European Pressphoto Agency The Chinese national flag flies in front of a building barely visible due to heavy smog in Beijing, December 02, 2011.

Agence France-Presse/Getty Images A traveler looks out at an airplane shouded in smog while it queues to take off at Beijing International Airport on December 5, 2011. Beijing authorities cancelled hundreds of flights and shut highways as thick smog descended on the Chinese capital on December 4 and 5, reducing visibility at one of the world's busiest airports.

NOTE: This post has been changed since it was first posted. See below.

Fog or smog?

For years, China’s meteorologists have been telling the country’s urban residents that what looks and smells suspiciously like that latter is, in fact, just the former. With no way to disprove the claim, most Chinese urbanites have appeared content to accept it as true and go about their business minus the facemasks and indoor air-filters they might otherwise bring to bear.

Lately, however, there seems to have been a fundamental shift in the willingness of Chinese city-dwellers to accept the government’s definition of fog at face value.

Case in point: A thick grey cloud currently enveloping northern China that has closed highways across multiple provinces and caused more than 300 flights into Beijing to be delayed or cancelled.

While state media have described the cloud as a “heavy fog,” millions of posts on popular Chinese microblogging service Sina Weibo and other Internet sites are treating it as something else. “How many thousands died because of London’s fog back in the day?” Weibo user Zheng Wuxie wrote on Monday. “Beijing is dangerous.”

“Friends in Beijing, are you OK?” wrote another Weibo user, CAPF Green, attaching a screenshot of a mobile app powered by the U.S. Embassy’s @BeijingAir Twitter feed showing dangerous pollution levels.

The U.S. Embassy in Beijing, which broadcasts readings from its own pollution monitoring equipment on an hourly basis through Twitter, has been instrumental in piercing the veil around air quality in China’s capital -- particularly in the month or so since celebrity real estate mogul Pan Shiyi cited its readings in calling for tougher air monitoring standards.

Authorities in Beijing and most other Chinese cities measure air pollution by counting only particles between 2.5 and 10 micrometers in diameter. The embassy counts particles smaller than 2.5 micrometers (PM2.5), which experts say make up the most of the city’s air pollution and cause more damage to the lungs. While Twitter is blocked in China, third-party developers have used the embassy's feed to build mobile apps that are accessible inside the country.

Agence France-Presse/Getty Images This combo shows photos of Beijing pollution (top) on December 5, 2011 compared to a blue sky day (bottom) on August 18, 2011.

The most recent online outpouring seems to have been set off on Sunday night, when the embassy published a PM2.5 air quality index reading above 500 – a level expats refer to as “Crazy Bad” – that contrasted sharply with the Beijing Municipal Bureau of Environmental Protection’s description of air pollution over the weekend as “light.”

While many Internet users have marveled at the discrepancy, others have been busy trading advice on the best types of facemasks to wear and what brands of air purifiers offered the best cost-benefit ratios.

“I searched for half the day and asked some experts,” Caijing magazine editor He Gang wrote in a Weibo post on the topic of facemasks that had been forwarded more than 4600 times by Monday evening. “Everyone recommends 3M masks. There’s all kinds – I hear the ones most suited to Beijing are No. 8264 and No. 8210, 3-5 yuan a piece. I’m going out to buy a few tomorrow.”

All this may be a boon to producers of purification paraphernalia, but it bodes poorly for the credibility of the government.

An article in the state-run Global Times tabloid on Monday quoted Yu Jianhua, director of the air pollution division of the Beijing environmental protection bureau, insisting that pollution in Beijing has not gotten worse since the 2008 Olympics. "If you compare the air quality on an annual basis, it is actually improving," he said, according to the paper. Yet the government’s pointed refusal last week to release PM2.5 readings – data that’s officially reserved for researchers and won’t be made public on a national level until 2016, according to current plans – has lately made that claim a hard sell.

The same holds for the government’s claim that what has kept planes from landing at the Beijing airport and blocked the city’s residents from seeing more than a couple blocks in any direction over the past few days is primarily fog. It’s a claim that might very well be true, but the online reaction suggests few are buying it.

Amateur video of what Beijing's street's looked like Sunday morning:

CORRECTION: The U.S. Embassy in Beijing publishes pollution readings through a Twitter account. A previous version of this post mistakenly said the embassy also publishes the readings through an iPhone app. The iPhone app, as well as other mobile apps that make use of the @BeijingAir Twitter feed, are not associated with the embassy.

-- Josh Chin. Follow him on Twitter @joshchin