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.

A man walks along the moat of the Forbidden City in Beijing, China, Feb 21, 2014. [Photo/Xinhua]

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.

Several steel makers and related enterprises in five cities of Hebei were closed on Sunday morning.

In Hebei, China's top steel producer, overcapacity has become the province's major concern in tackling pollution. The latest move reduced iron production capacity and steelmaking capacity by 6.71 million and 1.49 million tonnes respectively.

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.

Thirty-six enterprises have suspended production, another 75 reduced their output since Friday,according to Beijing Municipal Commission of Economy and Information Technology.

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.