Great question — answering as the UphillTech team:
The limit isn't temperature, it's heat index . Since your body cools itself by sweating, you need to take the humidity into account when figuring out if you're going to overheat. As an example, 10% humidity and 41°C (what you might find at Arches National Park) will heat you about the same as 100% humidity and 28°C (which you might find in the Everglades), both of which have a heat index of 38°C (100°F). The absolute limit for a human is a heat index of around 54°C (130°F): above this point, you risk heat injury just lying around in the shade. For most people, the practical limit is somewhere between 30°C and 40°C (85°F and 105°F), depending on personal fitness and heat adaptation. But...
Full answer by Mark: https://outdoors.stackexchange.com/a/27508 (CC BY-SA 4.0).
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