Exercise places stress on your body. So does heat – and the two can be a bad combination, especially if you’re working out in a hot, humid environment. Humidity makes it harder for your body to release heat through sweating. Fortunately, your body acclimates to exercising in hot weather over several weeks, but when hot weather first strikes, your risk for heat-related illness is higher. Here are three heat-related illnesses you can experience when exercising in the heat and ways to avoid them.
Muscle cramps are no laughing matter. They usually come on suddenly, especially if you’re not adequately replacing fluids and electrolytes you’re losing through sweat and force you to stop exercising. You’re more likely to experience them when you’re exercising for long periods of time, say a long-distance run rather than a 20-minute boot camp workout. They’re also more common in people who aren’t well-conditioned.
Dehydration, loss of electrolytes and muscle fatigue play a role in heat cramps, but the most compelling theory is that heat cramps are due to altered neuromuscular control. Your muscles receive excitatory signals that tell them to contract and inhibitory ones that send the signal to relax. Exercising in the heat seems to upset this balance, causing muscles to cramp and spasm.
Muscle cramps hurt, but heat exhaustion is a more serious problem from a health standpoint. It’s most likely to occur in people who aren’t acclimated to exercising in the heat and usually happens the first day or two of exercising in a hot or humid environment. Heat exhaustion is usually triggered by fluid loss from inadequate fluid replacement.
As fluid is lost through sweating, plasma volume, the amount of plasma in the blood vessels, drops. This reduces blood volume to the heart, causing an increased pulse rate, a drop in blood pressure and feelings of weakness. The drop in plasma volume makes it harder to eliminate heat, so body temperature may rise, but usually not too dangerous levels.
Other symptoms of heat exhaustion include nausea, dizziness, weakness, headache and moist, cool skin. Heat exhaustion can usually be reversed with fluid replacement and rest in a cool environment.
Heat exhaustion is typically not life-threatening, but heat stroke can be fatal. Heat stroke occurs when your body produces more heat than it can eliminate through sweating. Unlike heat exhaustion where body temperature isn’t extremely high, people with heat stroke can have temperatures of 104 degrees Fahrenheit or greater. People in the throes of heat stroke may be disoriented, have a rapid heart and breathing rate, experience nausea, feel dizzy or pass out. Unlike heat exhaustion where skin is usually moist and clammy, people with heat stroke have hot, dry skin since they can’t dissipate heat.
People with heat stroke need emergency treatment to bring down their core body temperature using ice packs immersion in cold water. It’s a “911” condition, and not one to take lightly. Like heat exhaustion, heat stroke comes from inadequate fluid and electrolyte replacement and is more common in people who haven’t adapted to exercising in hot weather. Children and older people are also more susceptible since they can’t regulate temperature as well.
Preventing Heat-Related Illnesses
The key to preventing heat-related illnesses when exercising in the heat is to hydrate well before, during and after exercise. Here are some other tips:
If you’ll be exercising for an hour or more, drink an electrolyte-rich drink instead of water.
If you plan on exercising in a hot environment and you haven’t acclimated, reduce the intensity and length of your workout for the first 10 days to give your body a chance to adapt.
Increase your salt and water intake a day or two before working out in a hot environment.
Wear light, cotton clothing that “breathes,” so sweat can evaporate more easily.
Stop exercising if you feel weak or lightheaded, and head to a shady spot to rest and sip fluids.
If the humidity level is high along with the temperature, find an indoor spot to exercise. Heat and humidity are a tough combination.
The Bottom Line
Heat and humidity add additional stress to your exercise routine. Take precautions to prevent these three common forms of heat-related illness. Most importantly, stay hydrated, replace electrolytes and know when it’s time to cool down and rest.
Br J Sports Med. 2009 Jun;43(6):401-8. Epub 2008 Nov 3. Exercise Physiology. Fifth edition. McArdle, Katch and Katch. 2001.