Estonia topped the list with the best air quality, Mauritius ranked second, and Canada tied with Australia for third.
The global health body on Monday released its list of measurements ranking 1,100 cities in 91 countries. The list is intended to highlight the need to reduce outdoor air pollution, which is estimated to cause 1.34 million premature deaths each year.
Tiny particles do big damage
WHO spokesman Gregory Hartl said the air quality in all cities is affected by combination of four main factors: private transport, power generation, indoor combustion of fuels, and industry.
He cautioned that the data is better examined at the city-level, as some countries have many more measuring stations. Although Mongolia is ranked lowest on the list of countries, it only has one station — in the capital Ulaanbaatar — whereas a large country like Canada has several stations.
"On average, only a few cities currently meet the WHO guideline values," the organization stated in a release. The vast majority of urban populations exceed the recommended maximum level of 20 micrograms per cubic metre.
WHO expressed concern over the prevalence of PM10 particles, which are particles of 10 micrometers or less that can penetrate the lungs and may enter the bloodstream. These tiny particles can cause respiratory problems and other diseases in humans, including heart disease, lung cancer and asthma.
Ahvaz, located in southwest Iran, earned the unfortunate distinction of worst air quality at the city level. Ahvaz has an annual average of 372 micrograms of PM10 per cubic metre.
Cities in Iran, India, Pakistan and Ulaanbaatar were among the worst on the planet for air pollution, whereas cities in Canada and the United States fared better.
Canada's annual average was 13 micrograms of PM10 particles per cubic metre of air but performance varied by city: Whitehorse did best with 3 micrograms of PM10 per cubic metre, whereas Sarnia fared worst at 21.
Most countries that ranked well benefit from lower population density, favourable climates and stricter air pollution regulation — conditions unavailable to those at the bottom of the list.
"Across the world, city air is often thick with exhaust fumes, factory smoke or soot from coal burning power plants. In many countries there are no air quality regulations and, where they do exist, national standards and their enforcement vary markedly," Dr. Maria Neira, WHO Director for Public Health and Environment, said in a release.
The organization said investments to lower pollution levels quickly pay off due to lower disease rates and health-care costs.