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.
I think the main factor would be the temperature at which the air would damage the membranes in your lungs. At that point you would no longer be able to breathe and would suffocate.
However, I don't know at what temperature the air would damage your skin to the point where you would die, the temperature may very well be lower and happen sooner!