AIR pollution in India and China is already a great threat to human health. By one estimate, northern Chinese lives are 5.5 years shorter than they should be, all because of foul air. The lives of nearly 700m Indians would, on average, be over three years longer if national standards for air quality were met. One big problem is indoors: particles from smoke from dung or wood fires, or from paraffin stoves and lamps. In India indoor air pollution is thought to cause 1m deaths a year. But the fastest growing threat is outdoor air pollution, especially in cities. Air pollution is caused by traffic fumes, industrial emissions, construction dust, burning rubbish and more. It is thought already to kill nearly 700,000 Indians a year, and that toll is likely to rise sharply in the coming decades.

India's poor fuel standards, poor-quality vehicles and badly planned cities help to explain why so many of its cities are choked with awful air. Even China, notorious for problems with smog and pollution, appears better off than India. Michael Greenstone of Chicago University points out that far more cities in India (with Delhi the worst, by far) endure very high average levels of small particulate matter known as PM2.5. This is particularly deadly, as particles settle deep in the lungs, causing cancer, heart attacks and strokes. Air pollution affects not just the big cities. Much of northern India along the Gangetic plain is draped in smoggy air for large parts of the year. This is also bad for crops. One study of wheat yields in India, from 1980 to 2010, suggests these are one-third lower than they should be because of high exposure to ozone and other air pollution.