Last July, NGT directed the transport department to de-register all such vehicles in the capital. Following the order, the authorities stopped issuing no-objection certificates to these vehicles and impounded those found on the road. The Union ministry of heavy industries then filed a plea against this order but their application wasn't discussed until Wednesday.

The Centre claimed that more than 10-year-old passenger cars running on dieselcontribute to only .225% of PM2.5 (fine, respirable pollution particles) emissions in the capital and, hence, they shouldn't be phased out. The matter was argued by additional solicitor general Pinky Anand who referred to IIT-Kanpur's source apportionment study released in 2015, another report called Breathing Cleaner Air — Ten Scalable Solutions for Indian Cities, and another study by Dinesh Mohan, a Volvo chair professor emeritus at IIT Delhi.

NGT was hearing Vardhaman Kaushik's petition against severe air pollution in the city. "You (Centre) say that every kind of fuel causes some or the other kind of pollution. Then by your logic either every type of vehicle should be banned, or everything should be allowed. What is the solution?" a bench headed by NGT chairperson Justice Swatanter Kumar said.

"We wanted to be heard. There has to be decision on old diesel passenger cars. That is why we have started our arguments now and to plead with NGT to reconsider its earlier order," said a lawyer representing the Centre. "We are saying the PM2.5 emissions from more than 10-year-old diesel cars is only .225% because the IIT Kanpur's report says among various transport modes cars contribute 10% of PM2.5 emissions. We have further calculated the contribution from old diesel cars which turns out to be only .225%," he claimed.

The bench will hear ASG's arguments on Thursday too.