The air quality in Beijing was “excellent' yesterday, according to a local government website whose launch last year was hailed as a breakthrough in transparency.

The US embassy website told a different story, however. The Americans have their own machine to measure pollution levels at their fortified compound in Beijing and, at 7pm last night, the reading was “unhealthy”.

To be sure, the new US embassy is not far from a motorway and the 7pm reading catches the end of rush hour, which might explain some of the difference. But the main reason is that they measure different things. Beijing monitors coarser particles while the US measures the finer pollutants that are also potentially more dangerous to human health. It is not uncommon for the official figures to give the city a clean scorecard – “blue-sky days”, as the government calls them – while the US embassy figures are screaming “health risk”.