NEW DELHI: Delhi is the most polluted city in the world when it comes to air quality, according to a WHO study released on Wednesday. However, Gujarat and the southern states have relatively cleaner air.The situation is so bad in Delhi that its air has PM2.5 concentrations of 153 micrograms and PM10 concentrations of 286 micrograms — much more than the permissible limits.In comparison, Beijing, which was once considered one of the most polluted cities, has PM2.5 concentration of 56 micrograms and PM10 concentration of 121 micrograms. Air quality is represented by annual mean concentration of fine particulate matter (PM10 and PM2.5, — particles smaller than 10 or 2.5 microns).The database covers the period from 2008 to 2013, with the majority of values for the years 2011 and 2012. Apart from the national capital, Lucknow, Kanpur, Firozabad, Agra, Gwalior and Raipur are among the other Indian cities with the worst air quality.Most of the 124 Indian cities included in the 2014 version of the Ambient Air Pollution database far exceed the safe limits of particulate matter. India, however, is not alone. The WHO database, which covers 1,600 cities across 91 countries, found the air quality in most cities fail to meet WHO mandated safe levels.