Honolulu

Residents of Honolulu and Santa Fe-Espanola, N.M., on the other hand, are in luck: Those two cities had air that is among the country's cleanest -- and they were the only two in the nation that had no days in which smog and soot levels reached unhealthy ranges...

Source: http://health.usnews.com/health-news/family-health/heart/articles/2011/04/27/report-california-cities-have-worst-air-pollution-in-us.html