10. Lucknow, India - 96 µg/m3 of PM 2.5
Lucknow, a city in northern India, starts off the top 10 cities with the worst air pollution levels list. It still significantly has a high average air pollution level that falls into the "unhealthy" category. Vehicle emissions are a major factor in Lucknow's air pollution problem.
9. Ahmedabad, India - 100 µg/m3 of PM 2.5
India's western city Ahmedabad gets its air pollution in part from the major construction happening in the city.
8. Khorramabad, Iran - 102 µg/m3 of PM 2.5
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.
7. Rawalpindi, Pakistan - 107 µg/m3 of PM 2.5
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.
6. Peshawar, Pakistan - 111 µg/m3 of PM 2.5
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.
5. Karachi, Pakistan - 117 µg/m3 of PM 2.5
Vehicle and industrial emissions combined with rapid urbanization puts Karachi as the city in Pakistan with the most air pollution.
4. Raipur, India - 134 µg/m3 of PM 2.5
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.
3. Gwalior, India - 144 µg/m3 of PM 2.5
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.
2. Patna, India - 149 µg/m3 of PM 2.5
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."
1. Delhi, India - 153 µg/m3 of PM 2.5
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.
In comparison, Beijing's average PM 2.5 is 56, though other calculations put it much higher.
(Source)