From yahoo,dot com
"Despite the improvements, 4 in 10 Americans live where pollution levels are often dangerous to breathe. Since the ALA began studying particle pollution, almost all of the most polluted cities have consistently remained among the worst. Pittsburgh has been one of the 10 most polluted cities since 2004. The cities of Bakersfield and Merced, in California, are the most polluted cities in the U.S. this year and have been among the 25 most polluted since at least 2004. They are among many California cities, including Los Angeles, that have struggled with pollution for some time.
The ALA’s 2013 “State of the Air” report measures cities based on low-lying ozone pollution, as well as both short- and long-term particle pollution. These particles, just 1/30th the diameter of a human hair, are capable of getting past our bodily defenses and cause physical harm, particularly to those who already suffer from pulmonary diseases, the very young and the elderly. The report measures both the total accumulated particle pollution over the course of a year, as well as the number of days that the air pollution hit unhealthily high levels."
1. Bakersfield-Delano, Calif.
2. Merced, Calif.
3. Fresno-Madera, Calif.
4. Hanford-Corcoran, Calif.
5. Los Angeles-Long Beach-Riverside, Calif.
6. Modesto, Calif.
7. Visalia-Porterville, Calif.
8. Pittsburgh-New Castle, Penn.
9. El Centro, Calif.
10. Cincinnati-Middletown-Wilmington, Ohio-Ky.-Ind.