Some cities in Iran, India and Pakistan have the worst air quality on Earth, according to a new report from the World Health Organization.

The Associated Press reports that Ahvaz, the capital of Khuzestan province in southwestern Iran, is one of the most polluted cities in the world, by a significant margin, in terms of PM10 air particles.

PM10 particles are produced mostly by power plants and car exhaust emissions, and they can cause cancer, respiratory infections and heart disease.

The WHO guidelines call for PM10 readings lower that 20 micrograms per cubic metre, but the annual average in Ahvaz was reported at 372 micrograms per cubic metre. Heavy industry and low-grade fuels are two of the main causes of pollution in this city, which has a population of 1.3 million.

Ulaanbaatar, the Mongolian capital, trails Ahvaz as the next-worst polluted city, followed by Sanandaj in northwestern Iran.

The WHO announced that there are different causes of pollution but the main culprits are industrialization, low-grade transportation fuels and energy production.

Some Iranian MPs have pointed to the use of low-grade gasoline as one of the major causes of pollution in Iranian cities.