The hardest thing about frostbite is that, when you have it, you can’t feel it. Your toes or fingers or the tip of your nose go numb, making it easy for it to go undetected and get worse. Full-fledged frostbite is caused by extremely cold temperatures and sustained exposure to the elements. You’re not likely going to suffer from frostbite after a quick stroll outside. But if you’re climbing a big peak, skiing all day, or helping with a prolonged rescue in the backcountry, you might be susceptible. For tips on how to spot, treat, and prevent frostbite, we called up Dr. Peter Hackett, director of the Institute for Altitude Medicine in Telluride, Colorado, who spent over a decade running the medical and rescue camp on Denali, the highest peak in North America.