The recently released WHO figures come from a study conducted predominantly in 2008-09, and has measured the density of PM10 particles in cities around the globe.
PM10 is a collective name for particles smaller than 10 micrometers in size, which can therefore penetrate the body and harm inner organs, causing for example asthma, heart decease and lung cancer.
The amount is measured in µg/m3, meaning micrograms per cubic meter of air.
The WHO study showed that the people of Whitehorse in Canada breathe the cleanest air, with only 3 µg/m3, whereas the figure for the Swedish capital Stockholm is 28. That is 8 µg over the recommended average.
Malmö is ranked as Sweden’s cleanest city out of the four measured, and the only one under the recommended average, with 18 µg/m3. On the same level is also Nottingham, Monaco and New Orleans.
Next is Umeå with 22 µg/m3, which compares to Chicago, Oslo and Geneva.
Gothenburg at 24 µg/m3 equals Marbella and Amsterdam, and Stockholm is found having the same air quality as Costa Rica’s San Jose, Thailand’s Phuket and Brussels in Belgium, 28 µg/m3.
The areas measured in Los Angeles were Long Beach, Glendale and Santa Ana, where the µg/m3 level reached 25.
The cleanest city in the U.S. was Clearlake in northern California, but also Miami, San Francisco and Denver rank above the first Swedish contester.
“The measurements used in the database are taken from monitoring sites in cities, including roadside, but excluding industrial and other recognized ‘hot spots’ that are not representative of the exposure of many people (e.g. crossings at highways) in order to avoid overestimates,” according to a fact sheet on WHO’s website.
The dirtiest city on the list is Iranian Ahwaz, with a value more than 20 times higher than Malmö. At 372 µg/m3, Ahwaz also beats second worst Ulaanbaatar of Mongolia, with almost 100 units.
Other Cities above 100 µg/m3, is Abu Dhabi, Sarajevo, Beijing, Mumbai and Islamabad.