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.

A former UN chief negotiator on climate change, Yvo de Boer, suggests that air pollution costs China the equivalent of a tenth or more of GDP and he warns India to avoid that fate. He urges India to “industrialise in a cleaner way”. And a study of agriculture in India from 1980 to 2010 found soaring levels of ozone and other air pollution, which has led to wheat yields a third lower than would otherwise have been expected.

India’s leaders are starting to act, pressed by anti-smog campaigns such as the one by the Times of India to “let Delhi breathe”. Prakash Javedekar, the environment minister, says that monitors have been installed on thousands of industrial chimneys to gather data on emissions. Now officials have identified the country’s 17 most polluting industries.

In theory, at least, every Indian city is now supposed continuously to measure air quality. But state governments are slow to enforce national orders, while the Central Pollution Control Board, India’s main environmental agency, does little. Mr Javedekar promises “aggressive action” to improve fuel standards, which would cover those belching lorries coming into Delhi. In March the Supreme Court may anyway order standards to be tightened, by reducing sulphur, as well as instructing carmakers to cut vehicle emissions.

Some good initiatives to improve air quality are under way. A research project near Patna in Bihar proposes retrofitting the chimneys of brick kilns in ways that reduce smoke. The scrapping of subsidies on petrol and diesel in recent years has had the welcome effect of raising the costs of running especially noxious generators, which may account for nearly a third of all installed electricity capacity. Given that most people, even in the city, still commute by foot, bus or bicycle—and that only 5% of households own cars—India still has time to set up systems for mass public transport before the car becomes king. Already 14 cities have or are building metros. As for farmers, Sunita Narain, a green activist, says they should be pressed to use modern harvesting machinery that renders it unnecessary to burn stubble in fields, a big cause of air pollution.

One experiment could prove particularly welcome. Three industrialised states—Gujarat, Maharashtra and Tamil Nadu—are about to launch the world’s first market for trading permits in emissions of particulate matter. In the town of Surat, in Gujarat, 300 textile plants, which typically burn coal to produce steam, are likely to be the first to trade such permits. Monitoring equipment has already gathered emissions data from these and other plants.

Factories could quickly cut emissions by a lot once they have incentives to do so. They could, for example, clean their equipment better or burn fuel more efficiently. The market can function once India’s central government gives the order. That is likely to happen, however, only once the law allows financial rather than criminal penalties for owners whose plants breach legal standards. No one knows how long the change will take. Lots of bright ideas exist for tackling air pollution. Their widespread implementation, however, depends to a great degree on how much the public makes a fuss about inaction. As lorry drivers might say, honk please.