One of the most popular iPad apps in Beijing at present is China Air Pollution Index. The app is both addictive and disturbing. When I checked into a Beijing hotel recently, I found that – even from the 40th floor – I couldn’t see further than one block because of the grey smog enveloping the city. So I checked the numbers and discovered that the AQ level, which measures fine particulates that are especially dangerous, was 250 – about five times the level deemed safe.

A few weeks earlier I had experienced Beijing at 350, a level that made my eyes sting and lungs ache, even inside a car. Back in January it went as high as 1,000, at which point the US embassy urges people to remain indoors with the windows shut.

In fairness, when the wind blows the pollution away from Beijing, the sky can be blue and it is suddenly a pleasure to walk around. But a good day in Beijing is often a bad day elsewhere in the country.