The government has shut schools, restricted road traffic and urged people to stay indoors as 24 cities across north-east China were put on “red alert” for extreme smog on Tuesday. China’s most severe bout of air pollution this year has hit 460m people, who are exposed to smog levels six times higher than the World Health Organisation’s daily guidelines, according to calculations by Greenpeace. The smog has lasted more than three days in many areas. As of 11am local time, 217 flights at Beijing Capital Airport had been cancelled — almost a third of the total scheduled for the whole day. Pollution has become a rallying topic for Chinese citizens. “The smog problem is a man-made disaster, local environment bureaux are not fulfilling their responsibilities,” wrote the top-rated online poster under a news article about the smog. Two weeks ago, Beijing’s city legislature considered classifying smog as a “weather disaster”. The move was questioned by environmental researchers who said it would help polluters escape responsibility for man-made pollution.