>>211638427 (OP)physics. thermodynamics. physiology.
your body needs to stay below high fever temperatures or else you're literally COOKED. all the chemistry in your body relies on proteins. heat ruins them.
your body will produce heat for any work you do. if your body can't get rid of that heat, it'll get hotter.
you are evolved to get rid of heat by sweating. when sweat evaporates, it cools you down. if sweat can't evaporate, you can't cool down.
sweat only evaporates if the air isn't too humid.
the hotter the air, the drier it must be to allow the sweat to evaporate. cool air of any humidity will do, but hot air has to be dry, or else you're COOKED.