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Smog in China is so thick that planes can't land, cars are banned and factories are shutby Circa News

Smog in some northern Chinese cities has gotten so bad, planes are unable to land, factories were shut down and cars are temporarily banned. 

A national "red alert" was issued on Friday and continues through Wednesday. That's the highest level on China's four-tier alert system for smog, and it's grounded flights and closed major roads.

Chinese newspaper Jilu Evening News reported that on Sunday, the city of Jinan "basically disappeared" in the haze.

Here's an aerial view at the smog in Tianjin. It's not pretty.

"It is safe to stay at home rather than go to school."

Li Jingren, 15

The air pollution has been blamed on China's reliance on coal and emissions from old cars. 

The smog isn't just ugly -- particles in the air can damage lung tissue. Hospital waiting rooms have been packed with children in face masks, The Associated Press reports. Winter tends to be smoggier than most seasons as people use more energy to heat their homes.

The PM2.5, a measure of dangerous particles in the air, has skyrocketed.

Here's a look at Beijing.

This graphic shows how quickly the smog descended on Beijing.

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