(AP Photo/Vahid Salemi, File)
The southwest 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 global body said investments to lower pollution levels quickly pay off due to lower disease rates and, therefore, lower healthcare costs.
The list, which relies on country-reported data over the past several years, 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. Heavy industry and low-quality vehicle fuel are the main causes of air pollution in this desert city of 1.3 million.
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.
WHO said the reasons for high pollution levels varied, but that often rapid industrialization and the use of poor quality fuels for transportation and electricity generation are to blame.
In India, major metropolitan areas such as New Delhi, Mumbai and Kolkata have banned the construction of new power plants within city limits, and existing ones are being shut down or relocated.
But at the same time, a lack of public transport has led to an explosion of privately owned cars and SUVs as the economy booms, with the number of heavily polluting diesel vehicles increasing 10-fold as diesel is highly subsidized by the government.
At the other end of the list are cities in Canada and the United States, which benefit from lower population density, favorable climates and stricter air pollution regulation.
Yukon territory's capital Whitehorse had a yearly average of just 3 micrograms of PM10s per cubic meter, while Santa Fe, New Mexico, measured 6 micrograms.
WHO also released a shorter table comparing levels of even finer dust particles, known as PM2.5s. The level considered harmful there is 10 micrograms per cubic meter.
This list contained no measurements from Asia apart from Ulan Bator, which again ranked worst with 63.0 micrograms.