Our body's principle means of cooling is via sweat. When sweating stops being effective, it's basically impossible to cool down, and you're going to die of heat stroke if you don't go somewhere colder and/or less humid. Sweating breaks down at a wet-bulb temperature of ~100°F, and hitting even dry-bulb temperatures in the 150°F range is going to start to be lethal.

It depends on the humidity. The maximum wet bulb temperature that healthy humans can endure long term is about 35 °C.

https://en.wikipedia.org/wiki/Wet-bulb_temperature#Wet-bulb_...