Heavy smog engulfed large parts of China for a fourth straight day on Tuesday, amid a spate of pollution "red alert" notices.
As many as 24 cities in northern China put out red alerts, with smog concentrations continuing to increase despite emergency measures.
Despite the alerts, the environment ministry warned that some firms in the region are continuing to flout emergency restrictions to curb the pollution.
So far, the smog has caused dozens of flights out of Beijing to be cancelled.
Some power plants and chemical producers in the region had not scaled back operations in line with regulations, while drivers in Beijing had also been flouting traffic restrictions, according to China Environmental News.
Red alerts are issued when the Air Quality Index (AQI) is forecast to exceed200 for more than four days in succession, 300 for more than two days or 500 for at least 24 hours.
In Handan, a major steel producer, the 24-hour average AQI at one monitoring station reached a record 780, according to environmental group Greenpeace.
"The scale of the red alert measures show that the Chinese government is taking air pollution seriously," said Greenpeace climate and energy campaigner Dong Liansai.
"However, the ongoing 'airpocalypse' is further evidence that China must implement far stricter limitations on coal consumption and accelerate the restructuring of the economy away from the heavily polluting sectors," Dong said.
Pollution alerts have become increasingly common in China's northern industrial heartland, especially during winter when energy demand - much of it met by coal - skyrockets.
China declared a "war on pollution" in 2014 but the government still faces significant challenges after decades of breakneck economic growth, much of it based on the coal-burning power sector and other heavy industry.