I'm a bit late to this heat discussion but it's my opinion that acclamation is an important factor in tolerating heat. A while back when I was still refereeing soccer games for the state of CT I was working in a steel rolling mill where the temperatures in the summertime were quite high. The week they laid me off I was scheduled to do 2 soccer games in a tournament that turned out to be on a very hot day. We had a number of professional referees to do the games as mids and assistants. These were short games, 25 min halves with a 5 min halftime and 5 min between games. We cautioned the coaches to watch their players closely and to sub them out for fluids often.

To make a long story short, the other referees were failing in the heat and I ended up doing 5 consecutive games as the referee while they served as assistants. I was enjoying the day, running around constantly. The only thing out of the ordinary was chugging down a bottle of power aid (provided by the tournament) every 30 min. It was so bad that I was sloshing for the first few minutes of each new half. At the end of the day I rubbed my hand across my forehead and cut myself with the dried on salt crystals.

But, because of the work in the mill, the upper 90 heat outside during the games seemed like no big deal to me. Cold on the other hand I can't stand.