Beijing and Tianjin issued yellow alerts for smog on the afternoon of Oct 26, requiring polluting companies to suspend production to lower emissions. The smog is forecast to linger until around noon on Oct 28, the national environmental monitoring authority said.

Shijiazhuang in Hebei province saw pollution rise to the most severe level, and many other cities－including Baoding in Hebei province, Beijing and Tianjin－reached their second-worst pollution mark by 4 pm, according to the China National Environmental Monitoring Center. The air quality index in Beijing, for example, hit 235.

“The lasting air pollution will cover a wide swath of the northern and eastern regions, and will peak on Oct 26,” the center said on Oct 25, adding that many cities will see severe pollution on Oct 27, including Beijing and Tianjin; Hebei’s Shijiazhuang, Tangshan, Baoding and Langfang; and Shandong province’s Dezhou and Liaocheng.

A cold front will bring wind on the morning of Oct 28, and smog will ease gradually from north to south, clearing the skies in Beijing. However, some cities downwind may see pollution worsen on Oct 28, the center said.

The current spell of air pollution, which is forecast to persist until Oct 28, is mainly the result of heavy emissions from local companies combined with unfavorable weather conditions that prevent dispersal, said Wang Shuxiao, professor of environmental studies at Tsinghua University.

In response, Beijing and Tianjin issued yellow alerts, the third-highest in the nation’s four-tier emergency response system－red (the highest), orange, yellow and blue－on Oct 26.

Under a yellow alert, construction sites are required to stop all activities that generate dust, and some polluting companies－such as furniture plants, foundries and glass factories－must suspend production.

Tianjin has toughened its restrictions on industrial production while alerts are in effect. For example, under a yellow alert, total industrial emissions will be reduced by 30 percent to clear the air pollution, with production halted at all foundries.

The severe air pollution typically seen in autumn and winter in northern regions has become a thorny issue, and the Ministry of Environmental Protection has launched a campaign with other ministries and provincial governments to reduce the number of days with severe air pollution by 15 percent from October to March.