The Finnish sauna cohorts, and what they actually found

Almost every confident claim about sauna and health traces back to one Finnish cohort. It is worth reading what it measured, because the claim that travels is usually stronger than the finding.

The 2015 study

The Kuopio Ischaemic Heart Disease cohort: 2,315 middle-aged Finnish men, aged 42 to 60 at enrolment, followed for a median of 20.7 years.

At baseline they were asked about their sauna habits. Those answers put them into groups, and the groups were compared on later outcomes.

Session length showed the same pattern: sessions over 19 minutes went with lower risk of sudden cardiac death than sessions under 11.

The typical participant went 2.1 times a week for 14.2 minutes — about 30 minutes a week, which is a good deal less than the internet’s impression of Finnish sauna use.

The 2018 study, which added women

A later sample of 1,688 men and women with a median of 30 minutes a week found more sauna time associated with lower cardiovascular mortality. More than 45 minutes a week against 15 or fewer gave a hazard ratio of 0.57 for cardiovascular death.

A third study from the same group looked at blood pressure: four to seven sessions a week was associated with about half the risk of developing hypertension compared with once a week.

What a hazard ratio of 0.37 means, and does not

It means that over the follow-up period, in that cohort, the rate of sudden cardiac death among four-to-seven-times-a-week users was 37% of the rate among once-a-week users, after the adjustments the authors made.

It does not mean your risk falls by 63% if you start going four times a week. Three things stand between those two statements.

It is observational. Nobody was assigned to a sauna group. People chose their own habits, and people who use a sauna four times a week differ from people who use one once a week in ways beyond the sauna — income, leisure time, social connection, general health, and whether they were well enough to go. The authors adjusted for what they measured; nothing adjusts for what nobody measured.

Habits were reported once, at the start. Twenty years of outcomes are attributed to a single self-reported answer from 1984-ish. People change.

Reverse causation is plausible. A man developing early cardiovascular disease is less likely to sit in a 79 °C room several times a week. Some of the gap between the groups is illness reducing sauna use rather than sauna use reducing illness.

None of that makes the finding uninteresting. It makes it a strong association in a well-conducted cohort, which is a real and useful thing, and a different thing from a demonstrated effect.

The generalisation the authors warn against

The cohort used traditional Finnish saunas at about 79 °C. The authors explicitly caution against applying the results to steam rooms, infrared cabins or cooler saunas.

That warning is routinely ignored. The proposed mechanism runs through the cardiovascular load of the heat: heart rate and blood pressure in a hot sauna match those of submaximal cycling, which is the reason anyone thinks repeated exposure might do something. A cooler room is a smaller stimulus, and nobody has measured how much of the association survives the change.

How Contrast uses it

The app counts your last four weeks. A sauna session is a day with at least five minutes of sauna in total; minutes per session is your sauna time on those days divided by the number of days. It then reports which of the study’s groups you fall into, on both axes.

That is all it does. It does not multiply your group by a hazard ratio, it does not project a risk, and it does not tell you to go more often. The metric page carries the same caveats as this post, in shorter form.

Hot baths are handled separately, against the Japanese tub-bathing cohort, because the Finnish results do not transfer to a bath.

The safety side of the same literature

The most cited review of sauna risk concludes that sauna is well tolerated by most healthy adults. It names unstable angina, recent myocardial infarction and severe aortic stenosis as reasons to avoid it, and notes that alcohol raises the risk of low blood pressure and arrhythmia.

That combination is the honest summary of this entire body of work: a stimulus with a real cardiovascular load, associated with better long-term outcomes in people who tolerate it, and genuinely contraindicated in some of the people who most want a cardiovascular benefit.