Noting that diesel is the prime source of air pollution over Delhi, the NGT - a judicial agency that hears environmental cases - had directed on April 7 that all diesel vehicles, whether heavy or light, that are 10 years or older will not be permitted on the roads of Delhi and the National Capital Region.

The ruling comes amid long-standing concerns among environmental experts and others over the health impacts of air pollution, including tiny soot particles (particulate matter) emitted by engines powered by diesel.

Air quality data from the Central Pollution Control Board suggest that over 137 cities and towns across India had during 2012 average concentrations of PM-10 (particulate matter 10 micrometres in diameter) that exceeded the prescribed standards.

The NGT, responding to a petition on high levels of particulate matter in the capital, has also directed the Delhi transport department and other concerned authorities to prepare comprehensive data on registration of all vehicles that are 10 years or older.

"A phase-out of old diesel vehicles will contribute to cleaner air even if owners buy new diesel vehicles," Avinash Kumar Agarwal, professor of mechanical engineering, and a specialist in engines, at IIT Kanpur, told this newspaper.

"Diesel vehicles manufactured 10 years ago or earlier are based on old technology associated with higher levels of emissions than diesel vehicles sold today which use new fuel injection technology that leads to lower emissions and higher fuel efficiency."

But environmental activists who have been urging the government to disincentivise the use of diesel in privately owned vehicles caution that buying new diesel vehicles to replace old ones would not yield the same gains in air quality as buying petrol cars would.

"Keeping one diesel vehicle off the road is the equivalent of keeping three to five petrol-run cars off the road," said Anumita Roychowdhury, executive director who runs the Right to Clean Air Campaign at the Centre for Science and Environment, an NGO in New Delhi.

The emission norms for diesel-run vehicles are higher than those for petrol-fuelled vehicles, she said. While the primary emissions of diesel vehicles are particulate matter and oxides of nitrogen, petrol engines emissions are mainly made up of hydrocarbons and carbon monoxide.

The CSE has been urging the government to disincentivise diesel cars through additional taxes - an additional tax upfront and an annual tax to nullify what it claims are the "perverse benefits" that car owners gain through low-taxed diesel.

Engineers say diesel engines will continue to remain the only option for heavy load-bearing vehicles such as trucks and buses. "The diesel engine is more sophisticated than the petrol engine, it can work under heavy loads," said Arindrajit Chowdhury, professor of mechanical engineering at IIT Bombay.