A dense blanket of smog covered most cities in northeast China this past week, reaching record pollution levels and grounding hundreds of flights at Beijing’s international airport. Public outrage over the quality of the air in the Chinese capital is rising high while the government insists that the problem is down to inclement weather and nothing to be overly worried about.

For most of this week the air in Beijing has been rated as “very unhealthy” and “hazardous” by the US Embassy air monitor, reputed as the most reliable indicator of pollution in the city. On Sunday it posted a new record: “beyond index”, as it registered 522 micrograms of particulate pollutants per cubic meter of air.

More and more Chinese citizens - and not just expats - are turning to the US Embassy's BeijingAir Twitter account for precise data on pollution, especially since Chinese authorities continued to describe the situation as “moderate” despite the thick cloud of smog – “fog,” according to them - that envelops the city. In fact, Beijing’s health authorities insist that the air is perfectly safe 80% of the time, even though the US monitor has rated the air as good only 13 days this year.

A street in downtown Beijing. Posted on YouKu.

There is a reason for such a big difference: the air monitor located on the US Embassy roof measures fine particles of 2.5 micrometers in size, deemed by scientists as the most harmful because they can penetrate the lungs easily. Local authorities only measure coarse particles of 10 micrometers in size, and average the results from several air monitors, including one 20 miles away from the city.

Pollution in Beijing has become one of the most hotly debated subjects on the Chinese web, with public outrage growing after stories surfaced that high-level government officials were equipped with expensive air purifiers. Authorities have tried to ease the controversy by promising to measure small particles by 2015 and opening one of their monitoring centres to the public. They have also questioned the credibility of the US air monitor.

It isn’t the first time the BeijingAir Twitter account has generated controversy. Two years ago, Chinese officials asked the US Embassy to stop tweeting about pollution in Beijing on the grounds that the information was “confusing” and could have “social consequences”, according to a confidential US State Department cable made public by WikiLeaks.