Regardless of the type of heat you're working out in, exercise is going to produce internal heat. "When your body starts to get warmer than is optimal for daily living, it needs to shed some of that heat," says Alex Harrison, PhD, sports physiologist and sport performance coach for Renaissance Periodization. The primary way to do this? Sweating, which provides a cooling effect. "That cooling effect is transferred deep into your core, because your blood is sent to your skin to a greater degree when you're hot," he says. "That increased skin blood flow transfers the cooler blood back into your internal organs and muscles to keep you cool enough to keep going."
One downside of working out in heat—whether it's humid or dry—is that more of your blood is sent to your skin, rather than the muscles that you're using to work out. "This means that you'll have a higher heart rate and probably perceived effort than if more of your blood was doing the work you wanted—going between your heart and your muscles—and not your heart and skin," says Dr. Harrison. The heat also causes you to sweat more, no matter the level of intensity. "So you lose more water per hour the warmer it gets," he says. "One of the consequences of heavy sweating and the dehydration that is bound to result is impaired absorption of gut contents, including fluid and electrolytes, which isn't good when you need it most."
Besides that, high temperatures can raise both your resting heart rate and shorten the time it takes for your heart rate to spike and stay there. "In the same way, higher temperatures cause a longer recovery time for your heart rate to return to normal," says Bethany Stillwaggon, fitness trainer and master coach for Row House. The biggest consequence you could face in this scenario? "With enough water and electrolyte loss, someone could start feeling the effects of complications associated with water loss like headaches or dizziness, muscle cramping, easy fatigue, and muscle weakness," she says. Or, in a worst-case scenario, you could develop heat exhaustion or heat stroke.
On top of these effects from the heat on the body, there are some differences that exist between exercising in dry heat vs. humid heat, specifically. Keep scrolling to learn more about each, plus how to keep yourself safe in each scenario.
Exercising in dry heat
The perk of dry heat is that it feels less hot than humid heat does, which makes it less taxing on the body. "This is from a purely physiological standpoint, because the evaporative effect of sweating works well to keep you cooler than if it were humid," says Dr. Harrison.
That said, the danger with this is that you may not realize how dehydrated you are. "It can be misleading because it may appear like you're not sweating all that much because it's evaporating so quickly off your skin, so you might not notice how much fluid you're losing," he says. The key is to pay special attention to your thirst signals, and be sure to have water and electrolytes on hand to avoid dehydration.
Exercising in humidity
When humidity is factored into your outdoor workout, there are more risks that you face. "Put simply, humidity compounds all of the cooling issues and decreased performance from hot temperatures during exercise," says Dr. Harrison. First of all, high humidity means that sweat stays on your skin longer and evaporates less quickly, so you don't get a relief of a cooling effect on your skin. "In cases of very warm and very humid environments, like 95 degrees and over 90 percent humidity, sweating may have almost no measurable cooling effect," he says. This makes your body increase sweat production even more because you are feeling increasingly hot.
High humidity can also make it harder to breathe. "Breathing in humid air tightens airways and nerves in the lungs, which can impact your oxygen turnover rate," says Stillwaggon. On top of exercising, during which your body needs more oxygen than when you're resting, humidity can make breathing feel strenuous or challenging to anyone without underlying breathing issues, let alone someone who does, she says.
How to stay safe
When you're working out in the heat—humid or dry—hydration is crucial. "Start drinking an electrolyte solution before exercising and immediately upon starting exercising," says Dr. Harrison, who says that if you wait 30 minutes or longer to start hydrating, you'll have a harder time absorbing the fluid and electrolytes that you need. "Target 800 to 1200 milliliters per hour, with 700 to 1500 milligrams of sodium per liter," he says. Also, drink fluids periodically during your training—Jason Machowsky, RD, a dietician and exercise physiologist at HSS, recommends four to seven ounces every 15 to 20 minutes.
To help combat the effects of sweat, Stillwaggon suggests using a towel or cooling cloth to stay cool and wear breathable clothing. As you're working out, be aware of the warning signs of heat illness. "Some signs and symptoms of dehydration, which can progress to heat illness, include impaired aerobic recovery, unusual fatigue, facing pulse even during a break, loss of coordination, hyperventilation, large cramping, wet, pale skin, chills, nausea or vomiting, and lightheadedness," says Machowsky.
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