On the serious end of the spectrum lies heat stroke, a problem that kills approximately 4000 people in the United States every year. There are two varieties of heat stroke. In classic heat stroke, the patient is usually elderly or sick, or both. Temperature and humidity have been high for several days, and the patient has dehydrated to the point where his or her heat loss mechanisms are overwhelmed. You might say they simply run out of sweat.
Skin gets hot, red, and dry.
They lapse into a coma and, if untreated, die. But more and more people are being killed of the second variety, exertional heat stroke. The victim is usually young, fit, and unaccustomed to heat, sweating but producing heat faster than it can be shed. Skin is hot and red, but wet with sweat. Rapid breathing and rapid heart rates are almost universal.