I've always assumed there is some sort of "acclimation" period, maybe even related to the conditions you grew up in. I much would rather spend a time outside in -40c (with proper outerwear) than 40c. I'm relatively healthy but I feel like my body shuts down at anything above 36c

My understanding is that full heat acclimatization requires elevated core body temperature for 30-60 minutes a day for several weeks to really kick in and it’s easily lost during winter. Someone going largely from air conditioned to air conditioned building during the summer months may not even build up much of a passive acclimatization.

I don’t do as well in heat so for the summer hiking months in California so I have to do a two week sauna or hot bath protocol, otherwise I struggle really badly during the season. YMMV of course.

Same. There definitely seem to be strong genetic factors (just based on my personal experience TBF). I also notice I adapt substantially after two to three weeks of consistent exposure. But it does have to be consistent - hiding out with AC 24/7 prevents it.

That’s because you pay attention to your body and you’re not in denial. Above 37 degrees, your body cannot regulate itself, and it starts being seriously uncomfortable before that. Going outside for a run when it’s 40 is unbelievably stupid.