There are 22,162 registered autos in Gurgaon, of which 15,666 run on diesel and 6,496 on CNG. According to an estimate, about 7,500 autos are older than 10 years.

The NGT dismissed the Centre's plea, saying emissions from diesel vehicles were quite harmful and equivalent to the emissions of 24 petrol vehicles.

The bench headed by Justice Swatanter Kumar said the tribunal had noted emissions from diesel vehicles were highly carcinogenic and quoted a CPCB study which said one diesel vehicle was equivalent to 24 petrol cars or 84 CNG vehicles on the road. "The report speaks loudly, that it (diesel) is the most harmful of the exhaust emissions, which releases particulate emission that are carcinogenic. Thus, the contention that there will be reduction of carbon dioxide levels by use of diesel is not a positive factor in favour of use of diesel as fuel," the order said.

The Haryana government, in July 2017, had already asked the RTA to draft a phase-out plan for diesel autos from Haryana NCR cities. Pradeep Dahiya, RTA and additional deputy commissioner (ADC), told TOI, "Diesel autos are a known source of air pollution. De-registration of superannuated vehicles is automated in our system. The ban will automatically be implemented, as soon as an old vehicle approaches the RTA for fresh fitness certificate. The inauguration of the e-rickshaw fleet in Gurgaon recently by Union roads and highways minister Nitin Gadkari is evidence the government is focusing on cleaner public transport."

Both HSPCB officials and green activists were happy with the move. "Diesel autos are a big reason behind air pollution. I'm glad NGT and Supreme Court are now monitoring the situation," said Jai Bhagwan, regional officer, HSPCB, which has been pursuing the issue with the RTA for a long time.

AnumitaRoychowdhury, executive director (research and advocacy), and head of air pollution and clean transportation programme at NGO Centre for Science and Environment, said, "Gurgaon's air pollution should reduce significantly if diesel autos are taken off. They emit gases that are extremely harmful for human health. It's high time authorities chalk out a plan to convert diesel autos into CNG. Introduction of e-rickshaws is a welcome step. We need a clean and robust public transport system for the city."

Phasing out 50% diesel autos should bring down air pollution by at least 30%, said experts. "The two-day strike by diesel autos in August 2016 bought down Gurgaon's air pollution by 40%. Phasing out half of these should reduce air pollution by at least 30%. Diesel autos are a potential source of many pollutants, including particulate matter (PM) 2.5, carbon monoxide, sulphur dioxide, nitrogen oxides, hydrocarbons and volatile organic compounds," said Sucheta Kumar, a retired environmental engineer.

Residents were more worried about availability of public transport.

"Even after e-rickshaws were introduced, we're dependent on diesel autos, because e-rickshaws ply in a specific area, whereas diesel autos cover all routes, especially in Old Gurgaon. They are also cheaper than CNG autos. We need alternatives in place before diesel autos are phased out," said Rashi Singh, a college student and Sector 4 resident.

More than one lakh people are dependent on diesel autos for their daily last mile commute.