Cities in Iran, India, Pakistan and the capital of Mongolia rank among the worst on the planet for air pollution, while those in the US and Canada are among the best, according to the first global survey, released Monday by the World Health Organization.

世界卫生组织周一发布的首次全球调查显示，伊朗、印度、巴基斯坦的城市和蒙古首都是全球空气污染最严重的，而美国和加拿大城市的空气质量是最好的。

The southwest Iranian city of Ahvaz walked away with the unfortunate distinction of having the highest measured level of airborne（风媒的） 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 premature¹ 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（呼吸的） problems in humans. They are mostly sulfur² dioxide and nitrogen dioxide from power plants, auto³ 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 density⁴ 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 the high pollution levels varied⁵, but that often rapid industrialization and the use of poor quality fuels for transportation and electricity generation are to blame.

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.

Washington, D.C., had a level of 18 micrograms, Tokyo measured 23 micrograms, and Paris had 38 micrograms of PM10s per cubic meter.