Vigorous exercise such as running, bicycling or even hiking uphill in intense heat can lead to heat stroke and possibly a heart attack with few warning signs, researchers say.
The danger becomes even greater when the air contains high levels of carbon monoxide and ozone, both of which put additional stress on the heart and lungs, one of the researchers said in an interview.
The researchers were interviewed after the recent disclosure that two men had died after competing in races in New York’s Central Park during this summer’s intense heat wave.
The two men reportedly were not runners and were not accustomed to exercising in the heat, which meant that they faced a special risk of injury or death.
“I don’t think it’s generally recognized that even in these short races, when you’re exercising very intensely and you’re not trained or acclimated, you can very quickly produce and store enough heat to kill you,” said Edward Coyle, director of the Human Performance Laboratory at the University of Texas in Austin.
“They were untrained, unacclimated and they were running very intensely for their level of ability,” he said.
In New York and many other parts of the country, the intense heat has often been accompanied by unusually high pollution and ozone levels.
Carbon monoxide can trigger a heart attack in exercisers who unknowingly have latent coronary artery disease, said Peter Raven, an exercise physiologist at the Texas College of Osteopathic Medicine in Fort Worth.
Ozone irritates the lungs, causing the tiny airways in the lungs to shrink and decreasing the amount of breath taken in with each inhalation, Raven said.
“We’ve had elite cyclists complain at very low levels of ozone that they couldn’t perform anywhere near where they’re used to,” he said.
Exercisers should be concerned whenever the ozone level is reported to be higher than one-tenth of one part per million, Raven said.
According to the American College of Sports Medicine, the amount of heat produced in muscles that are exercising hard is 15 to 20 times that produced in muscles at rest.
Even short races can raise body temperature from its normal 98.6 degrees to 104 degrees, which is the point where serious injury can occur, the college says.
Hiking Can Be Risky
This problem is not limited to runners, but can occur with any type of exercise that involves expending a lot of energy, including brisk walking and hiking, especially uphill, Coyle said.
Coyle, an expert on fluid loss and replacement during exercise, said that dehydration is one of the prime causes of heat stroke.
Unfortunately, dehydration is a difficult thing for an exerciser to be aware of, Coyle said. “It’s difficult to tell that you are in danger because you don’t often get sufficient warning time, or you don’t get warnings early enough.”
Exercisers who are seriously dehydrated will feel chilled and stop sweating, Coyle said. Their skin turns white and they will feel tingly sensations--the hair on the back of the neck stands up.
“At that point you know you’re in trouble. Stop exercising and seek some help to try to cool yourself down. Usually at that point you’re not thinking very clearly, so it’s important to try to get some help,” Coyle said.
Headache a Warning Sign
Raven noted that many exercisers will develop a headache, especially around the temples, as the first sign of impending heat stroke.
“People will have it but they tend to ignore it,” he said. “And that’s the danger signal.”
Researchers had these tips for people who want to exercise on warm days:
- Drink at least a quart of water per hour of exercise. Avoid drinks high in sugar and other carbohydrates because they stay in the stomach too long.
- Take a week to gradually increase your exercise program so that you become acclimated to the heat and pollution. The body adapts to cool itself faster. The more acclimated you are, however, the more water you need.
- Exercise before 8 a.m. or after 7 p.m.
- Watch children carefully, because they can become overheated more easily than adults.