Living in hot and humid Florida, I've had my brushes with heat exhaustion. Hydration is the key. I read an article on a theory of sweating. The traditional theory is the evaporation of sweat on the skin cools the body. But that theory breaks down when you are at 98% humidity at 90+ degrees F, and the skin is soaked in sweat and and there is little or no evaporation occurring.

This article said the body is actually moving heat out by throwing off the sweat. So theoretically, if you can keep enough liquid going in, the body can keep pumping out the heat in the sweat. The problem is, your body can sweat faster than you can absorb the liquid, even if you're drinking as much as your stomach can hold.