New Delhi: The "odd-even" formula introduced by the Delhi government last year to temporarily restrict the numbers of vehicles on the capital's roads reduces pollution in winter but has no perceptible effect in summer, scientists have said.

Their findings suggest that the Delhi government's decision on Saturday to abandon its plans to impose the odd-even formula for five days next week will deny the capital's population a small yet significant improvement in air quality, they said.

The Delhi government had imposed the odd-even formula - that uses registration numbers to allow vehicles to ply on odd and even days - twice last year.

"The odd-even scheme would have brought some respite - even if the air pollution levels are lower now than what they were earlier this week," said Santosh Harish, a policy researcher at the Energy Policy Institute at the University of Chicago's India centre.

Sections of environmental analysts have pointed out that restricting vehicles addresses only vehicular emissions which, according to multiple estimates, account for an estimated 37 per cent of inhalable particulate matter (PM) in the air.

Harish and his colleagues performed earlier this year what they claim is the most rigorous analysis of the impact of the odd-even formula on air quality measured through the PM levels.

Their study, relying on a statistical technique called "difference in differences", had found that PM levels dropped by 14 to 16 per cent when the odd-even formula was first imposed in January 2016, but had no effect on PM levels in April 2016.

Air quality measurements at various locations had pointed to drops in PM levels from 20 per cent to 40 per cent.

"It's not enough to compare pollution levels before, during and after the odd-even periods," Harish said. "All that it shows is an association - we've used a statistical technique that isolates the effects of odd-even on pollution loads - using three control locations outside Delhi."

Harish and his colleagues Anand Sudarshan and Michael Greenstone at the University of Chicago and Rohini Pande at Harvard University examined pollution levels at seven locations in Delhi and one site each in Faridabad, Gurgaon and Rohtak.

Their analysis found no effect of the odd-even formula on air quality when the government re-imposed it in April 2016. The scientists say the warmer temperatures in April than in January offer one possible explanation for their observations.

A warmer temperature expands the volume of atmosphere near the ground where pollutants are trapped.

"We don't know the exact reason, but it is possible that the reduced emissions in April did not translate into observable drops in pollution concentrations in the larger volume of air available for the dispersion of pollutants," Harish said.

"Imagine a large container with water and a smaller one - if the same amount of ink is dropped in both, the smaller one will look darker than the other."