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Commuters wear masks on a polluted day in Beijing on December 20, 2016. 
Heavy smog suffocated northeast China for a fifth day on December 20, with hundreds of flights cancelled and road and rail transport grinding to a halt under the low visibility conditions. / AFP / Greg Baker        (Photo credit should read GREG BAKER/AFP/Getty Images)
Commuters wear masks on a polluted day in Beijing on December 20, 2016. 
Heavy smog suffocated northeast China for a fifth day on December 20, with hundreds of flights cancelled and road and rail transport grinding to a halt under the low visibility conditions. / AFP / Greg Baker        (Photo credit should read GREG BAKER/AFP/Getty Images)

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