Why is this city the worst air polluter?

New Delhi has world's most toxic air
cnntoday new delhi worlds most toxic air_00001119

    JUST WATCHED

    New Delhi has world's most toxic air

MUST WATCH

New Delhi has world's most toxic air 02:35

(CNN)CNN's Sumnima Udas investigates why the Indian capital, New Delhi, has the worst air quality in the world -- so bad, it shaves three years off the lives of hundreds of millions of Indians. Which other world capitals are the worst offenders?