My wife used to do hot yoga and I was in the yoga studio once. Dear lord, it was like hell on earth. I know that they struggle to get the right environment. They have a couple of humidifiers cranking and if you get close to the floor to ceiling windows in there you'll be sweating on one side and freezing on the other. O.O
To be clear, these outfits do not keep the studio this way 24/7. They just do it for classes, so the moisture dissipates pretty quickly when they turn the humidifers off and the heat down, provided the system fan is still running.
I did the Polar Bear dip a couple of times with some friends when I was 18. Had a outdoor Sauna at Lake Mantario and cut a hole in the ice in the lake and would run out and jump in and run back a couple more times. Not sure my heart would handle it now but it was fun and swimsuit optional.
"Cut it twice and it is still too short".