In the 1990s, Delhi ranked fourth among the 41 most polluted cities of the world.
Air pollution problems in Delhi became so serious that a public interest litigation (PIL) was filed in the Supreme Court of India.
After being censured very strongly by the Supreme Court, under its directives, the government was asked to take, within a specified time period, appropriate measures, including switching over the entire fleet of public transport, i.e., buses, from diesel to compressed natural gas (CNG).
All the buses of Delhi were converted to run on CNG by the end of 2002.
CNG burns most efficiently, unlike petrol or diesel, in the automobiles and very little of it is left unburnt.
Moreover, CNG is cheaper than petrol or diesel, cannot be siphoned off by thieves and adulterated like petrol or diesel.
The main problem with switching over to CNG is the difficulty of laying down pipelines to deliver CNG through distribution points/pumps and ensuring uninterrupted supply.
Simultaneously parallel steps taken in Delhi for reducing vehicular pollution include phasing out of old vehicles, use of unleaded petrol, use of low-sulphur petrol and diesel, use of catalytic converters in vehicles, application of stringent pollution level norms for vehicles, etc
The Government of India through a new auto fuel policy has laid out a roadmap to cut down vehicular pollution in Indian cities.
More stringent norms for fuels means steadily reducing the sulphur and aromatics content in petrol and diesel fuels.
Euro II norms, for example, stipulates that sulphur be controlled at 350 parts-per-million (ppm) in diesel and 150 ppm in petrol. Aromatic hydrocarbons are to be contained
at 42 per cent of the concerned fuel.
The goal, according to the roadmap, is to reduce sulphur to 50 ppm in petrol and diesel and bring down the level to 35 per cent.
Corresponding to the fuel, vehicle engines will also need to be upgraded.
The Bharat Stage II (equivalent to Euro-II norms), which is currently in place in Delhi, Mumbai, Kolkata, Chennai, Bangalore, Hyderabad, Ahmedabad, Pune, Surat, Kanpur and Agra, will be applicable to all automobiles throughout the country from April 1, 2005.
All automobiles and fuel-petrol and diesel – were to have met the Euro III emission specifications in these 11 cities from April 1, 2005 and have to meet the Euro-IV norms by April 1, 2010.
The rest of the country will have Euro-III emission norm compliant automobiles and fuels by 2010.

Latest:

The Supreme Court of India has ruled that no Bharat Stage IV vehicle shall be sold across the country with effect from April 1, 2020.
Instead, the Bharat Stage VI (or BS-VI) emission norm would come into force from April 1, 2020 across the country.
The major difference between the existing BS-IV and forthcoming BS-VI norms is the presence of sulphur in the fuel. While the BS-IV fuels contain 50 parts per million (ppm) sulphur, the BS-VI grade fuel only has 10 ppm sulphur content. Also, the harmful NOx (nitrogen oxides) from diesel cars can be brought down by nearly 70%. In the petrol cars, they can be reduced by 25%.