China's weather observatory issued a yellow alert on Thursday for smog that has shrouded the country's northern and eastern regions, including Beijing and Tianjin.
Smog will blanket parts of Beijing, Tianjin, Hebei, Henan, Shandong, Jiangsu and Liaoning until Friday night, the National Meteorological Centre said.
Polluted air was reported in 163 cities across China as of midday on Wednesday, Xinhua news agency reported.
Air pollution has repeatedly hit China, especially the north, in the winter of 2015.
China has a four-tier warning system for weather changes, with red as the most severe, followed by orange, yellow and blue.