Population: 108,779
Bismark's air stays pristine for many reasons, but a lot has to do with the fact that the place is predominantly flat.
The plains surrounding the capital of North Dakota make it so air doesn't get trapped and stay in one place for very long. And prevailing westerly winds constantly refresh the skies, according to Terry O'Clair, the director of North Dakota's Department of Health, Division of Air Quality.
Also helping to clean things up: Under the local Regional Haze Program, power plants have upgraded their cleaning systems, adding scrubbers and "bag houses," which filter dust particles from smoke.
There is one growing concern, however: The oil boom that's taking place in the northwestern corner of the state. "A lot of that traffic is on gravel roads, which causes dust in the air," said O'Clair.
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