How to Tell If It’s TOO Hot Outside to Exercise

You know how effective and beneficial outdoor workouts are, and when the weather is just plain gorgeous, it’s hard to pass up the opportunity to sweat outside. Unfortunately, though, summer heat and humidity are not always ideal workout buddies. Find out how to stay safe and when it’s smarter to work out indoors:

The Dangers of Working Out When It’s Too Hot Out
Although hotter temperatures can help you warm up faster, the health risks far outweigh that little benefit, says Keri Peterson, M.D., a physician at Lenox Hill Hospital in New York City and a medical advisor for Women’s Health. For starters, when you work out in super hot temperatures, your body sweats a lot to cool itself. Then your blood rushes to the skin to cool it, which means there’s less blood in your muscles. That makes your blood pressure drop and your heart rate go up, which can sometimes cause you to feel lightheaded. As your body temperature climbs higher, you might feel nauseous and put yourself at risk for heat stroke, seizures, and heart rhythm problems. Overall, pushing yourself in this kind of heat is just a bad idea.

When You’re Better Off Staying Indoors
Thing is, there’s no official red-flag temperature, says Peterson. Why not? Because both heat and humidity impact how much work your body needs to do to cool itself. The reason humidity has such an effect on your workout is because moisture in the air prevents sweat from evaporating, which makes it harder for your body to keep your temperature under control, says Peterson. So she recommends using a heat index (here’s one on, which uses both the temperature and humidity level to calculate how hot it actually feels outside, to help you decide whether to lace up your shoes outdoors or not. Peterson says when the heat index hits 90 degrees, you should head to your air-conditioned gym instead. For example, though the thermometer might say it’s 84 degrees out, when the humidity hits 75 percent, it really feels like it’s 92 degrees outside, according to the heat index chart. That means you’re in the danger zone for working out.

MORE: 5 Ways to Get an Awesome Outdoor Workout

But That Doesn’t Mean You Don’t Have to Be Careful When It’s Cooler
Even if the heat index hasn’t hit 90, there are several things you can (and should!) still do to prevent dehydrationRachel Cosgrove, a certified strength and conditioning specialist, author of Drop Two Sizes, and creator of the Women’s Health Spartacus 4.0 Workout in the Women’s Health Personal Trainer subscription tool, says you should weigh yourself before your workout and again after to find out how much fluid you lost during exercise. Then, try to drink that amount of water throughout the day. You should also start prepping for a hot workout before you even pop your headphones in, says Peterson—she recommends drinking about 16 to 20 ounces of water about an hour before you plan to head outdoors. While you’re working out, try to drink at least four ounces of water every 15 minutes. Another thing Peterson says is worth noting: Now is not the time to test out that interval-training routine you’ve been dying to try. Instead, stick with workouts that your body is used to doing.

MORE: 5 Ways You’re Hydrating Wrong

With that in mind, assess your body throughout your sweat session for signs of dehydration, such as thirst, fatigue, and light-headedness. “If you’re thirsty, you’re already dehydrated,” says Peterson. And if one of Cosgrove’s clients were to start feeling overheated during a workout, she says she would probably end the workout completely and start cooling them down.