Dense, choking smog has blanketed Beijing over the past week, with the capital's air quality at the weekend plunging to one of its worst levels since before the Olympics three years ago.
It was so bad at one point on Sunday night that a pollution reading from the US embassy described it as 'beyond index', meaning that it exceeded the upper limit for a 'hazardous' day by US standards.
But figures by the central government indicated that the air was only slightly polluted, despite poor visibility throughout the city due to the thick haze, which frequently hangs over the capital.
Although there were no reports of the pollution causing disruptions at outdoor sporting events on Sunday, including the China Open women's tennis final and the final stage of the inaugural Tour of Beijing cycling race, local meteorological officials issued a rare warning advising residents to avoid going outdoors, citing health hazards.
While it is no secret that Beijing's air quality has markedly deteriorated since the 2008 Olympics, environmentalists have warned that the government's lack of transparency in monitoring air pollution and in its subsequent clean-up efforts have made the situation worse.
A World Health Organisation survey late last month ranked Beijing as one of the dirtiest capital cities in the world, as well as one of the most polluted cities on the mainland.
According to the hourly pollution data published on Twitter by the US embassy, levels of PM2.5, or particles smaller than 2.5 microns, surged past 500 micrograms per cubic metre against a maximum rating of 500 between 8pm and 10pm on Sunday.
By US standards, a level above 100 is considered unhealthy to those with lung or heart diseases, and readings above 300 are deemed 'hazardous for the entire population'.
The US data, based on a single monitoring station near the embassy in eastern Beijing, has been no stranger to controversy since being introduced in 2009.
Mainland authorities, who insist that Beijing's air quality has improved steadily with the city's decade-long pollution-control efforts, have frequently been embarrassed by the embassy's shocking pollution readings, which stand in stark contrast to those released by the city.
In November, air pollution readings that also hit the maximum 500 for several hours were described in one embassy tweet as 'crazy bad', sparking an outcry among expatriates and locals.
The embassy soon deleted the tweet and has replaced the controversial description with 'beyond index' ever since.