Starting in mid-December, Beijing will ban vehicles that meet only the Euro 1 and 2 standards for exhaust emissions in the event of severe smog.

According to the capital city's newly revised extreme weather emergency response mechanism, to become effective Dec. 15, these high-emission vehicles will be restricted from roads when the city issues red or orange smog alerts -- the city's two most serious weather alerts.

The restriction will become even tougher starting Feb. 15 of next year, when the city's 456,000 vehicles meeting Euro 1 and 2 standards will be banned on all work days within the city's fifth ring road, which forms a loop about 10 km from the city center.

China's National Emission Standards comply with the European standards for emission limits. Standard I, which is equivalent to the Euro 1 standard, allows an average petrol sedan to emit a maximum of 2.7 grams of carbon monoxide a kilometer.

Beijing became the first Chinese city to enforce the Standard IV or Euro 4 emission limits on all newly purchased and produced cars in 2008.

Automobile emissions contribute 31.1 percent of Beijing's average PM2.5 density. With a diameter of less than 2.5 micrometers, the particulate matter has been a primary factor behind hazardous smog.

Auto exhaust emissions consisting of nitrogen oxides and organic pollutants can help produce the particular matter, said Li Kunsheng, an emission control official with the Beijing bureau of environmental protection.

He said the tougher restriction is among a series of measures to reduce high-emission vehicles in the city.

Beijing has 5.7 million vehicles on the road, discharging 500,000 tonnes of air pollutants a year. The number of vehicles meeting Euro 1 and 2 standards for exhaust emissions makes up only 8 percent of the total, but they discharge 30 percent of air pollutants caused by emissions.