# How we grade the evidence on saunas and cold water

Source: https://saunabenefits.uk/about/evidence-grades/
Author: Kit Ellery · Updated 2026-11-04

**Short answer:** 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.

## 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.