TIANJIN -- Smog returned to parts of northern China Friday as red and orange alerts were issued for the new round of pollution over the weekend.

PM2.5 readings in many areas of Beijing, Tianjin and Hebei province exceeded 200 Friday afternoon. In Shijiazhuang, Hebei provincial capital, the index exceeded 400, meaning serious pollution.

Beijing started its orange alert, the second-highest level, Friday. The alert will continue Saturday and Sunday.

Tianjin began an orange alert Thursday night, banning heavy diesel trucks from roads and cutting 30 percent of pollutant emissions of key factories.

Eight cities, including Shijiazhuang in Hebei, activated red alerts, the highest level response, Thursday night.

Seven cities in Henan, including provincial capital Zhengzhou, began a red alert Friday. Zhengzhou suspended school classes Friday.

The new round of pollution is forecast to last until Jan 5 in Hebei and Henan.

The smog also hit Shandong Province, where Jinan, Dezhou and Liaocheng cities will be put on red alert Friday night.

Measures such as suspension of school classes and restriction of cars, will be adopted in red alert areas.