It’s that time of year when the days are getting colder and darker. And you’re probably slipping into more layers before you head out the door to run. Depending on where you live, the temperatures may even reach single digits or below zero over the next few months. Video above: Tips for surviving treadmill workouts during winterWhen temperatures dip below zero, you may have wondered: How cold is too cold to run? Whether the breathing challenges, wind chill, or numb fingers and toes keep you inside, it’s smart to set a cutoff for when you should still head out or stay safe indoors. To help you determine the best place to clock miles, we asked a pro, running coach, and physical therapist to weigh in to help you best tackle cold weather running this season.According to a physical therapistEveryone has heard the warning: “Don’t go out in the cold, or you’ll get sick!” But Natalie Markland, D.P.T., a physical therapist based in Boulder, Colorado says getting outside and being active in the cold can make your immune system more resilient—as long as you wear enough warm layers. “As an additional benefit, the exercise endorphins can help to offset SAD (Seasonal Affective Disorder) symptoms that frequently accompany the cold-weather seasons,” she notes. However, there are also certain factors for runners to consider on an individual basis. “If you have asthma or some kind of breathing condition, it’s not ideal to breathe in that harsh air,” she said. “Personally, if I feel it’s too cold and dry, my throat starts to burn and will sometimes be sore for the rest of the day.” If you experience the same thing, ditch the trail in favor of the treadmill. Keep in mind using a neck gaiter or face mask can help trap a bit more moisture and add humidity to the air, which can be helpful. Taking in breaths quickly and then exhaling slowly may also help keep small airways more open for those with asthma. Markland also warns that you’re on thin ice (figuratively—maybe literally) if you try to go out in extremely low temperatures: “When wind chill is around negative 20 degrees Fahrenheit, frostbite can set in within 30 minutes,” she says. In that case, it’s also best to stay inside. The coach’s approachKathy Butler, USATF level 3 certified coach and coaching education instructor, as well as a two-time Olympian, sees to it that her athletes stay safe in a variety of outside conditions. In her experience, the potential hazard is not necessarily the cold itself, but rather a lack of sufficient preparation for running in it. That’s why you want to start with short runs in the cold, so you can adjust and figure out your layering.“I have seen my group train well in temperatures as low as 10 degrees Fahrenheit, but there was no wind, it was sunny, and they were quite bundled up,” she says. “Getting injured or sick while running in the cold is usually the result of not wearing the correct clothing—and sometimes it’s from dehydration, as you don’t expect that when it’s cold.” Butler points out that another common mistake people make is waiting too long to get warm after a run. Athletes are encouraged to immediately get out of the cold and put on warm, dry clothes. Sipping a hot beverage can make a big difference in recovery, as well. What professional runners sayFor professional athletes who must train year-round in their sport, it’s critical to assess weather conditions as an injury avoidance measure. So, when do the elites opt for the treadmill over an outdoor run? “Having trained in Oregon, upstate New York, Michigan, and Colorado, I’m quite used to running in some pretty cold conditions,” says Sage Canaday, marathoner and professional mountain ultrarunner. “I’ve run in below zero a few times, but for the sake of my lungs and throat, I generally will run indoors if it is that cold outside—especially for intense workouts when I’m breathing harder.” Will Leer, a mid-distance runner, enjoys winter running but says it’s too cold to run outside when temperatures hit below zero. “At five degrees below zero (with wind chill), the propensity to emerge with frostbite goes up exponentially,” he says. “I’ve had frostbite, and you can take my word: It’s not worth it.”Both Canaday and Leer acknowledge the risks and the need for extra preparation when running in the cold. “If there is a lot of snow or ice on the ground, then lack of traction and slipping can be an issue, resulting in falls and other injuries,” Canaday said. “Also, it generally takes longer to warm up your muscles, so there may be an increased risk of a muscle tear or strain when trying to run hard.” (You can find some winter shoes now or add some traction to your own shoes.)Leer recommends getting the warmup done while in “climate-controlled bliss” to make sure all parts of the body are ready for exercise before heading outside. Consider finishing the warmup process in the cold, though, because there will inevitably be a shift in blood flow in the body that could set you back until you adjust to the lower temperatures.Leer also advises choosing the right gear for enjoyable, injury-free winter running, with a good pair of gloves topping the list. “Nothing ruins a run more quickly than freezing cold hands. Look for something insulated with wind-blocking properties,” he says. The bottom line on when it’s too cold to runBe smart with protective gear (covering up the chin, nose, and ears are important!), and use your best judgment according to how well you personally tolerate cold temperatures.“Everyone has their limits,” Leer says. “As I have gotten older, my limits have changed. My rule of thumb is that if I feel like the weather is an impediment, I run inside.”You don’t always have to just move your run inside, either. You can use the cold weather as an opportunity to work on strength and mobility training indoors, considering many runners put those training methods on the back burner, but winter offers a good time to focus on them.