And the temperature at which it’s *too* cold to be working out outside is…

Working out outside has a long list of perks: First of all, it is completely free. Then you get the added (scenic) benefit of being out in nature, a built-in subtle incline that revs up your run, and the opportunity to sightsee as you sweat versus, say, watching the Food Network or staring at an empty treadmill in front of you.

Of course, along with the perks comes one very real factor that can make or break your outdoor workout: the weather. Obviously, having anything falling from the sky means your workout isn’t happening outside of the gym. But the temperature is a whole other story. If you’ve ever gone for a run in the cold, I’m sure you’re familiar with having your lungs hurt. But besides that—which, in case you were wondering, happens because of the dry air hitting your lungs—sometimes it just feels too brutally cold to sweat out there in the elements.

Sure, you can pull on your warmest leggings and layer up the workout gear—but I consulted a doc to really see if there’s actually a cutoff point at which you should just not. exercise. outside.

“When the temperature falls below zero and if the wind chill is minus 17, it’s best to stay indoors—even if you have warm clothing,” says Robert Segal, MD and co-founder of LabFinder.com. “In these conditions, it only takes 30 minutes for frostbite to form on any exposed skin.” He stresses the importance of checking the wind chill factor, which takes into account the wind speeds that will be endured by your body while outside.

You might be thinking, well, 13 degrees doesn’t sound so ideal. And you may be right—you just have to listen to your body. “Even if the temperature isn’t below zero but you start to feel numbness, go indoors immediately,” says Dr. Segal. “Numbness is the first sign of frostbite.” So if you’re an outdoor workout devotee, just be sure to slip into the warmest clothes… until it’s zero degrees. Then, hey, it’s your recovery day.

Another key thing to know: How to protect your skin in the cold weather. By the way, it is good for you to exercise outside in the winter—these are the benefits of working out in the cold.