BEIJING — Weary of waiting for the authorities to alert residents to the city’s most pernicious air pollutant, citizen activists last May took matters here into their own hands: they bought their own $4,000 air-quality monitor and posted its daily readings on the Internet.

That began a chain reaction. Volunteers in Shanghai and Guangzhou purchased monitors in December, followed by citizens in Wenzhou, who are selling oranges to finance their device. Wenzhou donated $50 to volunteers in Wuhan, 140 miles inland. Officials have claimed for years that the air quality in fast-growing China is constantly improving. Beijing, for example, was said to have experienced 286 “blue sky” days in 2011, a statistic belied by the heavy smog smothering the city for much of the year.

But faced with an Internet-led brush fire of criticism, the edifice of environmental propaganda is collapsing. The government recently reversed course and began to track the most pernicious measure of urban air pollution — particulates 2.5 microns in diameter or less, or PM 2.5. It decreed that about 30 major cities must begin monitoring the particulates this year, followed by about 80 more next year.

The Ministry of Environmental Protection also promised to set health standards for such fine particulates “as soon as possible.” Last week, after years of concealing its data on such pollutants, Beijing began publishing hourly readings from one monitoring station.

Ma Jun, director of the Institute of Public and Environmental Affairs, a Beijing nonprofit group, credits the Chinese public for the breakthroughs. “At the beginning of last year, we had almost lost hope that the PM 2.5 would be integrated into the standards,” Mr. Ma said. “But at the end of the day, the people spoke so loudly that they made their voice heard.”

The fine particulates, caused by dust or emissions from vehicles, coal combustion, factories and construction sites, are among the most hazardous because they easily penetrate lungs and enter the bloodstream. Chronic exposure increases the risk of cardiovascular ailments, respiratory disease and lung cancer. The Chinese government has monitored exposure levels in 20 cities and 14 other sites, reportedly for as long as five years, but has kept the data secret.

It sought 18 months ago to silence the American Embassy in Beijing as well, arguing that American officials had insulted the Chinese government by posting readings from the PM 2.5 monitor atop the embassy on Twitter. A Foreign Ministry official warned that the embassy’s data could lead to “social consequences” in China and asked the embassy to restrict access to it. The embassy refused, and Chinese citizens now translate and disseminate the readings widely.

While China has made gains on some other airborne toxins, the PM 2.5 data is far from reassuring in a country that annually has hundreds of thousands of premature deaths related to air pollution. In an unreleased December report relying on government data, the World Bank said average annual PM 2.5 concentrations in northern Chinese cities exceeded American limits by five to six times as much, and two to four times as much in southern Chinese cities.
 

Nine of 13 major cities failed more than half the time to meet even the initial annual mean target for developing countries set by the World Health Organization. Environmental advocates here expect China to adopt that target as its PM 2.5 standard.

Wang Yuesi, the chief air-pollution scientist at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, estimated this month that Beijing needed at least 20 years to reach that goal. The embassy’s monitor showed that fine particulate concentrations over the past two years averaged nearly three times that level, and 10 times the World Health Organization’s guideline, said Steven Q. Andrews, an environmental consultant based in Beijing.

In fact, Mr. Wang told Outlook Weekly, a magazine owned by China’s official news agency, Xinhua, that Beijing’s PM 2.5 concentrations have been increasing by 3 to 4 percent annually since 1998. He said the finer particulates absorbed more light, explaining why Beijing so often is enveloped in a haze thick enough to obscure even nearby buildings. Air pollution in the city and in nearby Tianjin is so severe that “something must be done to control it,” he wrote on his blog.

Such sentiments are increasingly common on weibos, the Chinese version of microblogs like Twitter, especially among elites. International schools here are doming their athletic fields because pollution so often requires that students stay indoors.

In November, Pan Shiyi, a Beijing real estate tycoon, asked his seven million microblog followers whether China should employ a stricter air-quality standard. Shi Yigong, a molecular biologist who left Princeton University in 2008 to lead Tsinghua University’s life sciences department, complained in a December blog post that air pollution was the single “most upsetting and painful thing” about life in China.

Some Chinese citizens remain stoic or unaware. One afternoon last week when smog cloaked Beijing and the American Embassy monitor edged toward the top of the chart, parents flocked to the Capital Institute of Pediatrics, a children’s hospital in downtown Beijing, towing children with respiratory ailments.

One mother of a 6-year-old awaiting treatment for her child’s chronic cough said: “I think it’s good for the child’s immune system to be exposed to tough weather like today’s. It will make them tougher.”

Chinese statistics indicate that urban air quality has improved over the past decade as cities have relocated factories, reduced coal burning and adopted stricter vehicle emission standards. The World Bank’s analysis of the government’s data found that average concentrations of particulates measuring 10 microns or less — a group that includes both fine and coarser particulates — fell 31 percent from 2003 to 2009 in 113 major cities.

Still, only a few cities managed to meet China’s own toughest standard, which is twice as loose as the World Health Organization guideline. Mr. Wang, the researcher, contends that while Beijing’s PM 10 level fell nearly a third from 2006 to 2009 — for the most part, in the years leading up to the Beijing Olympic of 2008 — it has been edging up ever since.

Whether government statistics are reliable is another matter. While some argue that the release of ever more detailed data makes fudging ever harder, Mr. Andrews, the environmental researcher, contends that the government systematically manipulated data and standards to create more “blue sky” days. Although attention focuses on Beijing, at least 16 other cities are more polluted, the World Bank says. Their efforts to clean up the air are partly offset by rising populations, an avalanche of vehicles and never-ending construction.

Some experts contend that the government shies away from epidemiological studies on pollution’s health impact. “They are really unwilling to match it to the health data because that would be much more alarming,” said one specialist who spoke anonymously for fear of angering Chinese officials. “They want to get the counts down first.”

The World Health Organization estimated in 2007 that 656,000 Chinese died prematurely each year from ailments caused by indoor and outdoor air pollution. The World Bank placed deaths related to outdoor pollution at 350,000 to 400,000, but excised those figures from a 2007 report under government pressure.

Zhong Nanshan, a respiratory expert at the Chinese Academy of Engineering, told China Daily last month that without intervention, PM 2.5 particulates would replace smoking as China’s top cause of lung cancer. Beijing health experts told the newspaper that while smoking rates were flat, the city’s lung-cancer rate had risen 60 percent in the past decade, probably as a result of air pollution.

Feng Yongfeng, a Beijing father of a 3-year-old who founded a nonprofit environmental group called Green Beagle in 2009, argues that the Chinese should protect themselves by investigating their surroundings.

“If the data is real, officials keep it to themselves,” said Mr. Feng, whose organization began this July to lend two PM 2.5 monitors to anyone who completes an online application. “You should not wait for the ministry to tell you the truth. You can find it out for yourself.”

Only 30 people accepted the offer in the first five months. But Wang Quixia, the project manager, said interest had skyrocketed since publicity made PM 2.5 a household phrase in Beijing.

Now there is a two-month waiting list.

Mia Li contributed research.