in doubt.
More extreme versions of the sauna, which involve covering the body in multiple layers and sweating it out for hours in a hot room for the purpose of detoxification or spiritual rebirth, can actually be much more dangerous. Chantal Lavigne, a 35-year-old mother of two, infamously died of hyperthermia during a seminar called “Dying in Consciousness,” in which she was told to rapidly breathe in and out while her body was wrapped in plastic and blankets and her head was covered by a box. This went on for hours.
Sweating it out can be relaxing and temporarily relieve pain but taken too far in the service of some mythical detoxification, it can have fatal consequences.
Take-home message:
-The Finnish sauna has a very high temperature and a relatively dry air, with humidity being raised by throwing water onto heated rocks
-Even though there are clear changes in the body during a Finnish sauna session, such as higher heart rate and changes in hormones, they are short-lived and the evidence for long-term health benefits is questionable
-Sauna bathing and other practices based on sweating are often sold as a way to detoxify, but this is not possible and is quite dangerous when the body is allowed to overheat
