India's air monitoring centre Thursday dismissed data released by the World Health Organisation that showed New Delhi's air as the dirtiest worldwide, saying the finding was biased and misleading.
A study of 1,600 cities across 91 countries released Wednesday by WHO showed Delhi had an annual average concentration of airborne small particles of less than 2.5 micrometres in diameter, known as PM 2.5, of 153.
This was almost three times as high as the reading for Beijing of 56 despite the Chinese capital's reputation for smog, and 10 times that of London.
"We have data for New Delhi which is not biased," Gufran Beig from the state-run System of Air Quality Weather Forecasting and Research (SAFAR) told AFP.
"It takes into account the data taken from 10 air quality monitoring stations spread around the capital in an unbiased way. It is not misleading," Beig said.
The WHO used data for New Delhi from 2010 to 2013 from five monitoring stations in residential and other areas.
The data from China, where authorities are under pressure to be more transparent about pollution, was from 2010, the last year for which figures were available.
"We are now studying the data for 2011-14 which we have procured from the US embassy in Beijing. This will help us arrive at a more accurate comparison," Beig said.
The WHO stressed that its new air pollution database, which relies mainly on data gathered by the cities themselves, did not aim to rank cities, pointing out that "some of the worst ones ... are not collecting data regularly."
PM2.5 particles are very small in size and can easily enter the body and interfere with the functioning of the lungs.
They are also associated with increased rates of chronic bronchitis, lung cancer and heart disease.
WHO says concentrations of the larger PM10 particles should remain below 20 micrograms per cubic metre, averaged out over the year, while the limit for PM2.5 is set at 10 micrograms.
While Delhi ranked as worst on the PM2.5 scale, on the PM10 measure others were far more polluted.
Peshawar and Rawalpindi in neighbouring Pakistan trumped all other cities with readings of 540 and 448 respectively.
Delhi has had its air quality under scrutiny for some time now with a research by Yale University scientists in January this year also suggesting it was worse than Beijing.
A World Bank report last year that surveyed 132 countries ranked India 126th for environmental performance and last for air pollution.
State-backed Indian scientists have repeatedly denied the findings.
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