NYTimes

July 29, 2009, 10:30 am

How to Avoid Heatstroke

Patrik Giardino/Getty Images

A few years ago, two triathlons in Melbourne, Australia, held two months apart provided a fine if unconventional laboratory for studying heat illness. During the first event, early in the Australian summer, fifteen competitors were treated for heat illness. Three of them were diagnosed with life-threatening heat stroke. In the second similar triathlon held on another very hot, muggy day, no racers succumbed.

Scientists don’t yet know why some people become seriously ill while exercising in hot weather, and others don’t. At one time, it was believed that exercise-related heat illness was caused by the sizzling rays of the sun beating onto an athlete’s skin, causing overheating from the outside in, and contributing to dehydration, which was thought to be a primary cause of heat problems. But that theory doesn’t explain why athletes develop heat illness on overcast days, when sunlight isn’t directly reaching them. They’ve also been known to become ill on relatively cool days, when temperatures are below 80 degrees. And many collapse despite being fully hydrated. The triathletes who fell ill in the first Melbourne race weren’t desiccated, as a group of scientists studying the two events pointed out in a paper published in 2007. They’d sickened before dehydration set in.

“There’s still a lot we don’t understand” about heat illness, says Douglas Casa, an associate professor in the Department of Kinesiology at the University of Connecticut, who’s extensively studied athletic performance in the heat.

Heat illness itself is an omnibus term, covering “a spectrum of seven or eight different conditions,” that range from mild heat exhaustion to life-threatening heat stroke, Casa says. August is prime season for the illnesses, since, even as temperatures soar, many athletes must or choose to continue exercising outside. “Football teams start two-a-day practices at this time of year,” Casa says. Runners training for a fall marathon increase their mileage. So do cyclists and triathletes preparing for autumn events.

Scientists have a pretty clear picture of what happens inside these athletes as they exert themselves. They bake. Muscles in motion generate enormous amounts of energy, only about 25 percent of which is used in contractions. The other 75 percent or so becomes body heat.

According to a 2007 position paper from the American College of Sports Medicine about heat illness, exercising can raise core body temperature by almost 2 degrees every five minutes, “if no heat is removed from the body.” Meanwhile, sunlight and high air temperatures do contribute to the problem, although not to the extent once believed, by increasing skin temperatures. Humidity also plays a villainous role, slowing or preventing the evaporation of sweat, one of the human body’s main mechanisms for removing heat, Casa says.

The biggest issue in heat illness, then, is not the outside temperature, but the internal one. If a person’s core body temperature rises to about 105 degrees Fahrenheit, a critical threshold, the consequences can be dire. The body overheats and puts strain on the heart, pumping less blood to vital organs and bringing less of the rising core body heat to the skin’s surface. Blood pressure is affected. Symptoms such as dizziness and disorientation are common. At a cellular level, fluid volume and membrane permeability are thrown out of whack. Cells begin to die. These are hallmarks of acute, exertional heat stroke, which can be fatal. (People develop heat stroke without exercising, of course, but that seems to be, physiologically, a different condition. Afflicted exercisers are sweaty and pale; the inactive are usually flushed and their skin is dry.)

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Why someone has more difficulty dissipating body heat on one hot afternoon’s run than on another is still mysterious. But researchers do have a growing knowledge of some of the factors that predispose people to heat illness. Being out of shape or overweight, drinking alcohol before exercising, using certain supplements that contain ephedrine or ma huang, having a fever, or wearing too much, heavy clothing contribute. “I’m all in favor of naked practice sessions,” Casa says.” Unfortunately, sunburn also is thought to have an impact on your abilitity to dissipate heat.

The primary factor predisposing people — especially those in shape — to heat illness, though, seems to be lack of acclimatization to the heat. “It’s much harder for the body to cope with heat if it’s not used to it,” Casa says.

Since much of the United States had a relatively cool start to summer this year, many of us remain under-acclimatized to what are generally the scorching temperatures of August. It requires at least five days and perhaps as much as three months of regular exposure to heat to fully acclimatize. So be wary during workouts this month, Casa says. “If you feel at all off — dizzy or unusually fatigued or just, in any way, strange — ease back,” he says. If you’re running, slow to a jog. If you’re jogging, walk. Seek out shade.

Gulp the entire contents of your water bottle, too. “You don’t have to be dehydrated to develop heat stroke,” Casa says. “That’s a myth. But that doesn’t mean we want people to skip drinking. Hydration can be protective” against heat illnesses, for most people.

Lacing your water bottle with plenty of ice cubes may help, too. In a study released last September, cyclists who drank a beverage cooled to about 39 degrees could pedal much longer in a hot, humid laboratory than cyclists whose drink was warmed to about 98 degrees.

If, despite these precautions, you or one of your workout partners becomes dangerously hot and confused, get yourself or them into a tub filled with ice or ice-cold water as quickly as possible. That’s the best treatment shown to reduce core body temperature quickly enough to avoid severe internal damage or death.

But, in general, if you enjoy exercising outside at this time of year, continue, if cautiously. The vast majority of people adjust to heat rapidly and well, Casa says, a conclusion underscored by the researchers studying the Melbourne triathlons. By the time the second race was mounted, the researchers noted, after two extra months of hot-temperature training, the racers seemed to be ready for the heat. That race was held, by the way, in the Australian calendar’s equivalent of August.