Beijing, Dec 30.- China's northern cities: Beijing, Tianjin, Hebei and Henan as well as cities in the center of the country are covered today by smog.
On Thursday, the capital issued an orange alert for air pollution, which will remain in force until Sunday.
In light of the alert, highly polluting vehicles and construction transportation trucks have been forbidden from circulating and some manufacturing factories have been forced to reduce their production levels.
The orange warning means that the air quality index shows that levels of nitrogen dioxide in the atmosphere will exceed 200 micrograms per cubic meter for three days in a row.
Henan and Hebei, where the smog is expected to last until January 5, issued red and orange alerts respectively.
In Henan classes were suspended in nurseries and primary and secondary schools this Friday.
Eight cities, including Shijiazhuang, the capital of Hebei, are on a red alert and restrictions have been placed on traffic and industrial discharges have been reduced by 40 percent.
The World Health Organization states that exposure for more than 24 hours to harmful concentrations of more than 25 micrograms per cubic meter of nitrogen dioxide is damaging to health.