Heavy air pollution present in the Beijing-Tianjin-Hebei area and neighboring regions since Sunday is expected to last till Tuesday morning as a result of low wind speeds and temperature inversions, experts said.

The National Meteorological Center forecast that the smog will gradually weaken on Tuesday, from the north to the south, because of the arrival of cold air on Monday.

Beijing authorities announced the whole city area reached the fifth level of the air pollution scale on Monday, the second-highest level in the six-tier system.

That meant the average density of PM2.5, hazardous fine particulate matter, was between 150 and 250 micrograms per cubic meter of air.

A research study by Tsinghua University on the latest air pollution in Beijing showed pollutants from surrounding regions were the major source, contributing 61 percent of the pollution, with the rest produced in Beijing.

The China National Environment Monitoring Center said 27 Chinese cities, including Beijing, reached the heavy pollution level on Monday. Linfen in Shanxi province, Weifang in Shandong province and Huludao in Liaoning province had the three highest pollution readings.

The meteorological center announced a yellow alert for heavy fog on Monday morning, warning that southern Beijing, southern Tianjin, central and southern Hebei province, northern Henan province, northern Shandong province and many areas in Central China had heavy fog, with visibility of less than 1,000 meters.

Shandong and Henan issued a red alert for heavy fog on Monday, following Hebei on Sunday.

Xinhua News Agency said some cities in northern China affected by bad climatic conditions could hit the highest level on the air pollution scale.

Experts gathered by the national air pollution authority analyzed the reasons for the pollution and likely developments.

Wind speeds in the Beijing-Tianjin-Hebei area and neighboring regions were lower than 2 meters per second, which led to the accumulation of emitted pollutants, the experts said.

"The authorities should strengthen control of coal-fired power plants, gas-fueled vehicles and industrial emissions in the major polluted cities," they said.