In the early hours of a January morning, all lanes of a motorway into Delhi are crammed with hundreds of rumbling lorries. These are brightly painted, with signs imploring overtaking drivers to “Honk please”. Every night immense convoys like this one snake their way into the Indian capital, belching sulphurous diesel smoke. The sinking winter air presses the resulting smog tight over the city.

The lorries are a chief reason why Delhi’s air is now more toxic than any other city’s on earth. Admittedly Beijing has a worse reputation, with its visible smog from particulates of 10 microns or smaller, known as PM10. Delhi’s grim distinction is that it has even higher levels of PM10, as well as of the smaller particulates, PM2.5, that are more likely to kill because they go deeper into the lungs. Levels of PM2.5 in Delhi are routinely 15 times above levels considered safe by the World Health Organisation. New data suggest that, on this score, Delhi’s air has been 45% more polluted than that of the Chinese capital for the past couple of years.

Last year the WHO assessed 1,622 cities worldwide for PM2.5 and found India home to 13 of the 20 cities with the most polluted air. More cities in India than in China see extremely high levels of such pollution. Especially to blame are low standards for vehicle emissions and fuel. Nor, for different reasons, are rural people better off. Indoor pollution inhaled from dung-fuelled fires, and paraffin stoves and lights, may kill more than 1m Indians a year. The WHO says the vast majority of Indians breathe unsafe air. The human cost is seen in soaring asthma rates, including among children. PM2.5 contributes to cancer and it kills by triggering heart attacks and strokes. Air pollution is likely to cause vastly more deaths as Indians grow older and more obese. Indoor and outdoor pollution combined is the biggest cause of death, claiming over 1.6m lives a year.

Michael Greenstone of Chicago University has led research into pollution-affected lifespans in China that has implications for India. The lives of northern Chinese, he found, are 5.5 years shorter on average because of air pollution. In a forthcoming article, he applies the same methods to assess the 660m Indians most exposed to toxic air. He concludes that they would each live over three years longer, on average, if their air met national standards.