(i) In the 1990s, Delhi ranked fourth among the 41 most polluted cities in the world. Government of Delhi was asked by The Supreme Court to take strong actions, in a limited time frame.

(ii) All the buses of Delhi were converted to run on CNG (Compressed Natural Gas) by the end of 2002. CNG burns more efficiently than diesel or petrol. Also CNG is cheaper than any of the liquid fuels.

(iii) CNG can also not be adulterated or siphoned by the thieves.

(iv) The other steps included:

a) phasing out of old vehicles,

b) use of unleaded petrol,

c) use of low sulphur petrol and diesel,

d) use of catalytic converters in automobiles,

e) Application of stringent pollution-level norms etc.