BEIJING, Feb. 23-- China is taking measures to control air pollution, including limiting vehicles on the road and suspending factory production, as smog persisted in many areas for days.

The National Meteorological Center (NMC) issued a yellow alert for smog on Sunday morning, the fourth straight day such an alert has been announced.

North China's Shijiazhuang, capital of Hebei Province, went on orange alert for smog, the second highest, early Sunday morning, and an emergency response initiated.

The emergency response cuts private vehicles in use by 20 percent based on the last digit of the license plate. Vehicles with last plate digit of 1 or 6 must be off the road in urban areas on Sunday.

Beijing went on orange alert on Friday for the first time. Manufacturing plants in the city have suspended or cut production, building work is halted, and no barbecues are allowed.

At a construction site on the north side of Xinhua East Road in Tongzhou District in Beijing, construction work has been suspended since Friday. Reporters could see no trucks with construction waste or bare earth at the site.

A worker said work had been halted after receiving the smog alert.

Barbecue stands near Beijing's Minzu University of China were advised to close by staff from the Beijing municipal bureau of environment protection, according to a vendor.

The city's road sweepers have also extended their working hours and frequencies since Thursday.

"Besides road sweeping, we've also increased road spraying and cleaning work." said Liu Yi, a driver of a sprinkler.

Zhang Zengshou, general manager of Beijing Jinyu Concrete said the company had suspended production at two plants.

"The capacity of the suspended works both exceed 1.5 million cubic meters. They are the main production stations of our company, but we have followed the city's regulation and shut them down," Zhang said.

Physical education classes, outdoor exercise and sports meets at kindergartens, primary and high schools have also been called off. < According to the Ministry of Environmental Protection, almost all of north, east and central China are suffering. Beijing and the provinces of Hebei, Shanxi, Shandong, Henan and Liaoning reported severe smog.

In respond to the pollution, the Ministry said on Sunday said that it has sent 12 inspection teams to Beijing, Tianjin, Hebei and surrounding areas to check the response to the pollution.

According to the ministry, the teams will review the local government response to the pollution and their implementation of September's central government action plan.

They will inspect major polluting industries such as steel, coal chemicals, glass and cement and check construction sites.

Violations found by the inspectors will be publicized, according to the ministry.

China's meteorological disaster alerts are categorized as blue, yellow, orange and red in order of increasing severity.