CHINA is the world’s biggest polluter, so it is no surprise that it fares poorly on some measures of pollution in a new global index of environmental performance. The shock is that it also stands out for its world-beating greenness in other areas on the same index.

The Environmental Performance Index (EPI), a joint product of America’s Yale and Columbia universities, is the latest volume in a long-running biennial ranking of 178 countries on a variety of measures of environmental performance. New this year are assessments of performance in waste-water treatment and combating climate change, as well as the clever use of satellite data (to track trends in forestry and air pollution) in order to top up traditional computer modelling and official data.

The report’s conclusions are more cheerful than most green report cards. The experts believe countries are doing well in improving access to safe drinking water and sanitation, and in bringing down child mortality. However, the global trends are worrying in other areas like fisheries, wastewater treatment and air quality. Overall, Switzerland came out top. Somalia came last. China was 118th, a middling ranking that beats India (155th) but falls well below South Africa (72nd), Russia (73rd) and Brazil (77th).

However, that average masks a huge divergence in China’s performance in two areas. Using satellite data, the boffins worked out, for the first time, what global exposures were to fine particulate matter (known as PM2.5) from 2000 to 2012. China ranked at the bottom on air pollution, with nearly all of its population exposed to levels of PM2.5 pollution deemed unhealthy by the World Health Organisation (WHO). Though less frequently criticised than Beijing, Delhi’s air is also terrible—but China as a whole fares worse. In 2012 the average human exposure to PM2.5 for all of China was 48 micrograms per cubic metre, but the national figure for India was only 32 units (the WHO says anything above 10 units is unhealthy).