BEIJING, Dec. 14 (Xinhua) -- Construction is to be suspended and use of official vehicles limited in cases where Beijing air quality plummets to "extremely bad," under a new rule on curbing air pollution.

The rule, to be enacted on Jan. 1, 2013, categorizes air pollution as "bad," "seriously bad," and "extremely bad" -- depending on a basket of indexes that include measurements of both fine and coarse air particles and ozone levels, meteorological officials announced on Friday.

Yu Jianhua, an official with the Beijing meteorological bureau, said when pollution is 'extremely bad,' work on construction sites will be suspended, heavy-polluting factories will be ordered to cut emissions, and 30 percent of cars assigned to the Party and government officials pulled off the road.

The level of air pollution in Beijing has not been optimistic over the past few years. The city's sky is often shrouded by yellowish smog.

To curb pollution, the government has introduced a spate of measures including moving heavy polluters away from urban districts. It also started to use PM 2.5, a measurement of smaller air-borne particles which experts say can be more dangerous to human health, to accompany PM 10 to gauge pollution.