New Delhi may be the world’s most polluted city, but it’s making an effort to relinquish that title. With pollution from particulate matter at potentially lethal levels early last December, city officials took a drastic step: they announced that they would temporarily restrict the use of private vehicles by allowing owners to drive only on alternate days, based on the sequence of their number plates.

The initial results of that 15-day trial, which began on January 1, are now in. Although traffic actually increased in the first week of the ban, the levels of PM2.5 — parti­culate matter measuring less than 2.5 micrometres across—fell by roughly 10%. That is a victory not just for New Delhi officials, but also for the scientists who sprang into action to collect the data necessary to determine whether the test had achieved its goal.

“This experiment with ‘live research’ has been really quite exciting,” says Santosh Harish, assistant director of the India centre of the Energy Policy Institute at the University of Chicago (EPIC-India). EPIC-India and the New Delhi-based Council on Energy, Environment and Water (CEEW), an independent think tank, used video monitors around the city to document the types and numbers of vehicles on the roads. The groups had less than a month to collect baseline data before the driving restrictions began.

But they weren’t the only researchers interested in Delhi’s living lab. Economist Gabriel Kreindler of the Massachusetts Institute of Technology in Cambridge scrambled to secure human-study approval and funding for a survey of driver behaviour during the traffic restrictions. Within 18 days of the announcement of the driving ban, he had arrived in New Delhi to oversee a surveying team from the Abdul Latif Jameel Poverty Action Lab’s office there. Kreindler’s work eventually found that the alternate-day restrictions were well received by most drivers, who, in spite of the disruption, were willing to comply and alter their behaviour for short periods of time.

Other researchers built on work already under way. The Centre for Science and Environ­ment (CSE), a non-profit research and advocacy group in New Delhi, had been closely analysing government air-quality data since last October. By December, government monitors were recording daily levels of noxious PM2.5 in the range of 400–600 micrograms per cubic metre. This is much higher than the Indian legal standard of 60 micrograms (which itself is more than double the 25-microgram target threshold set by the World Health Organization).

PM2.5 particles cause more than 600,000 premature deaths in India each year, from lung cancer, asthma, and cardiovascular and respiratory diseases. There is no known safe level for this pernicious pollutant.

The CSE’s analysis found that, despite unfavourable weather conditions, the peak pollution during the driving scheme was lower than it would have been without the restrictions in place.

“The region is geographically disadvantaged,” says M. P. George, a scientist with the government’s Delhi Pollution Control Committee. In winter, particulate levels can be twice as high as during the summer, because ‘inversion layers’ of warm air trap cold air close to the ground. This prevents pollution from dissipating into the atmosphere. Emissions from vehicles and construction dust also combine with raised levels of black carbon generated from winter sources—fires for warmth, brick kilns that are lit in the autumn, and widespread field burning in neighbouring states.

“It’s a very simple math,” says Sarath Guttikunda, director of the independent research group Urban Emissions, which is registered in New Delhi. “In winter, your air volume is going down and your emissions are going up.”

Because atmospheric conditions such as wind and temperature can greatly affect particulate-matter measurements, researchers from EPIC-India and the Evidence for Policy Design initiative at Harvard University in Cambridge, Massachusetts, gathered data from air-quality monitors in New Delhi and placed monitors in three adjacent cities as a control. They found that the daily level of PM2.5 pollution in Delhi dropped by 10–13% during the vehicle restrictions. Hourly comparisons showed an even greater improvement, at times an 18% fall.

The question now is whether New Delhi, the capital of a nation with dozens of growing cities choked by pollution, can build on the experiment for long-term gains in air quality. “Delhi has to get it right,” says Namit Arora, a member of the pollution task force of the Delhi Dialogue Commission, a government initiative. This will require long-term strategies and coordination between local, regional and national efforts, he says, as well as a reduction in all sources of air pollution. Other researchers stress the need for more open-access data from a wide range of well-calibrated instruments.

But the driving-restriction experiment has given researchers a tantalizing glimpse of one possible future. “We need to re-imagine the way we think about cities,” says Hem Himanshu Dholakia, a research associate at the CEEW. “That’s the real opportunity.”

This article is reproduced with permission and was first published on February 17, 2016.