In such extreme cold, exposed skin can develop frostbite in as little as five minutes, said George T. Chiampas, an emergency medicine doctor and professor at Northwestern Memorial Hospital in Chicago.

The body’s first reaction to extreme cold is to restrict blood and oxygen flow from its extremities, in order to preserve major organs, Dr. Chiampas said. The first signs of frostbite including tingling or pain in the affected areas. If you think you have frostbite, you should immediately go inside and check yourself for any discoloration or other clear sign of frostbite. Fingers, toes and the face are most often affected.

People with frostbite sometimes don’t realize what is happening, because their fingers or other parts of their bodies go numb as it sets in. And if they are also experiencing hypothermia, which can be deadly, their judgment could be seriously impaired. (More on that below.)

Signs of frostbite include skin that has blistered or become discolored, or that feels unusually firm or waxy. It can result in permanent damage and amputation, and can be more dangerous the longer it goes without treatment.

Speaking on Tuesday afternoon, Dr. Chiampas said his department had treated 15 patients with weather-related complaints in the previous 36 hours, including one who had a finger amputated because of frostbite — a much higher number than is typical for this time of year. The hospital, which is operating a warming center, has prepared for a spike in such patients over the next two days.