Hanging out in the heat is pretty uncomfortable, but working out in the heat can be downright dangerous.

“In hot (and especially humid) environments, we dissipate heat by physiological cooling mechanisms, including sweating for heat loss via evaporation,” explains Elaine Choung-Hee Lee, Ph.D., associate director of Human Performance Laboratory at the University of Connecticut. “When you’re unable to dissipate enough heat to cool the body, that’s when you might experience heat exhaustion, heat-related illness, or, in extreme cases, heatstroke.”

Heat exhaustion will sideline you for a day or so, but heatstroke can be fatal—so it's not really something to take lightly. And whether you’re working out in the great outdoors or at your un-air-conditioned CrossFit box, you run the risk of overheating. Here’s how to ensure you don't run too hot—and how you can recover if you do.

1. Know the signs of heat exhaustion

Heat exhaustion is uncomfortable, but it’s also the first alarm bell to warn you it’s time to slow down and cool off. If you push past this, you enter the territory of heat illness, and then potentially the ultra-dangerous condition of heatstroke—so you need to know what those alarm bells look like.

The primary force behind heat exhaustion centers around your cardiovascular system—it has to shuttle blood not just to your muscles to support the physical work of exercise, but also to your skin to help keep you cool, explains Michael Sawka, Ph.D., professor of physiology at Georgia Tech.

When you heat up, your blood pressure is so high, your heart starts to race, you feel faint, and you feel exhausted before you normally would. “You can’t sense a high core temperature, but wet skin and a high heart rate are signs of thermal discomfort,” he adds.

And here’s the slightly alarming reality: Heat exhaustion is a possibility any time your skin temperature exceeds 81°, which, for all intents and purposes, is whenever the air is 81° or higher, according to Sawka.

Signs you’re passing the threshold of heat exhaustion onto something more dangerous: cold and clammy skin, nausea, dizziness and fainting, pale skin, heavy sweating, headaches, decreased coordination, chills, and irritability.

“It is challenging because many of these may merely be signs of working hard and challenging your body, but many of these together with the context of working in a hot environment can signal that the athlete should back off activity,” Lee adds.

2. Hydrate like crazy

Drinking water before and during a workout is always important, but during the summer it’s absolutely crucial. Hot temps already put a strain on your system, but being dehydrated reduces your blood volume, increasing its thickness and making it even harder to continue proper blood flow, Sawka explains. Because your heart can’t pump blood as efficiently, dehydration will unnecessarily boost your heart rate and possibly worsen your balance and coordination.

Furthermore, because less water means less sweat, dehydration can prevent your body from cooling itself, Lee says. As such, you want to not only go into the workout well-hydrated, but also continue to hydrate throughout the workout (and after), Lee adds.

And even though it sounds unnecessarily intense, consider measuring your water loss to know you’re hydrating enough: “If an athlete has an unusually high sweat rate, he can even lose up to four or five liters of water over multiple hours of exercise,” Lee says. (Remember that before your next multi-hour run in the heat, or a three-hour lifting session in a hot gym.) Weigh yourself before and after a grueling workout in the heat. Losing 1kg (2.2 lbs) during a workout is equivalent to about 1 liter of water (that's a pretty typical water loss for your hour-long workout, Sawka adds). Because you want to rehydrate 150% of what you've lost, aim to drink 1.5 liters of water over the course of recovery and into the next day, Lee explains.

3. Work out in the heat

The single best thing you can do to avoid heat exhaustion, other than hydrate properly, is actually to head straight for it. Why? While your body’s initial reaction to high temps is to essentially freak out—skyrocketing your heart rate, sweating buckets—it then becomes more efficient at managing blood flow and core temperature, as well as sweating and conserving sodium.

“It’s similar to strength or endurance training—you’ll adapt faster if you exercise in the heat versus just sitting in it, and the fitter you are the quicker you adapt,” Sawka says. He helped conduct a 2015 study, published in Sports Science Exchange, that determined that most people acclimatize to heat, and subsequently feel overheated less often, after working out for just a week or two for 90 minutes in hot conditions. We're not talking Death Valley, either—85° is enough to drive up your body temperature, he adds.

For example: In a recent study published in the Journal of Strength and Conditioning Research, runners spent five days biking for 90 minutes in 99° conditions. After the five days of heat acclimatization, the runners ran a 5K. Runners who had trained in the heat for five days ran the 5K 6.5% faster than other runners who cooled down their bodies ahead of the race. That’s because you gain more by acclimatizing to the heat rather than trying to cool off after, the study authors say.

One thing to note: The study authors told The New York Times that, theoretically, athletes could similarly acclimatize themselves by soaking in a hot bath for 30 minutes after a 30-minute run. Sawka, however, says that soaking in a hot bath is unlikely to offer the same benefits as training in the heat.

Taking a hot bath after exercise "is not a good idea, in my opinion," he says. "If an athlete has performed exercise heat stress, those adaptations will be much stronger then resting in hot water."

Furthermore, Sawka says, sitting in a hot bath after a workout can be dangerous. After a hot workout, your body sends more blood to the surface of your skin to cool off. By sitting in a bath, the warm water will encourage the body to shunt even more blood to the surface of the skin. Combined with even modest dehydration, the movement of blood to the skin can reduce the overall volume of blood being pumped by the heart—and that's "NOT a good combination," Sawka says.

And even if you feel like taking a hot bath after a hot workout—which, again, will likely do comparatively little to improve heat acclimatization—only take the bath if you clearly aren’t showing signs of heat exhaustion after a hot run. If you are feeling dizzy, excessively fatigued, dehydrated, or any other symptoms, climb into a cool bath instead to lower your body temperature—but not so cold that you put your body into shock.

4. Cool where you can

Remember, the main way your body combats a high core temperature is to sweat. Heading out on a long run mid-day at the height of heat is asking your body to fight against the highest level of resistance. Similarly, wearing heavy, thick, or synthetic clothing prevents air circulation and makes your body sweat a helluva lot more than necessary. Work out before or after the sun is out, stay in the shade, and wear breathable fabrics like cotton. It seems obvious, but how many times have you seen people out for a run at noon in July? Exactly.

5. Skip the workout if you’re sick

“To prevent exertional heatstroke or heat-related injury, it is important to generally begin exercise in a healthy state,” says Lee. Being sick or having an infection—especially one that involves a fever—makes it even harder for your body to cool itself and counteract the physiological load of a heated room. If you’re feeling sick, take a rest day.

6. Skip the pre-workout supplements

One factor that contributes to your risk for heatstroke: taking drugs or supplements that increase your metabolism and, thus, your metabolic heat production, says Lee. While we’re big fans of metabolism- and energy-boosting supps normally, the perks aren’t worth the risk on an outdoor workout day.

7. Find a breeze

Turn on the fan, open a door—do what you need to find airflow in your gym. “A common mistake of fitness instructors is not encouraging the use of fans,” Sawka says. It’s one of the best things you can do to both avoid overheating and maximize performance: Movement in the air helps to improve evaporative cooling of the skin (via sweat). And the cooler your body can stay in the heat, the harder you can hit it.

8. Be ready to call it quits

Two factors that increase your chances of going from heat exhaustion to full-blown heatstroke: prolonged exercise at a high intensity, and continuing beyond the point at which you should stop, recover, hydrate, and cool the body, says Lee. During a really intense workout, your body is producing so much heat on its own that it just compounds the environmental stressors. And not turning back once you hit the danger zone is just asking for trouble. “Based on what we observe in field studies, we see high rates of heatstroke in people competing at their maximum intensity and effort in a hot and humid environment in race conditions. But the folks who develop exertional heatstroke are the ones who may experience the cognitive symptoms, collapse, or feel a physical inability to perform, and yet they get up and attempt to continue over and over again,” Lee says.

Don’t be dumb—you know your limits better than any coach or instructor. And while you may think being the first to tap out of a circuit is the most embarrassing thing that can happen at the gym, we’d argue passing out or leaving in an ambulance is probably a lot worse.

9. Cool off in water

“For people suffering heatstroke, rapid cooling is critical in long-term outcomes and preventing fatality,” says Lee. The most effective cooling technique if you start to overheat: cold water immersion, or sinking up to at least the chest in a tub filled with cold (even ice) water. This increases the transfer of heat from the body to the surrounding water up to 70 times faster than just trying to use the air, she adds. If you can’t hit a tub or the shower, Lee advises covering your entire body in cold, soaked towels, and frequently trading them for fresh cold ones.

Just be careful not to submerge your head all at once—plunge from the heat into ice cold too quickly and you could put your body at risk for shock.

10. Stay on alert for 48 hours

Heat exhaustion and heatstroke exhibit the same initial symptoms—headaches, dizziness, and fatigue. Both involve the same treatments, too: rest, cool your body, and rehydrate.

But whereas you’ll recover from heat exhaustion within a day or two, heatstroke will continue to get worse and displays “profound central nervous system disturbances,” as Sawka says, which means things like getting combative, having seizures, and maybe even losing consciousness. If your symptoms aren’t getting better with rest, cooling, and hydration, you need to head to the hospital.

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