ZHENGZHOU, Jan. 18 (Xinhua) -- Zhengzhou, a central China city chronically plagued by smog, will raise the alert for air pollution to the highest level Friday, a local authority announced Thursday.

The red alert will be effective starting Friday, with no specific time to lift the warning. Local environmental authorities forecast heavy pollution to persist until Jan. 23 in the provincial capital of Henan.

Primary schools and kindergartens across the city are ordered to close Friday. The general air quality index readings in Zhengzhou hit around 400 Thursday.

China has a four-tier warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.