The most dangerous condition for exercisers, Martin says, is humid heat. The evaporation of sweat helps to cool the body, but humidity limits the amount of evaporation, he explains.
If you're exercising, that can be critical. Most of the perspiration we produce comes from blood plasma. So excessive perspiration can decrease the volume of blood just when it is needed in the muscles that are working.
In times of heat stress, the body also sends blood to the skin surface to cool the body by radiant heat loss. "But at the same time, you want that same amount of blood to go into your working muscles, so that you can provide nutrition and oxygen for metabolism. And if you have to share a smaller and smaller amount of blood, because of sweating, with these two major tissue masses -- the skin for cooling, but the muscles for working -- something has to give," Martin says.
This could hurt the athlete's performance, or it could lead to heat exhaustion or even heat stress. A person racing in the heat, for instance, can lose a liter to a liter and a half of sweat an hour. But the body can absorb about a liter of water an hour, so adaptations are necessary for those who must compete or work in the heat.
"One of the training adaptations is that training in the heat stimulates storage of more fluid, so your fluid volume increases so you have more available," Martin tells WebMD. But that kind of training needs to progress gradually. Martin says it usually takes about a month for an exerciser to "acclimatize" to the heat.
Most sports drinks do work, according to Martin, as long as they aren't too high in carbohydrates, "because this can actually lessen fluid absorption." He says exercisers should stay away from beverages that contain alcohol or caffeine because they can be dehydrating.
So if preventing and rectifying heat exhaustion is as simple as drinking enough fluids, why do overheated people keep showing up in emergency rooms?
Maintaining healthy hydration (and the all-important healthy diet) doesn't always go according to plan, Martin says. "It sounds so simple; it's amazing that it's so difficult to do."