The red alert may be the Beijing government’s response to withering criticism it received from many residents last week who wondered why it had issued only an orange alert, the second highest, during the most recent “airpocalypse.” On Dec. 1, a blog post by Zhang Kai of Greenpeace East Asia asked, “What will it take for Beijing to call a Red Alert on pollution?”

Starting Nov. 28, an expanse of toxic air smothered northern China. Pollution in Beijing reached hazardous levels the next day and surged well beyond those levels on Nov. 30, when the air in some parts of the city contained deadly particulate matter called PM 2.5 that was nearly 40 times the limit recommended by the World Health Organization. It was the worst pollution of the year. The smog did not begin dissipating until late on Dec. 1, when strong winds blew across the city.

Under the system announced in 2013 and strengthened this year, Beijing is supposed to issue an alert at least 24 hours before the onset of bad smog. Officials can predict pollution levels based on wind and weather forecasts.

A red alert should go into effect if there is a prediction that the air quality index will stay above 200 for more than 72 hours. The United States government rates above 200 as “very unhealthy,” and 301 to 500 as “hazardous.” At 7 p.m. Monday, the Beijing municipal reading was 253.

The red alert was an upgrade from an orange alert issued on Saturday. By then, official Chinese news reports had already said a period of bad smog would start on Monday. Orange means outdoor construction should be halted, as well as the operations of companies that emit heavy pollution.

Chinese cities, especially northern ones, have some of the world’s worst air pollution. Most of it comes from industrial coal burning, and some from motor vehicles. The leaders in Beijing can ensure clear skies by ordering factories to shut down, but they have done so only during international summit meetings here and signature events like the military parade on Sept. 3 for the victory over Japan in World War II.