Heavy smog prompted Beijing authorities to advise the city's 20 million residents to stay indoors if possible on Friday.
Older or ill people and children should remain in closed rooms, the environmental monitoring services said.
Levels of airborne fine particulate have been reaching spikes of 490 since Thursday, the US embassy said. Any reading over 50 is not considered good quality air.
Many people in the Chinese capital donned face-masks against the pollution.