What about tanning (or tanning studios)? I know there's a big difference between a sauna and a tanning studio, but since we're talking about heating up the body, would it have the same effect? (...despite causing skin cancer)
A sauna does not heat up the body enough to trigger protein denaturation (unless you get locked in and die, which happens to be among my worst nightmares).
Core temperature is tightly regulated, as well. It should be 37degC. Plus or minus just 2 degrees will severely impair function, plus or minus 5 degrees is reason to call 911.
Saunas and ice baths don't damage cells but actually have benefits. Don't quote me as this is a rephrase that makes it sound like bro-science, it actually activate some heat shock proteins and also leads to hormone release.
Better, sound science can be sourced from Dr. Rhonda Patrick a la Tim Ferriss podcast or Google Scholar
Core temperature is tightly regulated, as well. It should be 37degC. Plus or minus just 2 degrees will severely impair function, plus or minus 5 degrees is reason to call 911.