Two years ago, Delhi had the highest PM2.5 levels of 1,600 cities surveyed by the WHO. Last month, in an updated and expanded inventory, Delhi retained its status as the most polluted of the world’s largest cities, with an annual PM2.5 average of 122 micrograms per cubic metre (μg m−3) — three times the permitted Indian standard and greatly exceeding the WHO standard of 10 μg m−3. The pollution, which comes mainly from combustion of wood, coal, gas, diesel and crop residue, is worst in the winter, when wood-burning peaks and cold-weather inversions trap pollutants close to the ground and cause spikes in the daily average of above 600 μg m−3. Late last year, the levels prompted the Delhi High Court to declare the city a “gas chamber”.
“The reality is that the pollution in Delhi is very complex. There are a lot of sources. It varies from season. It varies by time of day. It varies by neighbourhood,” says Namit Arora, a member of the pollution task force of the Delhi Dialogue Commission, a government initiative in the city. But he insists that the city can make progress. “We can act, and we need to act, on multiple fronts simultaneously.”
Delhi is trying to do just that. Long before it was saddled with the mantle of having some of the world’s worst air, the city and the Supreme Court of India took several steps to alleviate pollution. Vehicle emissions came down in the early 2000s, thanks to decisions to remove lead from petrol, improve vehicle-emission standards and pull old commercial vehicles off the roads. Around the same time, the city implemented a monumental conversion of the public-transportation fleet, including buses and the city’s zippy three-wheel auto-rickshaws, away from gasoline and diesel engines to ones fuelled by cleaner compressed natural gas. In 2002, the Delhi Metro subway system opened its first line, improving public-transportation options. All but two of the coal-based thermal power plants in the city were converted to natural gas, and many industries, including brick kilns, were moved beyond Delhi’s bounds.
These efforts reaped big gains, yet they have been offset by the incessant growth of the megacity. Since 2000, Delhi’s population has nearly doubled. And the number of vehicles has almost tripled, from 3 million to more than 9 million, according to the government.