How clean is your air? Most U.S. cities with the dirtiest air are getting cleaner but half of Americans still live in in areas where it’s often difficult to breathe, the American Lung Association reports today.

Los Angeles remains the smoggiest metro area, although it’s improved significantly in the last decade, and Bakersfield, Calif., has the worst particle pollution such as soot and ash, both on a daily and annual level, according to the ALA’s 12th annual “State of the Air” report.

In fact, eight of the 10 cities with the worst ozone or smog are located in California and six of the 10 worst for particle pollution – both short term and annually – are also in the state.

In contrast, Honolulu and Santa Fe ranked as the two cities with the cleanest air overall, says the report, based on the most recently available data from the Environmental Protection Agency. (Lists of cities with the cleanest and dirtiest air follow.)

EPA’s 2009 data, released last week, shows total U.S. greenhouse gas emissions — primarily carbon dioxide — fell 6.1% from 2008, the largest decline in at least five years. The agency, which began a multiyear plan to regulate these emissions in January, attributed the drop to less polluting fuels and lower energy consumption because of the recession.

Some improvement in air quality began prior to the recession, says Janice Nolen, author of the American Lung Association’s 2011 report, which credits the Clean Air Act for the progress.

The report says ozone as well as particle pollution can lead to asthma attacks, emergency room visits and premature death. It finds:

•All 25 cities with the worst ozone pollution improved their air quality from 2008 to 2009. All but two with the most year-round particle pollution also improved, but only 12 of the 28 with the worst short-term levels had fewer unhealthy days.

•However, half of Americans, or 154.5 million, live in counties with unhealthful levels of either ozone or particle pollution.

The report comes amid congressional efforts to stop the EPA from using the Clean Air Act to regulate greenhouse gas emissions. With unanimous GOP support and 19 Democratic votes, the House of Representatives passed such a bill earlier this month, but the Democratic-led Senate rejected it.

“This legislation will remove the biggest regulatory threat to the American economy,” said the bill’s author Rep. Fred Upton, R-Mich., chairman of the House Energy and Commerce Committee, arguing it could increase gas prices and cost jobs.

David Hawkins, director of climate programs at the Natural Resources Defense Council, disagrees. He says U.S. government and academic studies show heat-trapping emissions can be cut without hurting the economy, and Americans will benefit with cleaner air and lower energy costs.

Here’s how the American Lung Association ranks U.S. cities on air quality:

10 Most Ozone-Polluted Cities

1. Los Angeles-Long Beach-Riverside, Calif.
2. Bakersfield-Delano, Calif.
3. Visalia-Porterville, Calif.
4. Fresno-Madera, Calif.
5. Sacramento-Arden-Arcade-Yuba City, Calif.-Nev.
6. Hanford-Corcoran, Calif.
7. San Diego-Carlsbad-San Marcos, Calif.
8. Houston-Baytown-Huntsville, Texas
9. Merced, Calif.
10. Charlotte-Gastonia-Salisbury, N.C.-S.C.

10 Cities Most Polluted by Short-term Particle Pollution

1. Bakersfield-Delano, Calif.
2. Fresno-Madera, Calif.
3. Pittsburgh-New Castle, Pa.
4. Los Angeles-Long Beach-Riverside, Calif.
5. Salt Lake City-Ogden-Clearfield, Utah
6. Provo-Orem, Utah
7. Visalia-Porterville, Calif.
8. Birmingham-Hoover-Cullman, Ala.
9. Hanford-Corcoran, Calif.
9. Logan, Utah-Idaho
9. Sacramento-Arden-Arcade-Yuba City, Calif.-Nev.

10 Cities Most Polluted by Year-Round Particle Pollution

1. Bakersfield-Delano, Calif.
2. Los Angeles-Long Beach-Riverside, Calif.
2. Phoenix-Mesa-Glendale, Ariz.
2. Visalia-Porterville, Calif.
5. Hanford-Corcoran, Calif.
6. Fresno-Madera, Calif.
7. Pittsburgh-New Castle, Pa.
8. Birmingham-Hoover-Cullman, Ala.
9. Cincinnati-Middletown-Wilmington, Ohio-Ky.-Ind.
10. Louisville-Jefferson County-Elizabethtown-Scottsburg, Ky.-Ind.
10. Modesto, Calif.

25 Cleanest U.S. Cities for Year-round Particle Pollution

1. Cheyenne, Wyo.
2. Santa Fe-Espanola, N.M.
3. Tucson, Ariz.
4. Great Falls, Mont.
4. Honolulu, Hawaii
6. Anchorage, Alaska
7. Albuquerque, N.M.
7. Amarillo, Texas
9. Redding, Calif.
10. Salinas, Calif.
11. Bismarck, N.D.
12. Boise City-Nampa, Idaho
13. Billings, Mont.
14. Cape Coral-Fort Myers, Fla.
14. Flagstaff, Ariz.
14. Fort Collins-Loveland, Colo.
14. Palm Bay-Melbourne-Titusville, Fla.
14. Sarasota-Bradenton-Punta Gorda, Fla.
19. Claremont-Lebanon, N.H.-Vt.
20. Port St. Lucie-Sebastian-Vero Beach, Fla.
20. Rapid City, S.D.
22. Duluth, Minn.-Wis.
23. Fargo-Wahpeton, N.D.-Minn.
24. Bangor, Maine
24. Burlington-South Burlington, Vt.
24. Orlando-Deltona-Daytona Beach, Fla.

Cleanest U.S. Cities for Ozone Air Pollution (no ranking)

Bismarck, N.D.
Brownsville-Harlingen-Raymondville, Texas
Brunswick, Ga.
Coeur d’Alene, Idaho
Dothan-Enterprise-Ozark, Ala.
Duluth, Minn.-Wis.
Fargo-Wahpeton, N.D.-Minn.
Honolulu
Laredo, Texas
Lincoln, Neb.
Monroe-Bastrop, La.
Naples-Marco Island,
Port St. Lucie-Sebastian-Vero Beach, Fla.
Rapid City, S.D.
Rochester, Minn.
Santa Fe-Espanola, N.M.
Savannah-Hinesville-Fort Stewart, Ga.
Sioux Falls, S.D.
Spokane, Wash.
Topeka, Kan.