The PM 2.5 level being cited by the government for the month of December is an average of the records from six fixed stations of the DPCC. These are located at AnandVihar, Punjabi Bagh, RK Puram, Mandir Marg, Civil Lines and IGI Airport. The current readings, on the other hand, are from mobile monitoring units. These have been assessing air quality for 20 minutes at spots like Connaught Place, Talkatora Stadium, Chanakyapuri, TeesHazari Court, Sadar Bazaar, Delhi Cantonment railway station and DhaulaKuan.

Experts are divided over Rai's claim. His critics feel the government is in a desperate hurry to prove that pollution has gone down and its methodology is erroneous while others believe there can't be any other way of assessing the situation.

A scientist, who refused to be named, said the comparison of air pollution levels monitored through hand-held devices or mobile units with averages recorded by fixed stations isn't "fair" or "accurate". He argued: "Pollution is very location specific. You may not get accurate results if you compare levels at totally different locations because the emissions will obviously be different. The Delhi government should be comparing a day's ambient air quality with another day's when the weather conditions were identical on a non-odd-even day. Two to three factors are very important while comparing levels — maximum and minimum temperature, humidity, wind speed and location." Another scientist pointed out the very low levels quoted by Rai.

A researcher, who was studying on-road emissions with a portable monitoring device in Delhi, had a different view. He said the Delhi government should monitor on-road emissions. "One would expect to see a larger effect of the traffic interventions at on-road levels than at the stationary sites. That's simply because of proximity -- vehicles have a larger effect on on-road PM2.5 levels than on ambient levels. Still, it might be difficult to conclusively say something about the effect of the odd-even restrictions on on-road pollution with only a few days' data," he told TOI.

Centre for Science and Environment (CSE) researchers, who said they were not aware of the process, did offer an analysis. "This winter, compared to the previous high smog episodes, the peak pollution during the odd-and-even programme has been the lowest. This shows that despite the hostile weather conditions - no wind and the presence of a western disturbance - the peak pollution level has been arrested by lowering traffic volume and congestion. This is an important lesson for health protection," said Anumita Roy Chowdhury, head of CSE's 'Clean Air' campaign.

Rai told reporters that at "all the 18 locations, PM 2.5 levels were less than 300 ug/m3. At nine of these 18 locations, PM 2.5 levels were less than 200 ug/m3." He claimed the average reduction in PM 2.5 levels was between 100 and 150 ug/m3, attributing it to the odd-even scheme. "However, the scheme has had no major effect in border areas where PM10 levels are extremely high. PM10, in any case, is not caused by vehicular pollution. It is mainly because of construction dust," he added.

The minister said the odd-even campaign was inspired by the car-free days. "The reduction in pollution levels on one stretch during the car-free days was 40-45%. So, we were convinced that the impact would be apparent during the odd-even plan," he said.

He urged people to fully utilise public transport, saying the buses and the Metro were running below capacity. "Eighty per cent of people are car-pooling," he claimed. "The examples set by the chief minister and the Chief Justice of India have inspired people," he added.