BEIJING - Thick smog shrouding Beijing and its surrounding areas becomes heavier as the capital has raised its four-tiered alert system to "orange".

The smog covered about 1.4 million square km of Central and East China on Friday. Beijing and provinces of Hebei, Shanxi, Shandong, Henan and Liaoning reported "severe haze," according to satellite remote monitoring by the Ministry of Environmental Protection.

Data showed that among 39 cities including Beijing, Tianjin and their surrounding areas, 20 cities suffered heavy air pollution on Friday, four more cities than those on Thursday.

Data from the ministry put levels of PM2.5 particles in 19 cities at above 150 micrograms per cubic meter, compared with 75 micrograms per cubic meter, China's air quality standard. Beijing and another six cities saw "severe air pollution" with their PM2.5 readings above 250, according to the data.

PM2.5 are particles with a diameter less than 2.5 microns that have been a major contributor to the smog shrouding China in recent years.

Heavy air pollution in Beijing and part of its neighboring Hebei Province was forecast to last until at least Thursday next week as no strong wind is in sight to help disperse it, the ministry said.

People wearing respirators are seen on a pedestrian bridge in downtown Beijing, capital of China, Feb. 22, 2014. [Photo/Xinhua]

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