BEIJING - Forty cities in north China, including Beijing and Tianjin, have issued alerts for air pollution, according to Beijing air pollution emergency management headquarters.

Beijing, together with the cities of Baoding, Handan, Langfang and Xingtai in the neighbouring Hebei Province, have issued red alerts, the most serious.

Beijing was hit with severe air pollution on Tuesday, with pollution levels expecting to reach grade six on a six-grade pollution gauging system in the southern part of the city later in the day, according to Beijing Municipal Environmental Monitoring Center (BMEMC).

"Affected by increased humidity and temperature inversion, the density of PM2.5 (particles smaller than 2.5 micrometers used to measure air quality) may exceed 500 micrograms per cubic meter on Tuesday," according to Li Yunting, an environmental expert from BMEMC.

PM2.5 stood at 330 micrograms per cubic meter in Beijing as of 2:00 p.m Tuesday, data from BMEMC showed.

The pollution level is predicted to decrease to grade four, still hazardous for health, on Wednesday, and the smog is expected to disperse on Thursday as a cold front arrives, said BMEMC.

China's northern port city of Tianjin and other 12 cities from provinces of Hebei, Henan and Shandong have issued orange alerts. Yellow alerts and blue alerts have been issued in 17 cities and 5 cities respectively, according to the air pollution emergency management headquarters of Beijing.

China has a four-tier color-coded weather warning system, with red representing the most severe weather, followed by orange, yellow and blue.

Tianjin will upgrade its orange alert to red from 0:00 a.m. Wednesday to 6:00 a.m.Thursday, according to a government statement.

During the red alert, cars will be allowed on the roads depending on whether their license plate ends in an odd or even number, enterprises and public institutions will adopt flexible working hours and large outdoor activities and construction work will be suspended.

Kindergartens, middle schools and primary schools will also cancel classes on Wednesday. Key polluting industries will cut production as continuous cleaning operations are conducted in the city's downtown areas.

Heavy smog has hit the country's Beijing-Tianjin-Hebei region on three occasions since late November. Low wind speed, high humidity and unfavorable wind conditions are the main causes of the smog, according to Li.