BBC: Pollution has soared to hazardous levels in Beijing, reaching 20 times the limit recommended by the World Health Organisation.
Since Saturday visibility has been low and many people were seen wearing masks. Experts say car emissions and construction in cities are to blame.
Parts of China experience heavy smog periodically.
After an intense bout in Beijing last year in January, the government pledged to improve air quality.
Last year Beijing experienced a smog crisis which continued for...