Pollution readings in Beijing soared off the scale again yesterday, with levels of one pollutant reaching 26 times the safe level recommended by the World Health Organisation.
Visibility was reduced to 500 metres, shutting roads and causing huge traffic problems as the peak travel season for Lunar New Year began.
Residents were warned to wear masks as the density of PM2.5 fine particles - considered most hazardous - reached 671 micrograms per cubic metre at 4am.
The reading, taken by the US embassy, was the highest for a year. The choking smog will linger in the city today, with the Beijing meteorological bureau upgrading the blue alert for smog - the lowest of four levels -to yellow last night.
Video: Beijing's air pollution level 'off the charts'
The pollution levels began to rise at 9pm on Wednesday.
The official Air Quality Index (AQI) issued by the Beijing environmental monitoring centre went beyond the upper limit of 500mcg briefly at midnight.
It remained above 400mcg all yesterday morning, meaning pollution was "hazardous".
Yang Heng, who works in Beijing but was heading home to Sichuan province for the holidays, said: "It's so weird that the air suddenly turned acidic and visibility reduced so quickly on Wednesday night.
"Where has it come from? I'm lucky I'll escape the city today."
The choking smog also blanketed nearby cities in Hebei province, with AQI readings in Xingtai, Dingzhou, Shijiazhuang and Baoding remaining off the scale for most of the day.
A yellow smog alert was also issued in Shenyang, Yingkou and Dalian in Liaoning province.
Flights at Beijing Capital International Airport were not affected. Still, pilots, even of small commercial aircraft, are receiving training in "blind landing" - or instrument-guided landings - for when they fly between Beijing and other big cities including Shanghai, Guangzhou and Shenzhen due to the rising frequency of smog.
Greenpeace climate and energy campaigner Huang Wei said heavily polluted days were more likely in the first two months of the year in Beijing as pollution from central heating was exacerbated by stagnant air conditions. Last year, the city had 15 days when PM2.5 went beyond 600mcg, 10 of which occurred in January and February, said Huang. In January alone, the city had 26 smoggy days, with PM2.5 density hitting a record high of above 1,000mcg.
The mayor of Beijing, Wang Anshun , has announced that the city intends to reduce the average level of PM2.5 by 5 per cent this year.
It is a step towards meeting the city's longer-term target of bringing the annual average density to 60mcg by 2017, even though this is still more than twice the upper limit recommended by the WHO.