Authorities in Beijing and Tianjin issued yellow alerts for smog Thursday afternoon.
In Tianjin, the Air Quality Index readings of PM2.5, the major air pollutant, hit 260 in some areas, according to the Tianjin Environment Monitoring Center.
The yellow alert warns when there will be an air quality index of more than 150 micrograms of harmful fine particulate matter per cubic meter for two consecutive days.
Tianjin urged industrial enterprises to cut major pollutants by at least 30 percent and stop certain work on construction sites.
In Beijing, the city's air pollution emergency response office forecast the air pollution to persist until Saturday.
Low visibility forced the city's transport bureau to close several highways out of Beijing starting Thursday morning.
Under China's four-tier warning system, red is the most severe, followed by orange, yellow and blue.
Smog often becomes severe in late autumn and winter. China's Ministry of Environmental Protection (MEP) warned in September that Beijing's smog was expected to arrive earlier this year.
The air quality has remained bad for most of October.
Wang Shuxiao, head of the Beijing Air Pollution Prevention and Monitoring Work Group, said the stagnant air conditions had made the air more humid.
He said that under similar conditions, Beijing would have seen very severe smog but due to pollution control efforts air pollution was only at a moderate level.
Li Ganjie, environmental minister, said earlier this week that the monthly average of PM 2.5 density in Beijing between March and August registered an all-time low this year.
To address the winter smog, the MEP has launched a campaign against heavy air pollution in the Beijing-Tianjin-Hebei region and surrounding areas. It focuses on curbing industrial pollution, cutting coal consumption and improving emergency responses to heavy pollution.
