The Chinese national flag flies on a heavily polluted day in Shijiazhuang, in northern China's Hebei province on December 21, 2016. - China's smoggiest city closed schools on December 21, as the country suffered through its sixth day under an oppressive haze, provoking public anger about the slow response to the threat to children's health. Shijiazhuang, the capital of northern Hebei province, was one of more than 20 cities that went on red alert Friday evening, triggering an emergency action plan to reduce pollution by closing polluting factories and taking cars off the road, among other measures. (Photo by Greg Baker / AFP) (Photo by GREG BAKER/AFP via Getty Images)