One of the coldest air masses in decades kept its icy grip on the US Midwest on Wednesday, breaking temperature records, causing several deaths and forcing hundreds of schools and businesses to close.

Many temperatures in the Midwest fell below the high of 10 degrees Fahrenheit (negative 12 degrees Celsius) on Wednesday in Fairbanks, Alaska, a city of 31,000 about 120 miles from the Arctic Circle.

Governors in Illinois, Wisconsin, and Michigan declared emergencies. About 220 million people — or 75 percent of the continental US population — will endure below-freezing temperatures this week, CNN reported.

More than 2,000 flights were canceled across the United States, most of which were heading into or out of the frozen Midwest, according to FlightAware.

At least seven deaths were linked to the cold weather, according to The Associated Press, including a man hit by a snow plow in the Chicago region, a man believed to have frozen to death in a Milwaukee garage, and a couple killed in a vehicle accident on an Indiana road.

In Minneapolis, Minnesota, officials warned that anyone venturing into the cold through Thursday morning could suffer frostbite in as little as 5 minutes.

Chicago Mayor Rahm Emanuel said, “These (conditions) are actually a public health risk, and you need to treat it appropriately. They are life-threatening conditions and temperatures.”

Hundreds of businesses and public schools and several large universities from North Dakota to Pennsylvania canceled classes Tuesday and Wednesday.

In Chicago, known as the “Windy City” for winds whipping off Lake Michigan, temperatures fell to minus 19 degrees Fahrenheit (minus 28 degrees Celsius) early Wednesday, breaking the previous record low for the day set in 1966. The cold even forced the Lincoln Park Zoo to close.

In many areas, the focus was on protecting the homeless and the elderly.

Chicago turned five buses into makeshift warming centers that moved around the city, some with nurses aboard, to encourage the homeless to come in from the cold.

“We’re bringing the warming shelters to them, so they can stay near all of their stuff and still warm up,” Cristina Villarreal, spokeswoman for the city’s Department of Family and Support Services, told The Associated Press.

In Michigan, homeless shelters were filling up in Detroit, and they were overloaded in Lansing, according to the mayor.

Native American tribes in the Upper Midwest worked to help members with heating supplies.

Chris Fairbanks, emergency assistance program manager for the White Earth Band of Ojibwe in Minnesota, called the situation “scary” because many houses are substandard.

Detroit’s outlook Wednesday was for overnight lows around minus 12 (negative 24 degrees Celsius), with wind chills dropping to minus 35 (negative 37 degrees Celsius).

The bitter cold is the result of a split in the polar vortex, a large area of low pressure and cold air at the Earth’s poles. During the winter in the Northern Hemisphere, the polar vortex expands, sending cold air south — jokingly called “Canada’s greatest export” by some in the US.

In Pennsylvania, the wind chill is expected to feel like 10 to 30 degrees below zero. In western New York, Thursday’s forecast is for a high of 5 degrees with a low of 1 degree below zero. Parts of upstate New York will feel like 20 to 30 degrees below zero.

In New York, a snow squall flashed through the city Wednesday afternoon, lasting about a half hour. Meteorologists compared the quick-hitting storm to a summer thunderstorm in brevity and intensity.

Freezing cold and wind gusts up to 40 miles per hour were forecast for Wednesday night. By dawn Thursday, the temperature in the city is expected to fall to 3 degrees, with wind chills around minus 15.

The Associated Press contributed to this story.