This bit of news has been touted as "Pittsburgh, the most polluted city in the US", but that's not true. The American Lung Association has 3 threshold by which they measure the level of pollution in a city. There's short-term pollution (those days out of the month or year when the air is just dirty) and there's also year-round pollution which measures how many days in a year does the air stay dirty. The third threshold is smog.

Los Angeles won the #1 spot as both the smoggiest and most polluted city all year round. Pittsburgh though knocked it to #2 in the "short-term pollution" rankings.

AMERICAN LUNG ASSOCIATION ISSUES STATE OF THE AIR REPORT
Top Ten U.S. Cities Most Polluted by Short-Term Particle Pollution: "a deadly cocktail of ash, soot, diesel exhaust, chemicals, metals and aerosols that can spike dangerously for hours to weeks on end" 1) Pittsburgh, Pa.; 2) Los Angeles/Long Beach/Riverside, Calif.; 3) Fresno/Madera, Calif.; 4) Bakersfield, Calif.; 5) Birmingham, Ala.; 6) Logan, Utah 7) Salt Lake City, Utah ; 8) Sacramento, Calif.; 9) Detroit, Mich.; 10) Baltimore, Md./Washington, D.C./Northern Virginia.

Top Ten U.S. Cities Most Polluted by Year-Round Particle Pollution: the most year-round presence of ash, soot, diesel exhaust, chemicals, metals and aerosols in the air.
1) Los Angeles/Long Beach/Riverside, Calif.; 2) Pittsburgh, Pa.; 3) Bakersfield, Calif.; 4) Birmingham, Ala.; 5) Visalia/Porterville, Calif.; 6) Atlanta, Ga.; 7) Cincinnati, Ohio; 8) Fresno/Madera, Calif. 9) Hanford/Corcoran, Calif.; 10) Detroit, Mich.

Top Ten U.S. Cities Most Polluted by Ozone: "Ozone or "smog," [is] a gas formed most often when sunlight reacts with vapors emitted when motor vehicles, factories, power plants and other sources burn fuel"
1) Los Angeles/Long Beach/Riverside, Calif.; 2) Bakersfield, Calif.; 3) Visalia/Porterville, Calif.; 4) Houston, Texas; 5) Fresno/Madera, Calif. 6) Sacramento, Calif. 7) Dallas-Fort Worth, Texas; 8) New York, N.Y./Newark, N.J.; 9) Baltimore, Md./Washington, D.C./Northern Virginia; 10) Baton Rouge, La.

Check out their maps at State Of The Air. Their list of cleanest city air year-round is here.