Beijing residents have been warned to stay indoors as the city's pollution index hit its highest level amid the worst conditions since coal-fired power plants kicked into gear for winter.
Every monitoring station in Beijing's urban districts recorded levels of particulates seven times higher than the safety standard, the Beijing environmental protection bureau said.
Sulfur dioxide and carbon monoxide levels, emitted by coal furnaces and cars, were also several times over standards, it said.
"The city environmental protection bureau is alerting citizens to reduce outside activities under such conditions. Frail people or those sensitive to pollution should take measures to protect themselves," it said.
The pollution levels were the worst in the capital since the winter heating season began in early November, when the city fired up its nearly 6,000 coal-fired furnaces, it said.
A heavy layer of fog covering much of northern China was resulting in the pollutants accumulating in the capital, which is also home to 2.8 million cars, one of the city's other main sources of air pollution.
In the run up to the 2008 Olympic Games, Beijing has vowed to clean up its air and this year set a goal of 238 "blue sky days," or days with only light pollution.
As of Tuesday, the city needed eight more to reach the annual goal, the bureau said. But official "blue sky days" are often hazy affairs with heavy pollution.