China smog around 50 times WHO recommendations (Update)
Pictures showed smog so thick that buildings in Changchun, the capital of Jilin province in the northeast, were rendered invisible.
One image showed a restaurant's neon sign seemingly floating in mid-air above traffic, proclaiming in yellow: "Northeastern Dumpling King".
An image circulating online showed a man biking through snow in Shenyang, capital of the neighbouring province of Liaoning, on Sunday wearing a vintage-looking gas mask, and the official news agency Xinhua quoted a hospital official in the city saying that his respiratory ward had been overwhelmed, with all its beds full.
Levels of PM2.5, the tiny airborne particles considered most harmful to health, reached 860 micrograms per cubic metre in Changchun, a city of around eight million, on Monday.
The World Health Organization's recommended maximum is a 24-hour average of 25 micrograms.
"Today's haze is pretty severe and choking - when I walked out the door I thought someone's house was on fire," said one poster in Changchun on China's Twitter-like Sina Weibo.
The Changchun city government said on social media it was initiating a "level three" emergency response, telling schools to stop organising outdoor activities, and reminding residents to stay indoors and "take health precautions", without further specifications.
China's chronic pollution problem has been linked to hundreds of thousands of premature deaths, and has become a major source of popular discontent with the government.
PM2.5 particulates can play a role in heart disease, stroke, and lung ailments such as emphysema and cancer.
Online commentators were furious.
"If heating companies dare to buy cheap, low-quality coal and pollute the environment, they should be discovered and immediately shot," said one poster.
Overall levels of PM2.5 particulates reached 1,157 micrograms per cubic metre in Shenyang on Sunday, data from the city's own environmental protection bureau showed.
They peaked as high as 1,400 in parts of the city according to state broadcaster CCTV, with visibility less than 100 metres.
The readings appear to be among the highest ever publicly recorded in China.
The extreme smog was caused by the city's coal-powered public heating system being switched on with the onset of winter, and by heavy pollution blown in from other provinces, city environmental authorities said on a verified social media account.
The explanation provoked derision online.
"Sweden also has central heating - why don't they have haze?" asked one poster.
Xinhua blasted Shenyang's emergency response as "useless" in the face of "such severe haze and pollution", which it said was the heaviest of the year.
"Fairyland or doomsday?" quipped Xinhua of the sorry winterscapes on its Twitter account.
Calls made to the environmental protection bureau in Shenyang, where around eight million people live, went unanswered.
Online commentators expressed concerns that governmental attempts to clean up the air would be too little, too late.
"It's like this every year," was a common refrain.
One user wrote: "Environmental pollution has made chronic diseases more and more common; we're becoming the sick man of Asia of the new century."
© 2015 AFP