Just a month before the start of the Beijing Olympics, the city is still failing to meet international air quality standards, the BBC has found.
When Beijing bid for the Olympics in 2001, it said its air would meet World Health Organization (WHO) standards.
The BBC put this to the test using a hand-held detector to test for airborne particles known as PM10.
We found that the city's air failed to meet the WHO's air quality guidelines for PM10 on six days out of seven.
These particles are caused by traffic, construction work and factory emissions. They are responsible for much of this city's pollution.
On one of these days, the pollution reading was seven times over the WHO's air quality guideline.
By comparison, recent readings done in London - the site of the 2012 Games - all fall within the WHO's guidelines.
Beijing insists that there is still time to get things right.
Later this month it is imposing a series of emergency air-quality measures which will take cars off the streets and shut down building sites.
One official has told the BBC that he is confident that Beijing will still fulfil its clean air promise.
But this city does not have all that much time left.