A dense wave of smog began in Northeast China, especially in major cities including Harbin, Changchun and Shenyang, as well as the surrounding Heilongjiang, Jilin, and Liaoning provinces on 20 October 2013.

[1] In Changchun, air pollution recorded at an all-time high and the levels of PM2.5 particulate matter rose to 845 micrograms per cubic metre on 22 October 2013.

Officials blamed the dense pollution on lack of wind, burning of crop waste in farmers' fields, and 20 October[3] start-up of Harbin's coal-powered district heating system.

[7] According to the National Environmental Analysis released by Tsinghua University and The Asian Development Bank in January 2013, 7 of 10 most air polluted cities in the world are located in China, including Taiyuan, Beijing, Urumqi, Lanzhou, Chongqing, Jinan and Shijiazhuang.

[2] Visibility of less than 50 m (160 ft) was also reported, but Changchun education bureau refused to close the school and was criticized by parents of the students and public.

[17] PM2.5 is the amount of particulate matter less than 2.5 micrometres in diameter in the air, with the World Health Organization recommending a maximum 24-hour mean of 25 micrograms per cubic meter (μg/m3).