More than 100 million people in China have been warned to stay indoors after at least 10 Chinese cities issued red alerts for smog, state media reported Thursday.
Pollution covered large parts of the country's east and centre as measures to curb the toxic haze were ordered to be implemented.
The alert came as broad swathes of the country suffered through their fourth wave of choking pollution this month.
In addition, the eastern province of Shandong, home to almost 96 million and some of the 10 cities under red alert, issued its first ever top-level warning Wednesday, the provincial environmental bureau said.
It is believed to be the first time an entire province has issued a red alert.
The ten cities include the sprawling industrial hub of Tianjin in the northeast.
Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- in one of the cities under red alert, central Henan Province's Xinxiang, were as high as 727 micrograms per cubic metre earlier Thursday, according to provincial authorities.
The reading is nearly 30 times the World Health Organisation's recommended maximum exposure of 25 over a 24-hour period.
The sharp rise in alerts follows Beijing's decision earlier this month to issue its first ever red alerts, the highest in its four-tier system, in response to scathing public criticism about the government's handling of the chronic haze.
The capital cancelled its red alert for pollution at midnight on Tuesday as a cold front blew away the foul air, state-run Xinhua news service said Thursday.
The notice saw factories ordered to close and half of all private cars pulled off the streets, among other measures.
In an announcement yesterday, the national environmental bureau has ordered six major cities, which were not named, to evaluate their warning systems, it said, adding that they should improve implementation "emergency emissions reduction" measures.
Beijing's decision earlier this month seems to have opened the floodgates for red alerts from other cities, many of which had long suffered silently through regular waves of smog.
Air quality for November and December was at a three year low, according to the state-run China Daily, despite measures to tackle the chronic problem.
"Soaring coal pollution" is to blame, it said, quoting air pollution expert Meng Fan at the China Research Academy of Environmental Sciences.
China's President Xi Jinping has said that the country's CO2 emissions, to which coal is a major contributor, will peak "around 2030".
However, China's state council has announced plans to reduce by 60 percent the amount of "major pollutants" coming from its coal-fired power plants by 2020.
Earlier this month, environmental organisation Greenpeace said Beijing had approved the construction of 155 new coal-burning power plants in 2015.
China's rise to the world's second largest economy was largely powered by cheap, dirty coal. As growth slows, the country has had a difficult time weaning itself off of the fuel, even as the pollution it causes wreaks havoc on the environment and public health.