Air pollution usually worsens in northern China during the winter, as coal is used to heat homes. Beijing issued a red alert for smog on Thursday, its first of the year, and 10 cities in Hebei have followed suit.

Under China’s four-tiered warning system, a red alert will force suspension of factory production and school classes, as well as traffic reduction measures.

A cold front was forecast to gradually dispel the smog this week, but the pollution hasn’t shown any sign of weakening. One Shijiazhuang resident told Xinhua the air outdoors smelled like burning coal but was no less choking indoors.

Pictures showing the darkened skies of Shijiazhuang have circulated on China’s social media. On Weibo, China’s version of Twitter, one use wrote: “Shijiazhuang is my hometown where I feel attached but I don’t know it as the haze gets worse … I cannot live without masks now and my body is starting to experience illness.”

In another post, one commentator wrote: “It is said that even staying inside the house is unsafe today as smog exceeds the maximum air quality index. Seems like I will have to purchase more air purifiers!”

Another commentator told the South China Morning Post she lived in the Shijiazhuang Hi Tech Industrial Development Zone and had travelled to the city’s Qiaoxi district, where she found the situation “equally horrible”.

“I feel my lungs are extremely uncomfortable after walking outside a few minutes,” she said.

In November last year, Shenyang, the capital of Liaoning province, saw its PM2.5 level exceed 1,000 micrograms per cubic metre.