BEIJING has endured a third day of pollution at hazardous levels, as authorities warned a thick cloud of smog may not lift from the Chinese capital until the middle of the week.
While those venturing outside wore face masks, dissident Chinese artist Ai Weiwei went one step further, posting pictures of himself on Twitter in a gas mask.
As the dense smog shrouded large swathes of northern China, flights were cancelled and traffic delayed as visibility was reduced to 100 metres in some areas.
The dangerously poor air quality was highlighted by a steady stream of news broadcasts on state television, many of which warned residents against venturing outside.
The pollution also provoked China's huge number of microbloggers to take to the internet, with some high-profile web users calling for a re-evaluation of China's rampant modernisation.
Rapid economic growth has led to a dramatic increase in the consumption of coal and clogged city streets with cars.
"The foreign media is laughing at us. I agree with their laughter," said Hu Xijin, the editor of the state-run Global Times newspaper on Weibo, China's version of Twitter.
"This is a warning to the Government and Beijing's citizens. We have to think about what kind of modernisation we want and how to manage it."
Hu said the pollution issue would be on the front page of his newspaper on Monday.
Beijing-based Ai, 55, an outspoken critic of China's communist government, posted three pictures of himself on Twitter standing against a white background wearing a gas mask, his beard frizzing out beneath. But he did not make any verbal comment.
Beijing's municipal environment warning centre issued its second alert in two days, warning people to avoid outdoor physical activity.
The centre also urged government officials to set an example to other residents, by not using their cars.
Air quality in Beijing showed small airborne particles with a diameter small enough to deeply penetrate the lungs at a reading as high as 993 micrograms per cubic metre on Saturday evening, the warning centre said.
The World Health Organisation says the figure for such particles, known as PM2.5, should ideally be no more than 25 micrograms per cubic metre.
High levels have been linked to health problems including respiratory disease, heart disease and lung cancer.
Official PM2.5 figures have only been monitored in China's major cities since the beginning of last year.
The lack of official data makes it difficult to compare the recent smog with previous cases of pollution, said Ma Jun, director of the Beijing-based Institute of Public and Environmental Affairs.
"But it is interesting to see that residents have been warned about the dangers of pollution by official media," Ma told AFP.
"This has never happened before, and is a result of the transparency that we now have with the figures."