A WHO study of 1,600 cities has found air pollution has worsened since a smaller survey in 2011, putting city-dwellers at a higher risk of cancer, stroke and heart disease.
New Delhi was found to have the dirtiest air, with an annual average of 153 micrograms of small particulates, known as PM2.5, per cubic metre.
But the Central Pollution Control Board of India has dismissed the claim, with government scientists saying the United Nations agency has overestimated pollution levels.
Thirteen of the dirtiest 20 cities were in India, the WHO said, with New Delhi, Patna, Gwalior and Raipur taking the top four spots.
Beijing was in 77th place with a PM2.5 reading of 56, little over one third of Delhi’s pollution level.
However, Gufran Beig, chief project scientist at the Indian Institute of Tropical Meteorology, said New Delhi’s air quality was better than Beijing’s, at least during the summer and the monsoon season.
Pollution levels in winter are relatively higher in New Delhi because of extreme weather events, Mr Beig added.
“The value which has been given in this (WHO) report is overestimating (pollution levels) for Delhi… the reality is that the yearly average is around 110 (micrograms).”
After the WHO study was released on Wednesday, Mr Beig said he analysed air pollution levels in Beijing using data available on the US Embassy’s website.
He found the Chinese city’s average to be around 100, nearly double the WHO’s estimates.
Air pollution killed about 7 million people in 2012, making it the world’s single biggest environmental health risk, the WHO said last month