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Copenhagen, Denmark

Copenhagen is fresh off of a 15-year campaign to improve air quality, put forward by Environment Minister Troels Lund Poulsen. Among his achievements in that time: designated environmental zones in large cities, tax breaks for diesel vehicles equipped with particle filters, and initiatives for cleaner boiler and furnace emissions. Denmark is now one of just three countries to meet and exceed the European Union’s requirements for minimizing dangerous particles in the air.

Scott R Barbour | Getty Images