The American Lung Association’s 2013 State of the Air report found that air quality across the U.S. is improving despite the cities with the worst air quality having more days of high ozone and short-term particle pollution than in the organization’s 2012 report. The 2013 report examines air quality statistics starting in 2009 and going up through 2011. The report from the association uses data from the Environmental Protection Agency for two types of pollution: ozone (from smog) and particle pollution (PM 2.5, a.k.a. soot).

Some key findings from this year’s State of the Air report include that more than 131 million people live in counties with unhealthy levels of ozone or particle pollution. Next, four cities ended up on all three of the cleanest cities lists, which the association said is the largest number ever to end up on all three lists. They include Bismarck, N.D., Cape Coral-Fort Myers, Fla., Palm Bay-Melbourne-Titusville, Fla., and Rapid City, S.D.

Next, seven cities ended up on the cleanest cities lists for ozone and for year-round particle pollution. Listed alphabetically, they include Bismarck, N.D.; Burlington, Vt.; Cape Coral-Fort Myers, Fla.; Duluth, Minn.; Palm Bay-Melbourne-Titusville, Fla.; Rapid City, S.D.; and Santa Fe-Espanola, N.M.

For year-round particle pollution, a total of 18 cities had lower year-round levels, including 16 cities with the lowest reported levels of pollution since the report started tracking these statistics. They include Bangor, Me.; Bismarck, N.D.; Cape Coral-Fort Myers, Fla.; Colorado Springs, Colo.; Farmington, N.M.; Flagstaff, Ariz.; Fort Collins-Loveland, Colo.; Lakeland-winter Haven, Fla.; Palm Bay-Melbourne-Titusville, Fla.; Prescott, Ariz.; Rapid City, S.D.; Redding, Calif.; Salinas, Calif.; Sarasota, Fla.; St. George, Utah; and Tucson, Ariz.

The following lists show the most-polluted cities and regions in the U.S., according to the 2013 State of the Air report:

Ozone Pollution, Most Polluted

#1: Los Angeles-Long Beach-Riverside, Calif.
#2: Visalia-Porterville, Calif.
#3: Bakersfield-Delano, Calif.
#4: Fresno-Madera, Calif.
#5: Hanford-Corcoran, Calif.
#6: Sacramento--Arden-Arcade-Yuba City, Calif. and Nev.
#7: Houston-Baytown-Huntsville, Texas
#8: Dallas-Fort Worth, Texas
#9: Washington-Baltimore-Northern Virginia, D.C.-Md.-Va.-W. Va.
#10: El Centro, Calif.

Year-Round Particle Pollution, Most Polluted

#1: Bakersfield-Delano, Calif.
#1: Merced, Calif.
#3: Fresno-Madera, Calif.
#4: Hanford-Corcoran, Calif.
#4: Los Angeles-Long Beach-Riverside, Calif.
#6: Modesto, Calif.
#7: Visalia-Porterville, Calif.
#8: Pittsburgh-New Castle, Pa.
#9: El Centro, Calif.
#10: Cincinnati-Middletown-Wilmington, Ohio-Ky.-Ind.

Short-Term Particle Pollution, Most Polluted

#1: Bakersfield-Delano, Calif.
#2: Fresno-Madera, Calif.
#3: Hanford-Corcoran, Calif.
#4: Los Angeles-Long Beach-Riverside, Calif.
#5: Modesto, Calif.
#6: Salt Lake City-Ogden-Clearfield, Utah
#7: Pittsburgh-New Castle, Pa.
#8: Merced, Calif.
#9: Fairbanks, Ala.
#10: Logan, Utah-Idaho

The association noted that tailpipe emissions are a major source of ozone and particulate pollution in its State of the Air report. Another report from the American Lung Association in early April found that by 2030, cleaner gasoline and vehicle standards could prevent more than 2,500 premature deaths each year from ozone and particulate pollution, prevent more than 3.3 million missed days at work or school , and result in $8.5 to $22 billion in annual economic and health care benefits.