Severe pollution blankets landmarks in the capital on Tuesday, including (L-R) the China Central Television building, Forbidden City, Beijing National Stadium and Beijing West Railway Station. The images provide a sharp contrast to days with a clear blue sky. Photos on iPad are provided to China Daily by Tao Yuan, Hu Qingming, Li Junfeng and Nipic.com. ZOU HONG/CHINA DAILY

A series of emergency pollution control restrictions have been introduced, ranging from closing industrial operations to reducing road traffic by half. Beijing's vehicle restrictions will last until noon on Thursday.

Although coal-burning is strictly banned in downtown Beijing and some of its suburban areas, coal is still being used for home heating in all rural areas of the smog-affected northern provinces.

Experts and the national environmental watchdog said the soaring use of coal and industrial pollution are the main reasons behind the severe smog affecting northern areas.

Xie Shaodong, an environmental professor at Peking University, said: "The main cause is the widespread use of low-quality coal in rural regions and in areas with a lack of environmental supervision."

Chai Fahe, deputy director of the China Research Academy of Environmental Sciences, said that even in distant suburban areas of Beijing pollution remains heavy because coal is used for heating and rules are not being enforced strictly.

Feng Yinchang, an environmental professor at Nankai University in Tianjin, said the use of low-quality coal in rural areas is a key source of winter pollution.

Tao Guangyuan, executive director of the Sino-German Renewable Energy Center, said vehicle exhaust emissions are "not that bad" because most Chinese vehicles use similar standards governing petrol that have been adopted in developed countries.

According to environmental authorities, the main source of pollution in Beijing is vehicle exhaust, while in Tianjin it is dust. Coal-burning is the chief culprit in Shijiazhuang.

The authorities have been leading an initiative for high-grade coal to be used along with less-polluting boilers in the countryside. But experts say it is hard to check the day-to-day situation in all villages.