Patna: In a recent report released by the World Health Organisation ( WHO ), Patna has been ranked at sixth place out of 795 cities in 67 countries across the globe in terms of air pollution . Four Indian cities have found a place in the top 10 list prepared by the WHO. These cities are Gwalior, Allahabad, Raipur and Patna. Iranian city Zabol has topped in the list of cities with worst polluted air. The WHO had compared a total of 795 cities in 67 countries for the level of small and fine particulate matter (PM10 and PM2.5) with the help of data of five years from 2008 to 2013 to prepare the list. The Bihar State Pollution Control Board (BSPCB), in December last year had issued an advisory for checking air pollution in the city. The steps suggested included stopping exposed burning of garbage and waste, especially by municipal bodies, covered transportation of building materials such as sand, cement, stone chips and mud, covering under-construction structures while the building work is in progress and to ensure that diesel vehicles older than 15 years are not allowed to run in the city. The board, in addition, also issued advisory for the regulation of timing of trucks carrying construction materials in the city. Even on Thursday, the air quality index (AQI) for Patna was measured at 153, which is considered unhealthy. The index level is considered satisfactory when AQI is below 100. In December last year, Patna's AQI had gone up to 378, which could cause respiratory ailments in case of prolonged exposure. The present situation of air quality can be considered good in comparison to the last year-end. The city's level of nitrogen dioxide on Thursday, according to the official website of Central Pollution Control Board (CPCB), was 108.07 micro gram per meter cube, while the prescribed level is 80. Similarly, the level for nitric oxide was 48 micro gram per meter cube while the prescribed level is 18.86. The CPBC monitors the city's pollution parameters with its station at Indira Gandhi Science Complex Planetarium.