Beijing's Municipal Bureau of Environmental Protection warned Monday that severe pollution would cloak the Chinese capital for several days, starting Tuesday morning.
The red alert -- the highest level in the system -- is due to be in force until noon Thursday local time.
According to the U.S. Embassy in Beijing, the air quality index stood at 250 this morning, classed as "very unhealthy" and 10 times higher than World Health Organization recommended levels.
The alert means extra measures will be enforced.
Car use is being halved by allowing only odd or even numbered license plates on the road at one time and heavy vehicles including garbage trucks are banned from the streets.
Other polluting industrial activity has been curbed, as have fireworks and outdoor barbecues.
CNN reporters in Beijing said the air pollution didn't feel quite as severe as last week, when air quality, as measured by the U.S. Embassy, went above 500 or "beyond index" on Monday and Tuesday. Some residents have questioned why the unprecedented red alert level wasn't issued then.
According to state-run news agency Xinhua, a red smog alert is only issued if heavy pollution is expected to last longer than 72 hours.
China, the world's largest greenhouse gas emitter, aims to halve its emissions peak by 2030.
Most of the country's carbon emissions come from burning coal to heat homes and to fuel power plants, a practice that spikes during cold winter months.
