The Finnish study that made doctors take the sauna seriously

In Eastern Finland in the mid-1980s, researchers began asking middle-aged men a long list of questions about their health and habits. Blood pressure, cholesterol, smoking, fitness, diet — and, almost as an aside in a country where saunas are part of ordinary life, how often and how long they sat in the sauna.

Then they waited. For decades.

The project is the Kuopio Ischaemic Heart Disease Risk Factor Study, usually shortened to KIHD. In 2015, a team led by Tanjaniina Laukkanen and Jari Laukkanen at the University of Eastern Finland published what those sauna answers turned out to predict. The paper appeared in JAMA Internal Medicine, and it is the reason you now hear the sauna discussed in cardiology circles and not just in spa brochures.

Here is what the study did, what it found, and — just as important — what it can't tell you.

Who was studied

The cohort was a population-based sample of 2,315 men aged 42 to 60 from Eastern Finland. Baseline examinations ran from March 1984 through December 1989. At that point, every man reported his usual sauna frequency and the typical length of a session.

Because almost every Finn uses a sauna, the researchers didn't compare users with non-users — there were too few non-users to make that meaningful. Instead they compared how much people bathed. Three groups emerged:

• Once a week: 601 men
• 2–3 times a week: 1,513 men
• 4–7 times a week: 201 men

Over a median follow-up of 20.7 years, the cohort recorded 190 sudden cardiac deaths, 281 fatal coronary heart disease events, 407 fatal cardiovascular disease events and 929 deaths from any cause.

What they found

The raw numbers already tell a story. Of the men who bathed once a week, 49.1% died during follow-up. Among those who bathed 2–3 times a week, it was 37.8%. Among the 4–7 times a week group, 30.8%.

Raw percentages can mislead, though — people who sauna more might also be different in other ways. So the researchers adjusted for established cardiovascular risk factors and reported hazard ratios: a hazard ratio of 0.5 means roughly half the risk at any given time compared with the reference group (here, once-a-week bathers).

For sudden cardiac death, the adjusted hazard ratio was 0.78 (95% CI 0.57–1.07) for 2–3 sessions a week and 0.37 (95% CI 0.18–0.75) for 4–7 sessions a week. Notice that the 2–3 times a week confidence interval crosses 1.0, meaning that result alone is not statistically conclusive; the 4–7 times a week result is. The trend across the three groups was significant (P = .005).

The authors report that similar, statistically significant inverse associations held for fatal coronary heart disease, fatal cardiovascular disease and all-cause mortality (P for trend ≤ .005).

Does session length matter?

The study also looked at how long each session lasted. Compared with men whose sessions were under 11 minutes, the adjusted hazard ratio for sudden cardiac death was 0.93 (95% CI 0.67–1.28) for 11–19 minutes, and 0.48 (95% CI 0.31–0.75) for sessions longer than 19 minutes.

Longer sessions were also associated with lower fatal coronary and cardiovascular events — but, interestingly, not with all-cause mortality. That nuance rarely makes it into headlines, and it's a good reminder to read past the abstract's first line.

The same cohort, more questions

Because KIHD tracked its participants for so long, the team returned to the data to ask about other outcomes. Three follow-ups are worth knowing:

• Dementia (2017). In the same 2,315 men, 4–7 sessions a week was associated with a hazard ratio of 0.34 for dementia and 0.35 for Alzheimer's disease compared with once a week (Age and Ageing).
• High blood pressure (2017). Among 1,621 men without hypertension at baseline, followed for a median of 24.7 years, 4–7 sessions a week was associated with a hazard ratio of 0.54 for developing hypertension (American Journal of Hypertension).
• Stroke (2018). In 1,628 men and women aged 53–74, followed for a median of 14.9 years, 4–7 sessions a week was associated with a hazard ratio of 0.39 for new-onset stroke after adjusting for cardiovascular risk factors (Neurology).

In 2018 the same researchers summarised the wider literature in Mayo Clinic Proceedings. Their review discusses proposed mechanisms — improved function of the blood vessel lining, reduced arterial stiffness, changes in the autonomic nervous system, favourable shifts in lipids and lower blood pressure — while describing these as postulated and calling out areas of uncertainty.

What the study can't tell you

This is where honest writing about saunas usually stops, and where it should keep going.

• It's observational. KIHD shows an association, not cause and effect. Men who could sauna most days may have had more leisure time, better housing, lower stress or other advantages that the statistical adjustments couldn't fully capture. The authors themselves conclude that further studies are needed to establish the mechanism.
• Habits were measured once. Sauna frequency was recorded at baseline in the 1980s. People's habits over the following two decades weren't continuously tracked.
• The highest group was small. Only 201 men bathed 4–7 times a week, and there were 10 sudden cardiac deaths in that group — enough to reach significance, but a small number on which to hang a big headline.
• It's a specific population and a specific sauna. The main mortality study included only middle-aged Finnish men, bathing in traditional Finnish saunas. Results may not translate directly to women, other ages, other populations or other forms of heat such as infrared cabins.

None of this makes the findings worthless — a long follow-up, a population-based sample and a consistent dose-response pattern across several outcomes are exactly what makes epidemiologists pay attention. It simply means the right takeaway is promising and consistent, not proven.

So what should you do with this?

If you already enjoy the sauna, the Kuopio data is a reassuring reason to keep the habit. If you're considering one, it's a reasonable argument that regular heat bathing belongs alongside the other things that tend to travel with good cardiovascular health — moving every day, sleeping well and not smoking — rather than as a substitute for them.

And if you have a heart condition, low or high blood pressure, are pregnant, or take medication that affects your heart or blood pressure, talk to your doctor before starting. Stay hydrated, skip alcohol before and during a session, and step out whenever you feel unwell.

The most striking thing about KIHD may be how ordinary its subject is. In Finland, the sauna is not a wellness trend — it's a weekday. Read more about how it got that way in our history of the Finnish sauna, or explore outdoor barrel saunas and cabin saunas built in that tradition.