Thirteen of the dirtiest 20 cities were in India, the WHO said, with New Delhi at the top.

New Delhi, May 10 (ANI): A World Health Organisation (WHO) study ranks New Delhi as the world's worst city for air pollution, with an annual average of 153 micrograms of small particulates, known as PM 2.5, per cubic metre.
The WHO study of 1,600 cities released on Wednesday found air pollution had worsened since a smaller survey in 2011, putting city-dwellers at a higher risk of cancer, stroke and heart disease.
"The number of vehicles has increased exponentially and the number of diesel vehicles has increased. Diesel is particularly, extremely toxic, it is also the key reason why there are such high levels of particulate pollution in the city of Delhi," said Director-General of the Centre for Science and Environment Sunita Narain.