These 10 cities have the worst air pollution in the world, and it is up to 15 times dirtier than what is considered healthy

India's western city Ahmedabad gets its air pollution in part from the major construction happening in the city.

Khorramabad, a city in western Iran, had the country's highest air pollution levels. One of the most populous cities in Iran, Khorramabad is an agriculture hub, which likely contributes to its air pollution problems.

Rawalpindi, the twin city of Pakistan's capital Islamabad, has a high average air pollution because of its heavy use of private vehicles. Rawalpindi just launched a mass transportation system in 2015, so maybe that will knock it off the top 10 list in the not-too-distant future.

With a population of 3.5 million people, Peshawar joins the list because of the usual suspects (vehicle and industrial emissions), as well as brick kiln emissions from the brick-making industry in the city.

Vehicle and industrial emissions combined with rapid urbanization puts Karachi as the city in Pakistan with the most air pollution.

Raipur, a city in central India, is the fourth most polluted Indian city in terms of average PM 2.5 levels. The reason for the high pollution is coal-powered plants and aluminum and steel production.

Of the top 20 most polluted cities, 13 were in India — including Gwalior. The Times of India attributes Gwalior's bad air pollution to coal-powered plants, cars, and inefficient energy use in buildings.

Patna, the second largest city in eastern India, is a major agricultural trade center, suffers from high pollution in part because of transportation, power generation and other industries. Its PM 2.5 levels are considered "unhealthy."

Delhi, is the only one on the WHO's list that qualifies as a "very unhealthy" average PM 2.5 level. Its high pollution level is due to motor vehicles and nearby factories.

(Source)