“PM2.5” seems an odd and wonky term for the blogosphere to take up, but that is precisely what has happened in China in recent weeks. It refers to the smallest solid particles in the atmosphere—those less than 2.5 microns across. Such dust can get deep into people's lungs; far deeper than that rated as PM10. Yet until recently China's authorities have revealed measurements only for PM10. When people realised this, an online revolt broke out. Such was the public pressure that authorities caved in, and PM2.5 data are now being published for Beijing and a handful of other cities.

World Health Organisation guidelines suggest that PM2.5 levels above ten micrograms per cubic metre are unsafe. The boffins have found (as the map shows) that almost every Chinese province has levels above that. Indeed, much of the country's population endures air so foul that it registers above 30 on the PM2.5 scale, with Shandong and Henan provinces topping 50. Because these readings reflect the average pollution that a typical resident in a province is likely to endure during a given year, they underplay the sharp spikes in pollution that are seen on particularly dirty days, when spot readings go much higher. That is why Beijingers should take little comfort from the fact that the capital's pollution measures only 35.

This approach is not perfect. Satellites are not great at taking readings over bright surfaces like snow and deserts, and cannot easily distinguish particles high up in the atmosphere from those closer to the ground. And the data also have to be adjusted to take account of the fact that pollution and people tend to coincide. (Otherwise uninhabited areas would drag the figure down, below the average atmospheric conditions actually experienced by the people who live in any given province.)

Such caveats aside, however, this study shows how far China still needs to go in cleaning up its act. Pollution and development have always marched hand in hand, and may even be regarded as tolerable so long as they mark only a temporary blip on the road to prosperity. What is intolerable is that it takes outside intervention to lift the lid on what is happening.