The latest wave of severe air pollution continued to smother China's northern regions including Beijing, Tianjin, Hebei and Henan provinces on Thursday, and is likely to last until Saturday.

The National Meteorological Center (NMC) upgraded northern China's smog alert from yellow to orange on Thursday afternoon. Most northern regions were severely affected. The NMC forecast that Beijing, Tianjin and Hebei Province will all suffer from dense smog that could bring visibility down to less than 200 meters in some places on Friday, NMC said on its website.

The smog, which first hit Henan Province on Tuesday, was said to be the worst in northern China since July.

According to Beijing's contingency plans, outdoor sport events and school activities should be cancelled while an orange alert is in effect and it is suggested that residents wear masks while outside and wash faces afterwards.

Some cities in Hebei Province, such as Langfang and Handan, began to limit the number of vehicles on the road by prohibiting cars with certain license plate numbers from being driven, in an attempt to curb air pollution.

Two thirds of Henan's cities have been shrouded by air pollution for several days this month, and the Department of Environmental Protection of Henan Province attributed the smog to farmers burning straw in their fields, which helps crops grow, the Guangming Daily reported.

However, environmentalists do not buy the explanation offered by Henan's government.

"The occurrence of the smog is by no means an accident," Du Shaozhong, former deputy director of Beijing Municipal Environmental Protection Bureau, told the Global Times.

China's current industrial production methods that rely heavily on traditional, polluting energy sources and vehicle exhaust fumes should be blamed for causing the smog, Du said.

Du called on the public and government to work together to change the production methods, to supervise emissions and to curb smoke produced by restaurants.

"Beijing, Tianjin and Hebei will never see blue skies if we do not deal with pollution caused by vehicles, burning coal and various industries," Du said.