Moderate to heavy smog has begun to settle on parts of northern China due to falling winds and rising humidity.
Affected regions include the provinces of Hebei, Henan, Shandong and Shanxi, according to the National Meteorological Center.
The smog will lift or dissipate from Thursday to Saturday under the influences of cold air and rain. From Sunday to Monday, more smog will appear.
Beijing and surrounding areas reported better air quality in the first month of 2018 as major air pollutants plummeted, the Ministry of Environmental Protection has said.
In 13 cities in the Beijing-Tianjin-Hebei region, "good air" days accounted for 64.5 percent of the average total last month, up by 28.3 percentage points from a year ago, the statement said. Beijing saw the biggest improvement as the proportion of days with blue skies reached 80.6 percent.