Update A blanket of toxic air that has blocked out the sun across most of north China is dominating social media and looming as a serious health and political challenge.
Visibility was reduced to a few hundred metres for much of the weekend in the Chinese capital and many of the city's 20 million residents went online to vent their frustration about "apocalyptic" and "post-apocalyptic" conditions.
The Beijing News covered the story with the headline "Blown the Charts", showing that several air monitoring stations were recording levels higher than their indexes could cope with.
Pollution was the top-ranking story on the country's most popular news portals.
Analysts said censorship of air pollution problems has eased since President Xi Jinping raised "a more beautiful environment" as a priority in his November 15 acceptance speech and then ordered official news outlets to report more genuine news.
The story was also the lead item on China Central Television's midday news on Sunday, in place of the usual reports of senior leaders in important meetings.
Even the Communist Party's staid official mouthpiece, The People Daily, headed a page-4 article today with "What's Going on With the Air?"
Monitors at the United States Embassy in the suburb of Liangmaqiao, in Beijing's inner east, said the concentration of airborne PM 2.5 particulates reached 886 micrograms per cubic metre at 8pm on Saturday night, believed to be the highest since it began measuring in 2008.
"I can tell you the machine is working properly," confirmed US Embassy spokesman Nolan Backhouse.
A concentration measure of 500 corresponds with an Air Quality Index of 500
An AQI reading above 300 is classed as "hazardous", according to US environmental standards, but the AQI does not compute concentrations above 500, which in developed countries is normally associated with bushfires.
PM 2.5 particulates, 2.5 microns in diameter or smaller, are considered the most harmful to health because they can penetrate easily into human tissue.
Last month a Lancet study said a record 3.2m people died worldwide from air pollution in 2010, four times the number in 1990, with 1.2 million of those deaths in East Asia including China.
This ranked pollution for the first time in the world's top 10 list of killer diseases, mostly because of vehicle exhaust.
The US Embassy pollution monitor is published on a Twitter feed, which is blocked by censors in China, but which is picked up and placed on several popular Chinese websites and iPhone applications.
Beijing temporarily improved its air quality for the 2008 Olympics including by improving vehicle emissions standards, banning coal stoves and shifting heavy industry to poorer parts of the country.
Much of those gains appear to have been offset by an explosion in the numbers of cars on the roads.
Most major cities were required to report standardised PM 2.5 readings from January 1.
Previously the Chinese Government measured but refused to release the data, publishing only the larger and less dangerous PM 10.
Concentrations of PM 2.5 particulates at an official monitoring station at Xizhimen, in Beijing's inner west, hit a staggering 993 at 7pm Saturday, according to an online report by the Beijing News,.
A faint breeze Sunday morning shifted the worst of the smog to Beijing's southern suburbs and neighbouring Hebei province, with the US Embassy reading dropping temporarily below 300 before midday Beijing time, which is merely "very unhealthy" rather than "hazardous".
The Hebei capital, Shijiazhuang, recorded a PM 2.5 reading of 960, according to People's Daily.
A resident plays with a diabolo in a park during a heavily hazy winter day in Beijing. Photo: Reuters
Chinese weather reports said cold, moist and windless atmospheric conditions could keep pollution at high levels until Tuesday.
The Beijing News report advised children to stay indoors, adults to avoid exercise and all citizens to eat more pears and fungus soup.
The US Embassy service ran into political trouble in 2010 when the reading soared above 500, beyond its existing health categories, and operators resorted to a new label: “crazy bad”.
The Chinese Government complained about the US Embassy pollution data and told it to halt publication, with a Chinese official calling it confusing, socially de-stabilising and insulting, according to a US diplomatic cable obtained by WikiLeaks.
Many Chinese citizens criticised authorities for prioritising the image of the Communist Party over public health, saying they had no choice but to base decisions on daily activities on the US measurement because the Chinese versions were misleading and untrustworthy.
Some days had been measured “beyond index” (or “crazy bad”) on the US index but “slightly polluted” on the official Chinese measure.
The World Bank estimates China to be home to 16 of the world’s 20 most polluted cities.