There are many variables that determine the max temp..... duration of exposure, wind, humidity, physical condition/makeup and age of subject etc.
One perfectly healthy 30yo male may die if contained in a room at 40�c, another one may be able to stand 50�c.
I remember seeing a show on TV many years ago where someone was contained in a cubicle for 1 minute and could stand a tremendous temperature, not sure of the actual temp anymore though. This person had 'trained' for this feat.