I had a punt on sunday afternoon, temperature must have been 30 degrees & it was an incall in a budget hotel with no air con. Fair play to the lady in question, she was wearing the stockings & sussies I asked for & it didn't seem too warm to begin with but after blowing my 1st load I suddenly noticed how hot it really was. Tried to pop a 2nd time but the heat was too much & couldn't manage it.
Lesson learned, I won't punt again in those temperatures unless the location definitely has air con. Like smiths said a fan on it's own just ain't enough.