It really depends on humidity. Humid heat is typically worse and can be really draining both mentally and physically. Dry heat is much more tolerable for humans. As a person who's experienced both I can concur, the 100F humid heat was borderline horrific.

38C/100F is probably fine (relatively) in Arizona but in Florida it'll be pretty terrible. Like when I was in the south for a week it was 98F and the walls were sweating.