Severe pollution is affecting large areas of northern China for a third consecutive day.
In the capital Beijing, a layer of smog blanketing the city remains at a level considered hazardous.
On Saturday, the pollution was literally off the scale at a figure well beyond that considered dangerous to human health.
According to an unofficial air quality monitor on the roof of the American Embassy in Beijing, the Air Quality Index on Saturday afternoon hit 886. Any figure above 300 is considered "hazardous".
By Sunday morning, the level had dropped to 391 which is still well into the hazardous bracket.
According to the World Health Organisation, levels of the smallest pollution particles, called PM2.5, should not be more than 25 micrograms (mcg).
At levels of 100mcg, the air is considered unhealthy. The official Chinese government reading by Sunday afternoon was 446, less than Saturday's 886 but still very high.
On Beijing's streets, it is hard to see more than about 150 metres. The skyscrapers which dominate parts of the city are barely visible.
The pollution is so bad that it is possible to smell the air and even taste it. Residents are being urged to remain indoors but few appear to be listening to the advice.
Beijingers are used to severe pollution but not usually on quite such a level.
Yu Jianhua, from the Beijing Environmental Protection Bureau, said the current weather conditions were preventing the smog from dispersing.
"Beijing has got warmer and wetter. We are facing large quantities of polluting emissions and they are not diffusing very quickly. The air is severely polluted," he said.
Mr Yu urged people to use public transport rather than their own vehicles to reduce emissions.
Even Beijing's underground network is affected. The smog was visible hanging in the air at a number of station platforms on Saturday.
Fumes from the ever-increasing number of vehicles on the city's streets contribute, but the factories surrounding the city also cause significant pollution.
Beijing is flanked on two sides by mountains. The air is dry, cold and hangs over the city. If there is no wind, all the factors combine to create the smog.
Experts also believe that the different pollutants can combine to create an even more toxic mix.
Vance Wagner is an American engineer who works on clean transportation for China. His blog Live In Beijing provides a detailed analysis of the issue.
"The atmosphere is quite the chemical soup. While direct emissions matter, a lot of air pollution is secondary, meaning that it is caused by directly emitted pollutants interacting with each other in the atmosphere, giving rise to new forms of 'secondary' pollution," he writes.
Last year, the Chinese government asked the American Embassy not to publish the figures from its monitor. The Americans refused, insisting that the information was for the benefit of its personnel.
"By recording pollution and publishing the results we are providing members of the mission community and the broader American community in China with information so that they can make better daily decisions about their outdoor activities," US spokesman Nolan Barkhouse told Sky News.
The Chinese authorities publish their own figures for the same air.
At the same time as the US monitor was recording a figure of 736, the official Chinese figure was 500 - still well beyond the hazardous level.
There is an important political dimension to the issue too, with anti-government protests around the country often connected to environmental concerns.
China's international image as a world leader is also severely dented by problems like this in its capital city.
Despite claims by the Communist Party's incoming leadership that it is trying to tackle the issue, there is still a level of denial.
The official government-controlled news agency Xinhua was describing the bad air as "fog" on Saturday.