If carried out properly, the ptemporary restrictions would affect many of Beijing's more than 20 million residents. From 7 am on Tuesday to noon on Thursday, schools will be required to close; cars will be allowed to drive only on alternate days, depending on their licence plate numbers; and fireworks and outdoor barbecuing will be banned (grilled kebabs are a hugely popular street food in the city). In addition, government agencies will have to keep 30 per cent of their automobiles off the streets.

An official signal of the announcement came around 6:30 pm Monday, when Xinhua, the state news agency, posted on its English-language Twitter account: "Beijing issues first red alert for heavy smog. Odd-even car ban imposed; schools suspended." The post included a photograph of the Bird's Nest, the iconic stadium built for the 2008 Summer Olympics, shrouded in charcoal-gray smog and barely visible.

Around the same time, an official website of the Beijing government posted advisories on how to respond to the alarm.

One big question was whether officials would strictly enforce the regulations immediately, especially given the late notice on Monday. Many residents were left scrambling to plan for the next morning: What to do with children not going to school; how to get to their jobs if they could not drive; and whether they should even go to work.

The city government did not explain the timing of its announcement. Thick smog had already settled over Beijing by Sunday afternoon, with pollution reaching what the United States government labels "very unhealthy," when everyone may experience the effects of toxic air and should avoid unnecessary outdoor activity. On Monday, even before the red alert, announcements in Beijing subway stations warned that a spell of toxic air was hitting the city and would last until Wednesday.