Outside magazine, August 1996
Exercising in scorching heat isn't just uncomfortable--it can be downright dangerous. When the air is warmer than your skin--around 95 degrees--your body's climate control mechanisms go postal. Convection and radiation, your otherwise stalwart protectants against roasting alive, become the enemy: Working in reverse, convection causes your body to absorb heat rather than expelling it into the air, and radiation, the flow of heat waves from your skin, has nowhere to go. You may still have evaporation on your side, but run amok, convection and radiation can raise your body temperature and possibly bring on heat exhaustion--or even life-threatening heatstroke. Lawrence Armstrong, a University of Connecticut professor of environmental and exercise physiology, says a red-alert day is "90 degrees with 90 percent humidity." He offers these tips for keeping such fair-weather factors in check and surviving the heat of summer with your health and fitness intact.
Timing is everything. Early mornings, as close to sunrise as possible, are cooler than late evenings and keep convection on your side. Avoid midday--especially 1 to 3 p.m., when temperatures and skin-damaging UV rays are most dangerous.
Hydration, hydration, hydration. After convection and radiation have turned on you, evaporation of sweat remains the primary line of defense against heat. But you have to be hydrated; Armstrong recommends filling up on water, starting four hours beforehand to avoid cramps, and drinking eight ounces of fluid every 20 minutes during your workout. Remember: If you're thirsty, you're probably already dehydrated. If you plan to be out longer than 50 minutes, swap water for an electrolyte-replenishing sports drink. Keep a pitcher of chilled water for post-workout rehydration--it can actually lower your body temperature by a degree or two.
Proper attire required. Choosing loose, light-colored, lightweight clothing keeps convection working in your favor. Long sleeves can keep you cooler by blocking the sun, assuming you opt for moisture-wicking synthetic garments, which aid evaporation (cotton, even though it gets soggy, won't let sweat evaporate). If you wear short sleeves, don't towel off that layer of sweat: It cools you as it evaporates. And go easy on the shoelaces, as your feet can balloon up a half-size in the heat.
Enough is enough. Even with these precautions, there's little defense for a 95-plus-degree day when humidity is above 75 percent, since the air is so laden with vapor that evaporation grinds to a halt. Such are the times when it's OK to hit the air conditioned gym.