Schools in Beijing are closed and outdoor construction halted as the Chinese capital's first ever pollution "red alert" came into effect.
The alert, the highest possible warning level, was issued late Monday and will last until midday on Thursday.
Limits have been placed on car use and some factories have been ordered to stop operations.
It comes as China, the world's worst polluter, takes part in talks on carbon emissions in Paris.
It is the first time China has declared a red alert under the four-tier alert system, which was adopted a little over two years ago, although pollution levels were far from the city's worst.
The World Health Organization recommends 25 micrograms per cubic metre as the maximum safe level.