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NEW DELHI: Almost all of the most polluted cities in India are located in the north with Uttar Pradesh, Punjab and Rajasthan dominating the list, according to a WHO report on the most polluted cities in the world published earlier this year.Incidentally, UP, Punjab and Delhi also lie in the fog belt of northern India and there's evidence to show that air pollution is worsening the problem. Early last year, TOI had reported on a study which found that average fog hours in the month of January in Delhi had increased by as much as eight hours since 1989 — a trend attributed to rising pollution.When the WHO report came out in May, much was made out of the fact that Indian cities dominated the list, with Delhi earning the dubious tag of the most polluted city in the world. But not as much attention was paid to which were the most polluted among Indian cities.Of the worst 30 cities, UP laid claim to nine, Punjab to five and Rajasthan to four. Madhya Pradesh and Maharashtra had three cities each while J&K, Bihar and Uttarakhand had one each. Raipur in Chhattisgarh, Ahmedabad in Gujarat and Delhi rounded out the list. In all, 124 Indian data stations – mostly in cities – were featured in the WHO report.Meanwhile, the 5 cities with the cleanest air were all located in south India. Two were in Kerala – Kollam and Pathanamthitta, one in Karnataka – Hassan, one in Tamil Nadu – Madurai, and Pondicherry.The WHO used the concentration of particles in the air under 2.5 microns to determine how bad air pollution is. Such particles cause the most damage to health because when inhaled, they can penetrate deep into the lungs, whereas larger particles are generally filtered out before they reach that far. Pathanamthitta, the cleanest city on the list, recorded a concentration of 10 parts per million of particles under 2.5 microns. Compare that to Delhi, which has a concentration of 153 parts per million of such particles – more than fifteen times that of Pathanamthitta.Dr Gufran Beig, the director and chief scientist of System Air quality Forecasting And Research (SAFAR) said that this is because many cities in the north are located on the Indo-Gangetic plains region, which stretches from Kolkata through Delhi into Pakistan."In addition to local sources of pollution, Indo-Gangetic cities also get pollution from neighboring regions and other parts of India, so as a result the pollution level becomes very high," Beig said.Because of the location of the Indo-Gangetic plains at the foothills of the Himalayas, climate conditions are such that air masses often converge in the plains, both in the summer and the winter."During the summer, winds sweep towards the Indo-Gangetic plains through south India and from the north through the Himalayas, converging in the plains," Dr. Beig said. "This means the polluted air from south and central India and from the north and Nepal all converge in the plains."Meanwhile, during the winter, winds from the northeast and the west come in and temperatures drop, causing the air over the Indo-Gangetic plains to become very calm."This creates a stagnant condition," Beig said. "The boundary layer which separates the ground atmosphere from the upper atmosphere, comes much closer to the ground because of the cooler temperatures. That makes it much harder for pollution to disperse."On the other hand, south and central Indian cities have more normal weather conditions, according to Beig, making pollution less of a problem there.In addition to climate conditions, there are more sources of pollution in the north that makes the air quality worse, according to Sarath Guttikunda, a emissions researcher and director of UrbanEmissions.info."In north India the winters are harsh, so most people have to burn things to keep warm. More garbage is being burned in the North," Guttikunda said. "Also, there are a large number of brick kilns in the Indo-Gangetic plains. Though you have them in the south as well, they are different in structure and technology – in the north they are coal-based."Thus, despite the south's reputation as a hub of manufacturing, such as Chennai's automobile industry, because of climate conditions and fewer sources of pollution south India tend to suffer less from bad air quality than north India. Kerala, which is both on the coast and has little industrial activity, with an economy dominated by service activities such as tourism and remittances, has many of the least polluted cities in the country.