He's also seen cases at the opposite end of the spectrum: He tested one man on a treadmill who lost 4.1 liters of sweat in an hour, in 72 degree heat. Bergeron advised him to steer clear of any races longer than 10K.
One thing humans share when it comes to temperature regulation: We're horribly inefficient at it. When we exercise, 20 percent of our energy goes into moving our muscles. The remaining 80 percent gets released as heat. Since that percentage remains the same regardless of the temperature, the environment helps determine how our bodies get rid of the heat. If it's cool and breezy, for example, sweat will evaporate quickly and keep body temperature in control. But if you're in a hot, humid environment with no breeze, a vapor barrier can prevent sweat from evaporating.
And body temperature isn't the only physiological effect of exercising in the heat: Your body undergoes cardiovascular, metabolic, central nervous system and behavioral changes as well. In fact, the cardiovascular changes may be the most significant, said Mike Sawka, who researched heat stress physiology at the U.S. Army Research Institute of Environmental Medicine and now teaches at Georgia Tech. When the skin dilates in the heat, he says, you've got the same volume of blood, but it needs to move to a bigger space -- so the heart works harder.