India’s capital city of New Delhi has the dirtiest air among the world’s 1,600 cities, says a World Health Organization (WHO) study released on May 7. It is followed by the Indian cities Patna, Gwalior and Raipur. Beijing ranks 77th. The cities with the cleanest air are in Canada, the US and Europe. Cities with dirty air have become death traps, as air pollution killed about 7 million people in 2012, said the WHO.