The Iranian city of Ahvaz walked away with the unfortunate distinction of having the highest measured level of airborne particles smaller than 10 micrometers.
WHO released the list to highlight the need to reduce outdoor air pollution, which is estimated to cause 1.34 million premature deaths each year.
The list measures the levels of airborne particles smaller than 10 micrometers — so-called PM10s — for almost 1,100 cities.
WHO recommends an upper limit of 20 micrograms for PM10s, which can cause serious respiratory problems in humans. They are mostly sulfur dioxide and nitrogen dioxide from power plants, auto exhausts and industry.
Ahvaz's annual average of PM10s was 372 micrograms per cubic meter.
The study found that the Mongolian capital Ulan Bator had an annual average PM10s density of 279 micrograms per cubic meter, followed by another west Iranian city, Sanandaj, with 254 micrograms.
Cities in Pakistan and India, such as Quetta and Kanpur, as well as Botswana's capital Gaborone, also ranked high on the pollution scale.
