Huge swaths of north and central China have been living under a pollution “red alert” since last Friday when a dangerous cocktail of pollutants transformed the skies into a yellow and charcoal-tinted haze.

Greenpeace claimed the calamity had affected a population equivalent to those of the United States, Canada and Mexico combined with some 460m people having to breathe either hazardous pollution or heavy levels of smog in recent days.

A picture from Henan province, showing more than 400 students sitting an exam on a football pitch after their school was forced to close, was widely circulated on social media…