The yellow alert and above will trigger a series of compulsory emergency responses that can include the suspension of outdoor construction operations to reduce dust, the restriction of heavily polluting vehicles and the suspension of production at smokestack industries.

The Ministry of Ecology and Environment has sent a notice to the governments of Hebei, Shanxi, Shandong, Henan and Shaanxi provinces, as well as to Beijing and Tianjin, urging them to take emergency response measures, including production restrictions, to help curb the pollution.

As of 8 am on Monday, 25 cities had taken steps as required, according to the ministry.