Exercising outside when it’s very hot can be risky business, since it’s harder for the body to cool itself. And it’s not just the heat but also the humidity that takes its toll. Our bodies cool primarily when sweat evaporates off the skin. But as humidity rises, this doesn’t happen as efficiently, potentially causing core temperature to rise, which in turn increases the risk of heat-related ailments such as heat exhaustion and heat stroke.
Here are some tips for exercising in the heat. They are based on research done mostly with competitive endurance athletes and for military training operations, but should apply to average exercisers as well. The first three may help lower your core body temperature before you work out (called precooling), so you can exercise longer and better in the heat. They can also be done during exercise to blunt the rate of heat gain and after exercise to help you cool down.
- Drink cold or icy beverages before exercising to precool internally. Though beverages around 40°F are generally recommended, consuming an icy drink (a slurry) has more of a cooling effect than cold water alone, according to a review paper in BMC Medicine in 2012. Cold beverages (though not too cold) also tend to be more palatable, so you’re likely to drink more—which helps replace sweat losses. But be aware that large amounts of icy fluids can cause abdominal discomfort while exercising, and rapid consumption of them can lead to “brain freeze” headaches—so find a temperature that works for you. To make an ice slurry, mix 70 percent crushed ice with 30 percent water. If you want to sip very cold fluids during your workout, freeze bottled water or a sports drink ahead of time to take with you.
- To precool externally, apply icy cold towels to your face, neck, arms, thighs, and other body parts for 5 to 20 minutes before exercising. Several studies have found this to be effective, including a small 2011 study in Medicine & Science in Sports & Exercise, in which runners applied icy towels to their head and neck before running and for 5 minutes during a break (with even better results seen when other parts of the body were also cooled). To do this, dunk a small towel in very cold water or in an ice slurry and wring it out; cool a damp towel using ice cubes or crushed ice; or place a damp towel in the freezer for a few minutes. You can also buy reusable “cooling towels” and “ice bandanas,” or use a flexible frozen gel pack. Don’t count on costly cooling garments, however—studies show them to be of limited value.
- Immerse your hands and forearms in cool or cold water as another way to precool externally. This has been found to be more effective than pre-cooling with beverages, but is also less practical. As noted in a 2015 paper in the Scandinavian Journal of Medicine & Science in Sports, which looked at the effects of pre-cooling on 30-minute runs, cold-water immersion can reduce heat stress. And a 2013 paper in the Journal of Thermal Biology, which reviewed the research on extremity cooling in military training operations, noted that immersing hands in cold water can lower core temperature and might reduce heat-related cardiovascular strain by lowering heart rate. But overdoing it or using water that is too cold may cause blood vessels in the hands to constrict, which could prevent heat dissipation. Use water that's 50° to 68°F and limit immersion to about 10 minutes. An easier (though also likely less effective) alternative: Carry a bottle of cold water in each hand.
During your workout
- Wear loose-fitting, lightweight clothes that provide good ventilation. And choose light colors, since dark colors absorb more heat. “Wickable” garments move sweat away from your skin and through the clothing so the moisture can evaporate quickly. But despite considerable research, there is still much controversy about what type of fabric is best, and even whether synthetics (such as polyester) advertised as “cooling” are actually better than natural fibers (such as cotton). According to a review of the literature in Sports Medicine in 2013, “synthetic fabrics seem to offer no thermal balance advantage over natural fabrics during exercise.” Keep in mind that what may matter most is how the clothing feels against your skin when you sweat. Fabrics that cling can make you feel clammier—and lighter and thinner materials are largely deemed more comfortable. “Next-generation” cooling fabrics are under development, including ones that let mid- and far-infrared thermal radiation pass through (the human body emits heat primarily in that part of the electromagnetic spectrum).
- Don a lightweight, light-colored hat or cap with a broad brim if you work out in direct sunlight. Though hats may make your head feel hotter and sweatier, they also shield you from radiant heat and skin-damaging effects of the sun. For extra cooling, dampen the hat with water or ice beforehand, or pour cold water over the hat while you work out. A 2010 study in the Textile Research Journal found that a hat made of polyester in a twill weave trapped the least heat and humidity in people exercising under hot weather conditions, probably because it wicked away sweat best. If you wear a helmet (such as for bicycling and inline skating), a light-colored one that has air vents (especially close to the front) and reflective material on top may help you stay cooler.
Other ways to keep your cool
How well you function when it’s hot out depends on many variables, including your individual physiology and whether (and how much) you have acclimated to exercising in the heat. If you are not used to hot-weather exercise, start off slowly over one to two weeks, gradually increasing the length of your workouts. Try to exercise during the coolest time of the day, seeking as much shade as possible and taking frequent breaks to cool down. On very hot and humid days (check the heat index), consider working out indoors in air conditioning or taking the day off—or swimming, if possible. And be sure to stay adequately hydrated by drinking enough fluids and not waiting to drink until you’re thirsty (urine should be light in color). Children, older people, and those who are overweight should take extra precautions because they are less heat tolerant in general and thus more susceptible to heat-related illness. Early warning signs may include muscle cramping, fatigue, nausea, headache, and heavy sweating. More serious signs such as dry red skin, confusion, and fainting could indicate heat stroke, which requires emergency treatment.
A word about sun protection: Lightweight, light-colored clothes may help keep you cooler than heavier, darker clothes, but they may not protect you as well from the sun. If you spend a lot of time exercising outside in the sun, it might be worth investing in specially made garments that carry UPF (ultraviolet protection factor) labels; some come in lightweight material. The Skin Cancer Foundation recommends garments with a UPF of at least 30. Also, be sure to apply sunscreen to exposed skin. Though some people may not like the feel of sunscreen when exercising, there is no convincing evidence that it impairs the body’s ability to regulate heat—and the skin protection benefits of sunscreen outweigh any discomfort it may cause.
Also see 9 Safe Exercise Strategies.