Expressways, schools and an airport remained closed on Tuesday in northeast China's Harbin City due to lingering heavy smog.

Meteorological authorities in Harbin issued a red alert for the fog at 5 a.m. Tuesday. Visibility in some areas of downtown was less than 20 meters.

Kindergartens, primary and junior middle schools were ordered to suspend classes for a second day on Tuesday in Harbin, the provincial capital of Heilongjiang Province. The Harbin Taiping International Airport has been shut down since Monday.

Meteorological authorities in Harbin issued a red alert for the fog at 5 a.m. Tuesday. Visibility in some areas of downtown was less than 20 meters.

All expressways in Heilongjiang remained closed on Tuesday.

The severe smog continued to shroud other major cities such as Changchun, capital of Jilin Province and Shenyang, capital of Liaoning Province on Tuesday. Meteorological authorities in the two provinces also issued a red alert for the thick fog.

Traffic control has been implemented since Monday evening for the section of the Beijing-Harbin expressway that passes through Jilin Province.

The heavy fog began to hit the vast northeastern region on Sunday.

Medical experts warn that 3 to 5 days after the smog will be a peak in people suffering from respiratory symptoms.

"The impact of air pollution on people will be gradual. There won't be a sudden outbreak of symptoms, but normally three to five days after the smoggy weather occurs, there is a peak in the number of people seeing doctors," said Deng Ying, a doctor at the Second Affiliated Hospital of Harbin Medical University.

On Tuesday, the Ministry of Environmental Protection released air quality data of 74 cities in September and the three-month period from July to September.

The ministry listed the 10 cities with the worst air quality in the third quarter of the year: Xingtai, Shijiazhuang, Tangshan, Handan, Hengshui, Baoding, and Langfang in north China's Hebei Province; Jinan in east China's Shandong Province; the municipality of Tianjin; and Zhengzhou in central China's Henan Province.