Cities in Iran, India and Pakistan have some of the worst polluted air on the planet, while Canada has some of the best, a new report from the World Health Organization finds.

WHO unveiled its list of air pollution measurements from some 1,100 cities on Monday. The rankings looked at how many tiny air pollutant particles were recorded floating in the air in each city.

Ahvaz, in Iran's southwest, had the worst air pollution of any of the cities surveyed.

Ahvaz had the highest measured levels in the world of PM10 particles, which are nearly-invisible air particles that measure 10 micrometres or less in diameter.

PM10 particles come mostly from power plant and auto exhaust emissions, and can penetrate into the lungs where they may cause cancer, asthma, and acute lower respiratory infections. They can also enter the bloodstream, leading to heart disease, research suggests.

WHO's air quality guidelines call for PM10 readings that are lower than 20 micrograms per cubic metre.

But Ahvaz had an annual average of 372 micrograms of PM10 per cubic metre.

Meanwhile, Canada as a whole had an annual average of just 13 micrograms of PM10 particles per cubic metre of air.

Whitehorse had a yearly average of just 3 micrograms of PM10s, while Fredriction had just 6.

Canada's worst air quality was recorded in the industrial Ontario city of Sarnia, where PM10 particles were measured at 21 micrograms per cubic metre of air. But even that was only marginally above the WHO recommended limit.

Ulaanbaatar, the capital of Mongolia, by contrast, had an annual average of 279 micrograms of PM10 particles per cubic metre of air.

Here are the annual mean PM10 measurements per square metre of air in some other Canadian cities:

  • Metro Vancouver 8
  • Halifax 8
  • Calgary 9
  • Winnipeg 9
  • Edmonton 11
  • Toronto 13
  • Montreal 19

Most cities in Canada benefit from lower population densities, favourable climates and stricter air pollution regulation.

The data on particulate matter were compiled from national records, websites and scientific articles. The database covered the period from 2003 to 2010, with the majority of values for the years 2008 and 2009.

WHO released the report to highlight the need to reduce outdoor air pollution, which is estimated to cause 1.34 million premature deaths each year.

The UN agency said investments to lower pollution levels quickly pay off due to lower disease rates and lower health care costs.

With reports from The Associated Press