Most Polluted U.S. Cities By Year Round in 2009

#1 Bakersfield
#2 Pittsburgh
#3 Los Angeles
#4 Visalia
#5 Birmingham
#6 Hanford
#7 Fresno
#8 Cincinnati
#9 Detroit
#10 Cleveland
#11 Charleston
#11 Huntington
#11 Louisville
#14 Macon
#14 St. Louis
#16 Weirton
#17 Atlanta
#18 Indianapolis
#18 Rome
#20 Canton
#20 York
#22 Lancaster
#22 New York City
#24 Hagerstown
#24 Houston

In compiling this list, the American Lung Association ranked U.S. metropolitan statistical areas (MSAs)--geographic entities defined by the U.S. Office of Management and Budget for use by federal agencies in collecting, tabulating and publishing federal statistics--using the highest weighted average for any county within that MSA. Grading was based on the Environmental Protection Agency's determination of violations of the national ambient air quality standard.

Though overall air pollution is down compared to previous years, the problem is still widespread. Visalia and Fresno, two mid-size towns in central California ranked high for short-term and year-round particle pollution. Birmingham, Ala., and Cincinnati were listed in the top 10 of metro areas with unhealthy levels of year-round particle pollution. In the Southwest, the Houston, Dallas and Phoenix metro areas had high ozone levels.

Residents of Los Angeles, Bakersfield, Calif., and Pittsburgh, Pa., might want to pay close attention to the ALA's rankings. These metro areas were deemed the most polluted in the nation.

The Los Angeles metro area--known for its thick smog--ranks this year as the most ozone-polluted. Pollutants produced by car exhaust and smokestacks form the raw ingredients for the production of ozone.

Nearby Bakersfield, Calif., ranked as the city with the most year-round particle pollution and had ozone levels second only to Los Angeles. The Pittsburgh metro area, an industrial hub in western Pennsylvania, had the highest short-term particle pollution and the second highest year-round levels. Short-term and year-round particle counts reflect the amount of tiny solid and liquid particles in the air, most often emitted from diesel-powered vehicles, steel mills and coal-fired power plants, among other sources.
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