What really matters is the wet bulb temp. For example, the Death Valley high today was 114°F (45°C) but at an RH of only 3%. That gives a wet bulb temp of ~ 65°F which isn't a problem with acclimation and adequate hydration.
Now if it had been 50% RH, the web bulb temp would be > 96°F which is not survivable by humans for very long because no amount of sweating in that humidity will cool you down.
That's 114°F in the shade. There is no shade in the desert though ...
Don't want to be projecting, but first time I went to Death Valley happened to be in August. I saw dunes from the car and thought I had to walk up there. Fifteen minutes later I had an unbearable headache and quickly headed back. Sombreros don't look all that ridiculous to me anymore.
Now if it had been 50% RH, the web bulb temp would be > 96°F which is not survivable by humans for very long because no amount of sweating in that humidity will cool you down.