BEIJING • A swathe of China has been blanketed with dangerous acrid smog after levels of the most dangerous particulates reached around 50 times World Health Organisation maximums, with energy use for heating blamed as winter sets in.

Pictures showed smog so thick that buildings in Changchun, capital of Jilin province in the north-east, were not visible.

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, yesterday.

The WHO'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 3" emergency response, telling schools to stop organising outdoor activities and reminding residents to stay indoors and "take health precautions", without further specifications.

The official news agency Xinhua quoted a hospital official in Shenyang, capital of the neighbouring province of Liaoning, saying that his respiratory ward was overwhelmed, with all its beds full.

Overall levels of PM2.5 particulates reached 1,157 micrograms per cubic metre in Shenyang on Sunday, data from the city's environmental protection bureau showed.

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 explained on social media.

The explanation provoked derision online: "Sweden also has central heating - why don't they have haze?" asked one poster.

AGENCE FRANCE-PRESSE