Moderate pollution is forecast until Friday in Yinchuan, which was shrouded in grey, photos from China News Service showed, with a chance of severe pollution on Thursday.
The city sought to slow or even stop industrial production, and strengthen pollution controls for key industries such as power stations and petrochemical, chemical, metallurgical and cement plants.
Authorities of both cities stopped most earthwork and demolition operations at construction sites, spray and paintwork as well as production and transportation of bulk materials using heavy-duty trucks at concrete plants, and in coal processing, stone-cutting and mining activities.
Both cities expanded restrictions on using high-emission vehicles including diesel vehicles, and advised people to avoid idling their vehicles.
The national weather observatory warned of low visibility in the dense fog of less than 200m in parts of the northern and eastern provinces of Hebei, Shandong, Jiangsu, and Anhui as well as south-western Chongqing city, potentially disrupting transport and traffic during peak travel before the New Year holiday.
It cautioned people with respiratory and cardiovascular illnesses, as the pollution is mainly due to fine particles that can travel deep into the lungs and even the bloodstream.