As an Arctic blast bombards the Northeastern United States, temperatures have plunged, and people are bracing for extreme wind chill. Those who will be affected by the cold should stay inside as much as possible. If you have to go out, protect yourself from the risks associated with icy conditions, including falls and car accidents. The unanimous advice from experts: Exercise extreme caution.

You’ll also want to bundle up and cover any exposed skin to avoid frostbite and hypothermia. Here’s how to spot the signs of those conditions — and what to do if you think someone has developed them.

In extreme cold, exposed skin can develop frostbite in as little as five minutes, said George T. Chiampas, an emergency medicine doctor and a 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, he said. And 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 signs of the condition. 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 impaired.

Watch for signs of frostbite, including skin that has blistered or become discolored or that feels unusually firm or waxy. The condition can result in permanent damage and amputation and can be more dangerous the longer it goes without treatment.

If you think you have frostbite, avoid using a heating pad or hot water, the Centers for Disease Control and Prevention warns: If the affected area is numb, you could get burned. Until you can see a doctor, immerse the area in warm water, change into dry warm clothes and use blankets and body heat, such as tucking fingers into armpits.

The C.D.C. warns against walking on frostbitten feet or toes or massaging affected areas, which can increase damage.

When the body is exposed to the cold for prolonged periods, it begins to lose heat faster than it can produce it, according to the C.D.C. Wet conditions are especially dangerous, even in relatively warmer temperatures. A low body temperature renders major organs incapable of functioning properly and can be deadly. Older adults and others with poor circulation are particularly vulnerable.

In the early stages of hypothermia, people often become disoriented or drowsy. Their judgment may be compromised. Fumbling and slurred speech are telltale signs. This makes hypothermia especially dangerous, according to the C.D.C., “because a person may not know that it’s happening and won’t be able to do anything about it.”

The agency advises taking the person’s temperature if you notice any of those symptoms. A temperature below 95 indicates an emergency, requiring immediate medical attention.

Until you can get medical assistance, get the person inside, remove wet clothing and gently warm the body, the agency says. You can give the person warm beverages, but avoid alcohol, as it causes the body to lose heat more rapidly.

In cases of severe hypothermia, the victim may be unconscious — and may seem not even to have a pulse, or to be breathing. But some hypothermia victims who appear dead can be resuscitated, the C.D.C. says. Call 911 and administer CPR if possible.