An effort by the World Health Organization to measure pollution in cities around the world has found New Delhi admits to having the dirtiest air, while Beijing's measurements, like its skies, are far from clear.

The study of 1600 cities found air pollution had worsened since a smaller survey in 2011, especially in poorer countries, putting city-dwellers at higher risk of cancer, stroke and heart disease.

Pollution in New Delhi has been named the worst of any city by the World Health Organisation. Credit:AFP

Air pollution killed about 7 million people in 2012, making it the world's single biggest environmental health risk, the WHO, a United Nations agency, said last month.

Thirteen of the dirtiest 20 cities were Indian, with New Delhi, Patna, Gwalior and Raipur in the top four spots. The Indian capital had an annual average of 153 micrograms of small particulates, known as PM2.5, per cubic metre.