THE capital’s “airpocalypse”, the choking smog that descended on Beijing in the winter of 2012-13, galvanised public opinion and spooked the government. The strange thing is, though, that information about air pollution—how extensive it is, how much damage it does—has long been sketchy, based mostly on satellite data or computer models. Until now. Responding to the outcry, the government set up a national air-reporting system which now has almost 1,000 monitoring stations, pumping out hourly reports on six pollutants, including sulphur dioxide, ozone and (the main culprit) particulate matter less than 2.5 microns in diameter, or PM2.5. These are tiny particles which lodge in the lungs and cause respiratory disease. The six are the main cause of local pollution but have little to do with climate change, since they do not include carbon dioxide, the main greenhouse gas. Scientists from Berkeley Earth, a not-for-profit foundation in America, have trawled through this recent cloud of data for the four months to early August 2014, sieved out the bits that are manifestly wrong (readings where the dial seems to be stuck, for instance) and emerged with the most detailed and up-to-date picture of Chinese air pollution so far.

Pollution is sky-high everywhere in China. Some 83% of Chinese are exposed to air that, in America, would be deemed by the Environmental Protection Agency either to be unhealthy or unhealthy for sensitive groups. Almost half the population of China experiences levels of PM2.5 that are above America’s highest threshold. That is even worse than the satellite data had suggested.

Berkeley Earth’s scientific director, Richard Muller, says breathing Beijing’s air is the equivalent of smoking almost 40 cigarettes a day and calculates that air pollution causes 1.6m deaths a year in China, or 17% of the total. A previous estimate, based on a study of pollution in the Huai river basin (which lies between the Yellow and Yangzi rivers), put the toll at 1.2m deaths a year—still high.

The sliver of good news is that pollution levels are better in some places than in others. They are worst in the corridor between Beijing and Shanghai and least bad in the south (see map—the study covers China east of 95ºE, accounting for 97% of China’s population), probably because that area was washed by monsoon rains during the period of the study. More importantly, levels of PM2.5 in large western cities such as Chongqing and Chengdu are about half the national average. Figuring out what they are doing right would be a first step towards reducing the smog elsewhere.