The humidity is the issue. Wet-bulb temperatures exceeding body temperature means sweating no longer cools you off.
That's why steam rooms peak at ~110 and saunas at ~150. A steam room at 110 is almost unbearably hot, it's hard to last more than 5 or 10 minutes in one.
Think about a human having a fever of 108—that's the temperature at which you'd start to have convulsions.
I wonder about that myself. I know my limits and can't stay in a sauna for too long, but I've heard that folks in Finland take it really far. But... reading up on the World Sauna Championships, it seems that even the most heat-adapted Finns tapped out after 16 minutes or so. That was in a ludicrous 110C with a steep humidity ramp, and they stopped running the competition after a contestant died and another one nearly died after about 6 minutes.
I've tried to look for better statistics about how much folks in Finland can actually tolerate, but all I found was this spectacular batshit: https://www.reddit.com/r/finlandConspiracy/comments/op97qm/t...
Re:Wet-Bulb Temperature
by alvarlagerlof on Wednesday March 02, 2022 @11:43AM
(#30529328)
The important thing is the moisture level. You need to be able to sweat, which you usually do in a sauna.
As someone who enjoys sitting in a sauna I wonder what physics are at play here. Heat stroke trying to work in >100* heat I get, but death even at rest?