Temperatures plummeted so low in parts of the nation this week that many schools and businesses closed, concerned that people would get stuck outside for too long in such frigid weather.

In Minneapolis, where I live, Monday dawned with a wind chill of 45 degrees below zero Fahrenheit. One of our cars started but the other was stuck for two days because the garage door froze shut.

Inconveniences aside, how dangerous is the cold, really?

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The answer is slightly different for everyone, experts say, depending on body size, body shape and layers of warm clothes worn. But overall, people don’t have a lot of biological tools for adapting to cold conditions. When wind chills fall into the double digits below zero things can go bad quickly.

“In general, humans are not very well adapted to be able to do well in the cold, short of having a very large brain,” said Christopher Minson, an environmental physiologist at the University of Oregon, Eugene. “There are some physiological responses that help, but the main thing we do is put more and more clothes on.”

When you step outside on a cold day, your body initially reacts by constricting blood vessels in the skin and diverting blood from the periphery to your core in an attempt to limit heat loss by lowering the temperature gradient between skin and environment.

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As your core temperature drops below the normal average of 98.6 degrees Fahrenheit, you start to shiver, which generates a little heat, Minson said, but not a lot.

At a core temperature of 95 degrees Fahrenheit, you’ll feel uncomfortably cold. That’s when mild hypothermia settles in and the body begins to have trouble maintaining its internal temperature.

When it drops to 91 degrees Fahrenheit, Minson said, people develop amnesia. They become irrational and do strange things like taking all of their clothes off, thinking that they’re burning up instead of freezing cold.

At internal temperatures of 82 to 85 degrees Fahrenheit, people lose consciousness. Death comes at a core temperature of 74 degrees Fahrenheit.

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One reason hypothermia is so dangerous is that the body’s enzymes and nerve signals work best in a warm system. When the system gets colder than it should be, processes slow down.

Frostbite is an even bigger concern during cold snaps like the one we’re experiencing now, said John Castellani, of the U.S. Army Research Institute of Environmental Medicine in Natick, Mass.

When wind chill is below -17 degrees Fahrenheit, there becomes a very real risk that uncovered skin tissue can freeze. Ice crystals that form in skin cells can cause damage, including cell death. The early stages of frostbite is called frostnip, which is freezing at the surface of the skin.

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In at least two small but notorious experiments, Castellani said, researchers observed what happened to people who sat in a chamber that was cooled to various temperatures. At -17 degrees Fahrenheit, results showed, it took about 30 minutes of exposure to cause frostbite.

But frostbite struck after just 10 minutes at -30 degrees Fahrenheit, and in just five minutes at -50 degrees Fahrenheit. The American College of Sports Medicine recommends “heightened surveillance” of athletes exercising outdoors when wind chills get down to -18 degrees Fahrenheit or below.

Wind chill measurements are more important than air temperatures when it comes to evaluating the dangers of cold weather, Castellani said, because the wind chill describes the temperature your skin is actually experiencing.

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Fingers, toes, noses, earlobes and cheeks are most vulnerable to frostbite, he added, because peripheral areas are the first to experience vasoconstriction and blood loss after exposure to cold.

After being chilled for a while, many people experience something called cold-induced vasodilation (CIVD), which dilates the blood vessels in the extremities, causing a sudden hot, burning sensation.

People with good CIVD responses deal better with cold than those who easily lose circulation to their fingers and toes. People with a condition called Reynaud’s phenomenon, which disproportionately affects women, do not experience CIVD at all and are particularly vulnerable to frostbite.

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Another concern on super-cold days is that frostbite can happen almost instantly if, say, you touch a frozen metal door handle or come into contact with gasoline, which can get down to -40 degrees Fahrenheit or colder and remain a liquid. Rolling down your car window to adjust the side window while driving fast on a cold day is also a bad idea.

“I was in Alaska a few years ago and even getting in and out of the car, you had better have gloves on,” Castellani said. “When the outside metal is -20 degrees Fahrenheit, you can get instantaneous frostbite touching supercooled materials.”

To protect yourself from the elements during cold fronts, Minson added, it’s important to avoid getting wet. The body loses heat 25 times faster when it’s wet than it does when it’s dry.

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Most important of all is to dress well and cover exposed skin if you’re going to spend any time outside in the bitter cold.

“People have been able to survive in -80 to -100 degrees on Arctic expeditions as long as they were adequately protected and stayed out of the wind,” Minson said. “If you’re not protected, you’re in real trouble.”