Mr. Pope, writing on Twitter, pegged the air quality index, a measure of the pollution, above 400 around the time of the video. The United States government rates readings of 301 to 500 as “hazardous.”

The environmental website Treehugger.com noted that the timing of the video coincided with a sharp increase in readings for PM10, suspended particles 10 micrometers or less in diameter.

The past few weeks have been especially bad in Beijing. Officials declared a red alert for the city on Dec. 16, the most severe warning in a four-tier system, forcing older, high-emissions motor vehicles to stay off the road and schools to close. Hundreds of flights were canceled and highways were shut down because of low visibility.

Adding to the disruptions, the government on Tuesday issued its first-ever national red alert for fog, saying visibility would be as low as 50 meters, or about 160 feet, across northern China. And Xinhua, the state news agency, said the smog in Beijing was expected to last until a cold front arrives next week.

While China has taken steps in recent years to reduce the burning of fossil fuels that contribute to such smog, a combination of factors in Beijing, including weather patterns, vehicle exhaust and the city’s geographic position near mountains that trap emissions, have made progress tough.