The Beijing-Tianjin-Hebei region will begin to see a moderate to severe air pollution starting Saturday due to unfavorable weather conditions for dispersing pollutants, the top environmental authorities said on Friday.

The pollution is expected to last for three days and reach its peak on Sunday. While cold air from the north will help clear the pollution on Monday, moderate and short-term heavy pollution may linger in cities in central and southern parts of the region on the day, said China National Environmental Monitoring Center.

From Friday night, the density of PM 2.5, a major air pollutant, in the central and southern parts of the region and western part of Shandong province, began to accumulate because of unfavorable conditions for pollutant dispersal, including high humidity and stable atmosphere. Some cities around the Taihang mountain will witness heavy pollution level, the center said.

China has a six-level air pollution system depending on the air quality index (AQI). Excellent air requires index levels below 50, and if the index falls between 151 and 200, the day is classified as moderately polluted. Days when the index rises over 300 are considered severely polluted.