Beijing issued its first ever red alert for smog on Monday, urging schools to close and invoking restrictions that will keep half of the city's vehicles off the roads.
An odd-even car plate system will be enforced, constructions sites and factories will shut, and schools are being advised to close during a three-day period in which a toxic cloud is predicted to linger over the Chinese capital.
The system enforced means limiting cars to driving every other day depending on the last number of their licence plate.
The environmental bureau urged people to “reduce outdoor activities” or to “wear a mask or take other protective measures" if they had to venture outside.
Beijing was engaging in “real battle conditions”, a local environmental official told Caixin news website, referring to the “rigorous test” for authorities organising a response in a city with thousands of schools, more than five million cars and almost 23 million citizens.
“How are cars going to be restricted and school classes suspended, and how will officials investigate the source of this pollution?” he asked.
Levels of dangerous tiny particles known as PM 2.5, which can embed themselves deep in the lungs, reached 220 micrograms per cubic metre at 9pm on Monday – far exceeding the World Health Organisation safe level of 25, but still way below readings above 600 which Beijing experienced last week.
That bout of smog saw officials issue an orange alert, the second most severe of a four-tier warning system that was introduced two years ago.
The decision not to issue the highest warning sparked a wave of criticism from citizens and campaigners, who claimed Beijing was not doing enough to raise awareness of the health hazards of smog.
Hundreds of thousands of people are thought to die prematurely each year in China due to pollution.
This month’s heavy bout of pollution comes as China, the world’s biggest emitter of greenhouse gasses, is at the centre of climate change talks in Paris.
Beijing has previously enforced traffic restrictions and factory closures, usually to ensure blue skies during high-profile events such as the 2008 Olympics or a parade last September to mark victory over Japan in the Second World War.