India air index: In world's most polluted city, Delhi
India has begun monitoring air quality in ten of its cities, in an effort to fight some of the world's worst urban pollution.
Last year, the World Health Organization said that 13 of the 20 most-polluted cities in the world were in India - with Delhi considered the worst.
Some environmental activists have criticised the new air-quality index, saying that simply measuring pollution levels is not enough.
The BBC's Sanjoy Majumder reports from Delhi.
06 Apr 2015