Three cities in north China's Hebei Province had raised the alert for air pollution to the highest level Sunday.
The red alert will be effective from Sunday with no specific time to lift. Local environmental authorities forecast heavy pollution would persist until Jan. 17 in the three cities in Hebei, which neighbors Beijing.
Another seven cities raised their alert to the orange.
Industrial enterprises were ordered to halt or reduce production. An odd-even number plate rule was imposed in certain areas to take vehicles off roads.
Hebei recorded no heavy pollution days from Jan. 1 to 13, with the average density of PM2.5 only 60 micrograms per cubic meters, down 60 percent year on year.
A new round of smog began to affect Beijing and neighboring regions from Friday, according to the China National Environmental Monitoring Center. Beijing and Tianjin issued orange alerts Thursday for the coming days. The smog will be dispersed when a cold front arrives on Jan. 18.
China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.
