The first study shows a 0.37 hazard ratio for frequent sauna use. That’s better than a lot of the HRs reported for physical activity. It’s even better than not smoking. It just doesn’t really fit.

Also, the fact that there are practically no sauna related studies outside of Nordic populations is suspicious too. I bet that with those hazards ratios a lot of people have tried to study the effects more, it’s suspicious there’s practically nothing out there.