The World Health Organization (WHO) has ranked an Iranian city as the worst for air pollution in the world.

The WHO said the dirtiest air in the world was in the city of Zabol, located in Iran's restive Sistan-Baluchistan Province near the border with Afghanistan that suffers from months of dust storms in the summer.

The next four most polluted cities in the world were all Indian: Gwalior, Allahabad, Patna, and Raipur.

The WHO said world pollution levels worsened by 8 percent between 2008 and 2013, although most cities in developed countries improved the state of their air over the same period.

The WHO said more than 7 million premature deaths occur every year due to air pollution, 3 million of them due to outdoor air quality.

Tiny particulate matter can cause lung cancer, strokes and heart disease over the long term, as well as triggering symptoms such as heart attacks that kill more rapidly.

The WHO admitted that many of the world's most polluted cities do not have a monitoring system and were not included in its ranking.

Based on reporting by AFP and The Guardian