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At least 21 people have died in one of the worst cold snaps to hit the US Midwest in decades.

Ninety million people – a third of the US – have seen temperatures of -17C (0F) or below. Some 250 million Americans overall have experienced the “polar vortex” conditions.

Hospitals have been treating patients reporting frostbite as parts of the country ground to a halt.

Temperatures are expected to swing to above average over the weekend.

Who are the victims?

Homeless people have been particularly at risk, with warming shelters set up across cities.

But some still braved the freezing conditions and one woman, aged 60, was found dead in an abandoned house in Lorain, Ohio.

• Kindness melting hearts in the polar vortex
• The city with no homeless on its streets

A hospital in Chicago has already…