BISMARCK, N.D. (AP) – A deadly arctic deep freeze enveloped the Midwest, forcing widespread closure of schools, offices and prompting the U.S. Postal Service to take the rare step of suspending mail delivery to a wide swath of the region because of the cold.

Many normal activities shut down and residents huddled inside as the National Weather Service forecast plunging temperatures from one of the coldest air masses in years. The bitter cold is the result of a split in the polar vortex that allowed temperatures to plunge much further south than normal.

Officials throughout the region were focused on protecting vulnerable people from the cold, including the homeless, seniors and those living in substandard housing.

Some buses were turned into mobile warming shelters to encourage the homeless to come off the streets in Chicago, where the forecast for Wednesday night called for temperatures as low as minus 21 degrees (negative 29 degrees Celsius), with wind chills to minus 40 (negative 40 degrees Celsius).

Major Chicago attractions including the Lincoln Park Zoo, Art Institute and Field Museum weren’t opening Wednesday. Governors in Illinois, Wisconsin and Michigan declared emergencies as the worst of the cold threatened on Wednesday.

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

A wind chill of minus 25 (negative 32 degrees Celsius) can freeze skin within 15 minutes, according to the National Weather Service.

In Michigan, homeless shelters in Lansing were becoming “overloaded,” Mayor Andy Schor said. They also were filling up in Detroit.

“People don’t want to be out there right now,” said Brennan Ellis, 53, who is staying at the Detroit Rescue Mission Ministries.

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

At least four deaths were linked to the weather system Tuesday, including a man struck and killed by a snow plow in the Chicago area, a young couple whose SUV struck another on a snowy road in northern Indiana and a Milwaukee man found frozen to death in a garage.

A popular saying goes: “Neither snow nor rain nor heat …” will stop the mail from being delivered. But extreme cold will on Wednesday.