If exercising in hot weather leaves you confused, weak, and headachy, you're suffering heat stroke and need immediate help. And then there's the smog. ...

WebMD Feature

Reviewed By Michael Smith

In Phoenix, locals call it a "cold snap" when the thermometer drops below 105 degrees. After spending time at a construction site, Arizona builder Albert Ayala found his head swimming and noticed he had stopped sweating. "I felt terrible," he recalls. He went home and a couple of hours later, experienced fierce muscle cramps. "I thought I knew heat," he says, "but it got me."

"It can happen to anyone," says Andrea M. McCauley, director of communication for the American Red Cross in the Phoenix area. Although becoming acclimated gradually to heat helps (in Phoenix, they say if you can get through one summer, you won't notice the heat as much), acclimation is not the whole answer.

How Hot Is Hot?

"When should you worry about exercising in the heat?" asks William O. Roberts, MD, a sports medicine specialist with MinnHealth in White Bear Lake, Minn., (not in a desert, notice -- this can happen anywhere). "That's a moving target. A lot depends on the humidity. With no acclimation, 70 degrees with high humidity can be dangerous. Your sweat can't evaporate."

Heat, Roberts repeats, is not something you can instinctively gauge. "People often don't realize how hot and humid it is until they are already in trouble."

If the body cannot carry the heat given off by exercising muscles to the surface of the body fast enough -- and once it's there, if the surrounding air is not cooler or evaporating sweat does not cool the body-- one's innards literally stew, destroying and shutting down organ systems. It's not a matter of discipline or will, it's a matter of heat exchange -- physics and physiology, not physical endurance.

Symptoms of Trouble

The most common symptoms of heat illness are nausea, vomiting, headache, weakness, or an altered mental state (confused, raving, aggressive, rambling incoherently). Body temperature can spike up to 105 degrees or more (110 to 114 degrees is not unknown). If the sweating mechanism shuts down as it did in Albert's case, over time the body loses all hope of cooling itself and the brain and other organs begin to "cook." Heat stroke is the term for this latter condition and can result in death.

But it's not that simple. "I usually don't want to say heat exhaustion is a first stage of heat stroke or that it can go from one to the other," Roberts says. "They are two different things. You can get exercise exhaustion in the heat, but it's usually from the exercise not the heat."

Giving it a name is not that important. "You may suddenly get tired, sick, headachy, thirsty, or faint," sums up McCauley.

Hydration Not the Whole Answer

The most common piece of advice about exercising in the heat is drink, drink, drink -- water, not caffeine-loaded sodas or beer. Roberts says you can get heat exhaustion even if you are hydrated, though. He recommends determining your sweat rate instead and replacing that, without overdoing it.

To determine your sweat rate, weigh yourself nude before your workout, then towel down and weigh again. The difference in ounces is the fluid you lost. "Replace that, not twice that," Roberts says. "Too much fluid can be bad, too."

McCauley, on the other hand, recommends drinking a quart before running or exercising outside, and a quart after. "Drink even if you are not thirsty," she says.

What about salt tablets? "They got a bad rep because they were thought to contribute to high blood pressure," Roberts explains. However, he still recommends augmenting with salt. "You know when you first exercise and the sweat drips in your eyes and it stings?" he asks. "Well, after two or three weeks of exercising in the heat, your salt level will go down. So I recommend eating a few more salty foods, pretzels, potato chips, or salt your food."

Time and Place for Exercise

How can one exercise smarter?

Run only between 4 a.m. and 6 a.m., McCauley says. That's when traffic is lightest and air quality the best. Sucking in poison to overload your already taxed system is not a good idea. Pollution of over 0.15 parts per million usually warrants an advisory -- so be advised! Be especially careful in cities with the worst ozone pollution. For 2003, according to the American Lung Association, they are (in order) Los Angeles; Fresno, Calif.; Bakersfield, Calif.; Visalia-Tulare-Porterville, Calif.; Houston; Sacramento, Calif.; Merced, Calif.; Atlanta; Knoxville, Tenn.; and Charlotte, N.C.
Wear light-colored, loose, absorbent clothing (lose the Spandex or even cotton, which can stay wet).
Don't engage in strenuous workouts, even in a heated pool (you can get overheated and dehydrated in water.)
Carry a frozen water bottle in the back of your shorts (feels good, too).
Seek shaded pathways.
Exercise moderately -- 60% to 70% of maximum heart rate. Take breaks. Walk.
Drink a couple of cups of room temperature water before leaving and more when returning. In between, slug back a cup or two every 20 minutes.
When it's over 90 degrees, hit the gym instead.

What to Do If the Heat Gets You

Despite all your care, what if the heat sneaks up on you or a companion anyway?

You need to cool off fast! "I toss my athletes in ice water," says Roberts. Cool, wet cloths, sips of water, shade, and if the person is still fire-hot or raving and incoherent, call the paramedics. "The idea is to lower the temperature as quickly as possible to stop the cooking process," Roberts says. "Temperature vs. time."

The best thing, however, is to prevent trouble. "I still see people in dark clothes running along a roadway during evening rush hour," McCauley says with a sigh.

"You can't get some people to do anything smart," Roberts says.

Star Lawrence is a medical journalist based in the Phoenix area.

Originally published June 24, 2002.

Medically updated May 20, 2003.

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