The National Capital stood sixth in the country in terms of air pollution in January, with Muzaffarpur and Varanasi topping the list, according to the latest data published by the Central Pollution Control Board (CPCB).<br/> The data compares pollution levels of 24 cities on the basis of their Air Quality Index (AQI). Muzzaffarpur in Bihar and Varanasi in UP are coded as 'severe' in terms of air pollution during January with an AQI value of 409 each. Delhi was 'very poor' at 362.<br/>The AQI is determined on the basis of concentration of eight pollutants, including Particulate Matter, sulphur dioxide, nitrogen dioxide, carbon monoxide, ozone, ammonia and lead.<br/>The World Health Organization (WHO) had earlier termed Delhi to be the most polluted city in the world in terms of air pollution.