In one of the World Health Organization’s most recent global studies on air pollution, measuring so-called PM 2.5 and PM 10 particles, major Australian and Canadian cities shared the top ranks. Noticeably absent from that list were most European capitals, while cities such as Brisbane, Toronto, Vancouver, Adelaide and Melbourne did well in multiple statistics and managed to stay way below recommended thresholds.

That makes their residents a minority in the world. According to some estimates, more than 90 percent of the world’s population live in cities or areas with pollution beyond recommended levels, even though there is no consensus on the exact thresholds.

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Two terms often used for comparisons are associated with the size of particles in the air, with PM 10 referring to particles smaller than 10 micrometers in diameter and PM 2.5 referring to particles smaller than 2.5 micrometers. Neither of those measurements can offer a complete analysis of a place’s air pollution, but they are one way that scientists compare figures internationally.

Any comparison can be tricky, though, and looking at only one of those factors can easily be misleading. While big Australian and Canadian cities had relatively few PM 2.5 and PM 10 particles, for instance, their U.S. counterparts managed to get into the top spots only for the fewest PM 10 pollution rates and did significantly worse when PM 2.5 particles were also analyzed. While one would assume that Sydney is a healthy city to live in based on WHO’s data for both types of particles, Greenpeace recently challenged that assumption, writing in a report last year that the city is a hot spot for nitrogen oxide emissions, or NO2, in several Chinese and Indian districts, among others.

NO2 is being released when biomass, oil or coal are being burned, which explains high emissions in industrial regions and especially high pollution in winter.

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Doing well in driving down the prevalence of some pollutants in the air and maintaining higher rates for others can be a dangerous strategy, Greenpeace warned, writing: “Long-term exposure to NO2 is associated with increased mortality rates worldwide.” Sydney in particular also frequently exceeds ozone standards, creating further headaches for air quality authorities.

But overall, researchers agreed that Australia has done a better job at reining in air pollution than many other nations.

“The air quality in Australian cities has cleaned up a lot over the last few decades,” said Martin Cope, a researcher with the Commonwealth Scientific and Industrial Research Organization. “For example, banning backyard burning in urban areas would have had a big impact on fine particles; also ongoing tightening of motor vehicle emission controls is another big factor,” he wrote in an email.

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Cope added that Australia’s “areas of highest population are clumped into a few major cities along the coastline, and there does not appear to be a great deal of interaction between the air pollution generated by each city.” In other countries, the clustering of huge cities together creates an interlocking mesh of pollution. In Canada and Australia, meanwhile, population density — even in big cities — is also lower than in most other parts of the world.

The director of the International Laboratory for Air Quality and Health, Lidia Morawska, cautioned that it would be difficult to apply the Australian lessons elsewhere. While power plants might pose the biggest challenge in some places, other types of pollutants such as vehicle emissions are far more relevant elsewhere.

City planning and moving residents away from high-pollution areas appear to be the primary lessons from the world’s cleanest big cities.

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“The ‘old’ paradigm of setting thresholds and being satisfied if levels are below these is not really adequate for health protection,” said Guy Marks of the University of Sydney. In Australia and elsewhere, smarter urban planning has been a long-overlooked measure — one that may not, for example, dissolve smog in India anytime soon, but that could one day offer important lessons.