BEIJING (AP) ― Dangerously high pollution levels shrouded Beijing in smog Tuesday for the second time in about two weeks Tuesday, forcing airlines to cancel flights because of poor visibility and prompting the city government to warn residents to stay indoors.

The outlines of buildings in the capital receded into a white mist as pedestrians donned face masks to guard against the thick, caustic air. The U.S. Embassy reported a level of PM 2.5 ― one of the worst pollutants ― at 526 micrograms per cubic meter, or “beyond index,” and more than 20 times higher than World Health Organization safety levels over a 24-hour period.

The Beijing city government advised residents to stay indoors as much as possible because the pollution was “severe.” It said that because there was no wind, the smog probably would not dissipate quickly. Visibility was less than 100 meters in some areas of eastern China.

Air China cancelled 14 domestic flights in or out of the Beijing airport, and an airport in the eastern city of Qingdao was closed, cancelling 20 flights.

The disruptions came in the first week of the country’s peak, six-week period for travel, linked to the Feb. 10 Lunar New Year. Every year, China’s transport system bursts at the seams as tens of millions of people travel for the holiday, in the world’s largest seasonal migration of people.

Celebrity real estate developer Pan Shiyi, who has previously pushed for cities to publish more detailed air quality data, called Tuesday for a “Clean Air Act” and said he would use his status as a delegate to the National People’s Congress to propose such legislation.