Each claim gets one of four grades. Strong means good controlled trials or large long-term studies backed by trials agree. Moderate means large long-term studies or small trials point the same way but cause and effect is unproven. Weak means small, short or mixed studies. None means no sound human evidence.
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What do the four grades mean?
| Grade | What it means | What would move it up |
|---|---|---|
| Strong | Several good-quality randomised trials, or large long-term studies whose results are backed up by trials measuring the same thing directly. | Already the top grade. It can move down if new, better studies disagree. |
| Moderate | Large long-term studies point the same way, or several small controlled trials agree, but we can’t be sure the sauna caused the difference. | Randomised trials long enough to measure the outcome itself, or results repeated in another country. |
| Weak | Small or short studies, surveys of sauna users, or results that are mixed. | Larger trials with a proper comparison group. |
| None | No sound human study supports the claim, or the studies that exist found no effect. | Any good-quality positive trial. |
Why can’t sauna studies prove cause and effect?
You can’t hide from someone that they are sitting in a hot room, so sauna trials can’t be blinded the way drug trials are. And the biggest studies are cohort studies: they ask people how often they use a sauna and follow what happens to them. People who take frequent saunas may also be healthier, wealthier or more active in ways a study can’t fully measure. Researchers adjust for these factors, but adjustment is never perfect.
So when we say regular sauna use is “linked with” a lower risk, we mean exactly that. We give the extra weight to findings that short controlled trials can check directly, such as the fall in blood pressure after a session.
What else do we check in every study?
- Who took part. Many studies only included middle-aged Finnish men.
- What kind of heat. A Finnish sauna at 80°C, an infrared cabin at 50°C and a hot bath are not the same exposure.
- How big the effect was, not just whether it was “significant”.
- Who paid. We note when a study was funded by a company selling the product.
First published 4 November 2026. Spotted an error? Tell us and we'll correct it. Read our editorial policy.