Airlines in Beijing and elsewhere have been forced cancel flights because of poor visibility and there have been government warnings for residents to stay indoors.
The outlines of buildings in the capital receded into a white mist yesterday as pedestrians donned masks to guard against the thick, caustic air.
The problems stranded passengers during the first week of the country's peak six-week period for travel surrounding the Chinese New Year on February 10.
The US Embassy reported a peak level of PM2.5 – one of the worst pollutants – at 526 micrograms per cubic metre, or "beyond index", and more than 20 times higher than World Health Organisation safety levels over a 24-hour period.
Beijing's official readings for PM2.5 were lower than the embassy's – 433 micrograms per cubic metre.
Even that level is considered "severe" and prompted the city Government to advise residents to stay indoors as much as possible.
It said that, because there was no wind, the smog would probably not dissipate quickly.
Visibility was less than 100 metres (328 feet) in some areas of eastern China, the official Xinhua News Agency reported.
More than 100 flights were cancelled in the eastern city of Zhengzhou, 33 in Beijing, 20 in Qindao and 13 in Jinan.
Celebrity property developer Pan Shiyi, who has previously pushed for cities to publish more detailed air quality data and who is a delegate to Beijing's legislature, called for a Clean Air Act.
His call has received widespread support in the country.