Here’s how to stay safe as a polar vortex puts the Midwest into a deep freeze this week: Do. Not. Go. Outside.

Experts warn that even a short time in the blistering cold can be extremely dangerous, with the risk of hypothermia and frostbite increasing every minute.

Of course, many of us have no choice: We have to go to work, take care of others, clear snow, get supplies. If you’ll be outside for any amount of time, you should dress warmly and cover any exposed skin.

Here’s what can happen if you don’t.

[Read more about the polar vortex.]

The severity of the threat was expected to be the worst on Wednesday, when actual temperatures could be in the minus double digits, with wind chill as low as minus 50 or 60, forecasters say. Wind aside, the high temperature in Chicago was expected to be minus 15, the low minus 26, according to predictions Tuesday evening.

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.

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Commuters in Detroit on Tuesday.CreditJeff Kowalsky/Agence France-Presse — Getty Images

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.

“These are absolutely dangerous environments,” Dr. Chiampas said. “If people listen to this really, really important message, hopefully we can avoid some really unfortunate and sad outcomes.”

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. Seniors and others with poor circulation are particularly vulnerable.

Extreme cold can also shorten your phone’s battery life. Your best bet is to keep it in a pocket close to your body.CreditAaron Doster/USA Today Sports, via Reuters

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. (Warm beverages are good. Alcohol is not, 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.

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Extreme cold can play a role in many other health issues — the most obvious, of course, being the danger of falls, car accidents and other ice-related injuries. The unanimous advice: Exercise extreme caution.

But it can also play a role in blood clots that cause heart attacks and strokes, and blood pressure tends to rise with exposure to cold. Researchers have found that blood becomes more concentrated and thicker in the cold, as blood flow to the skin is reduced to conserve body heat.

Extreme cold can also affect cellphones. (No, it’s not technically a part of the body, but what if you need it to call for help?)

Most manufacturers recommend using cellphones in temperatures ranging from 32 to 95. Otherwise, the battery’s ability to power the device is compromised, and it may shut down. Keeping the phone close to your body, say in a pants pocket, should keep it warm if you’re outside. And it should work fine once it’s back in normal temperatures.