The World Health Organization (WHO), the United Nations' health authority, says cities in Iran, India, Pakistan, and Mongolia have the most unhealthy air in the world.
In a new global survey, WHO says the southwest Iranian city of Ahvaz had the highest measured level of airborne particles smaller than 10 micrometers.
The particles -- sulfur dioxide and nitrogen dioxide from power plants, auto exhaust, and factories -- can cause respiratory problems in humans.
For health reasons, WHO recommends an upper limit of 20 micrograms for such particles.
The new survey says Ahvaz's annual average of the particles was 372 micrograms.
According to the survey, the next four most unhealthy cities include the Mongolian capital, Ulan Bator, the Iranian city of Sanandaj, the Indian city of Ludhiana, and Quetta in Pakistan.
The survey said the healthiest air was found in cities in the United States and Canada, which benefit from lower population densities and stricter air pollution regulations.