If it's 90 degrees Celcius, I don't think you could sit there for an hour. If it's 90 american degrees, what kind of sauna is that?
Perhaps we are missing some details with respect to humidity?
If it's a very humid sauna at 90°F, then your body can't regulate itself with sweat evaporation.
weird huh, in neither system does 90 degrees make for a useful sauna. that's either lukewarm, or cooking temp.
I have a dodgy thermometer on the wall, and I tend to enter the sauna when it shows 80, by the time I leave it's usually been showing 90 for half the time. Celcius.