When I lived in Indiana with the humidity, above 95 I'd start finding other things to do.
When I lived in dry west Texas, that went up to 110 or so, but at some point it seems like hot is just hot. I did a 10k TT at 115, and that was just too damned hot.
I guess I'd say both these were above my desirable limit, so I moved to Portland, OR. Where, by the way, it's 99 this week