Half of America's 10 dirtiest cities, listed by particle pollution as measured by the American Lung Association in its 2005 report, are located in California. If measured by smog, all but two are located in that state. Los Angeles The area of Long Beach and Riverside takes the top prize for the most polluted zone in the U.S.--an area of frightful traffic and heavy industrial emissions that has the bad luck of air inversions that hold the bad particles right where they hurt. Visalia-Porterville, Calif. The strip cities of Visalia-Porterville can thank a good deal of their particle problem on Route 99, with being located in a valley with hot, motionless air doesn't help either. Bakersfield, Calif. In addition to Route 99, which it shares with Visalia, Bakersfield has Interstate 5 to compound the particle problem: lots of traffic, valley heat and air inversion. Fresno, Calif. It may be north of Bakersfield and Visalia, but Fresno is almost in a league with them in particle count and ozone. It has an organization called 1,000 Friends of Fresno, who are working to improve their city's air quality. Pittsburgh The cities of Pittsburgh and Newcastle count on the winds from Ohio and West Virginia to bear the majority of their fine particles away, but the city's industries, as well as the major highways like Route 79, still contribute to poor air quality.