In the damp and dreary Pacific Northwest, where moss climbs the trees and moisture dribbles down lamp posts and seems to seep through almost everything, the reward for surviving another year wrapped in oppressive mist and grey clouds used to be a lush, bright, sky-blue late summer.

"Most folks I know tend to try not to leave Seattle in August," says Karin Bumbaco, Washington State's assistant state climatologist.

But for much of the last two weeks, air over three of North America's dampest, cleanest cities was dingier than the skies over Mumbai, Jakarta, and every major industrial city in China. For a brief period Wednesday evening, when it comes to the fine particles of pollution that can lodge in your lungs