In a rare bow to public pressure, the Beijing local government has begun using a more stringent measure for air quality, and the first publicly announced readings Thursday showed the air was “hazardous” in at least two areas of the polluted capital city.

The release of the data followed online protests and complaints that the U.S. Embassy in Beijing was providing a more accurate gauge of Beijing’s air than the city government, which typically tries to downplay the pollution as mere “fog.” And it capped a week in which Beijing has been shrouded in a particularly thick, low-hanging haze of pollution, the result of too many cars and too many nearby factories burning coal.

Several flights to and from the Beijing airport Thursday were canceled or delayed. Traffic has been backed up more than usual because of the low visibility, and several highways were closed. Parents have been keeping their children indoors. Residents have been racing to buy air purifiers, oxygen generators and face masks.

In a bit of black humor making the rounds here, people joke that you can smell China’s GDP in the air. But the official reaction to the pollution problem provides a sharp illustration of the challenges facing authorities as they try to maintain China’s enviable levels of growth, while also meeting the demands of increasingly informed urban residents for a better quality of life — including clean air to breathe.

The U.S. Embassy gauges the air quality from a monitor on its roof, and posts the results hourly on a Twitter account, BeijingAir. Postings over the past several days repeatedly declared the air “Hazardous” to those exposed to it for 24 hours, with several measurements so high as to be deemed “Beyond Index.” A respite came early Friday morning when pollution readings were deemed only “Unhealthy to Sensitive Groups” and then fell to “Moderate” levels.

Residents long accustomed to a polluted, congested capital are starting to openly complain.

“It's a fact that air pollution can damage your personal health,” said Wang Xi, 29, a computer engineer who said he has been riding a bicycle in the city for 10 years, first to school and now to work. He started wearing a high-tech mask after experiencing a sore throat.

The capital sits ringed by mountains on its north and west, so when a haze of pollution lumbers in, it just sits, and sits, and sits, until either strong winds or rains come along to push it off to the east.

‘The major problem is coal’

Technically, the stuff in the air is “particulate matter,” defined by the U.S. Environmental Protection Agency as dust, dirt, soot and smoke that comes from cars and power plants, like those in the provinces that surround Beijing.

According to a Jan. 8 report by the Xinhua News Agency, research by Beijing authorities found that 60 percent of the smallest particulate matter in the city’s air comes from coal burning, car emissions and industrial production; 23 percent from dust; and 17 percent from the use of solvents.

“The major problem is coal,” said Zhou Rong, a climate and energy campaigner for the environmental group Greenpeace, who wears a face mask when she goes outdoors and bought masks for her colleagues.

“Cars are easier to control,” Zhou said. “It is really hard for any Chinese government body to say ‘no more coal.’ ”

The U.S. embassy measures the smallest particulate matter, which is PM 2.5, meaning particles less than 2.5 micrometers in diameter — about 1/30th the width of a human hair, and so fine that they can lodge deeply in human lungs.

The Beijing municipal government, with more than two dozen monitoring stations around the city, has long measured PM levels of 10 micrometers in diameter, much larger than what the embassy measures. This has led to huge discrepancies, with Beijing authorities routinely reporting “clear blue skies” and only “slight” pollution on days when the American mission is declaring the air hazardous by international standards and warning children and the elderly to stay indoors.

The gap was so great that, according to a State Department cable released by WikiLeaks, a Chinese official in 2009 tried to pressure the U.S. Embassy to stop making its readings public, saying the confusion could lead to “social consequences.”

Over the past few months, more Beijingers — using their handheld devices and other means to access Twitter, which is officially blocked here — began closely following the U.S. figures. BeijingAir has been widely considered a truer reflection of what was in the air than the rosier government estimates. In December, even the party-run Global Times questioned the official numbers, and why the embassy’s appeared to be more accurate.

“Most people just have huge distrust of the government,” Zhou said.

Catching up

This month, the local government said it had been collecting the finer measurements and would make them public. On Thursday, in its first release, the city’s meteorological office said the air posed hazardous conditions in two districts, Tongzhou and Fangshan.

According to Xinhua, Beijing officials also announced new steps to try to clean up the air, including preventing dust from drifting from the city’s myriad construction sites, and new regulations to control the release of industrial pollutants.

What residents find most frustrating is the knowledge that the government is capable of cleaning up the air. It was done in 2008, before the start of the Beijing Olympics, when factories were shut down and tough restrictions were imposed on cars. Shanghai did the same around the time of the Shanghai World Expo last year, and Guangzhou cleaned up in time for the 2010 Asian Games.

But as Zhou from Greenpeace said, “After these big events, the activity is reversed.”

Researcher Liu Liu in Beijing contributed to this report.