Meteorological authorities in Harbin city issued a red alert for the fog, with visibility in some areas reduced to less than 20 meters.
Kindergartens, primary and junior middle schools were ordered to suspend classes for a second day in Harbin, the provincial capital of Heilongjiang Province.
The Harbin Taiping International Airport has been shut down since Monday.
All expressways in Heilongjiang remained closed on Tuesday, state-run Xinhua news agency reported.
The severe smog continued to shroud other major cities such as Changchun, capital of Jilin Province and Shenyang, capital of Liaoning Province. Meteorological authorities in the two provinces also issued a red alert for the 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.
"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.
Meanwhile, 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 northern Hebei Province; Jinan in eastern Shandong Province; the municipality of Tianjin; and Zhengzhou in central Henan Province.