Smog Towns, U.S.A.

Hazy, dirty air on a hot summer day is a visible sign of air pollution in America's cities. But you can't see ozone -- the main component of smog -- and a lung irritant that aggravates asthma and COPD. In the top 10 worst cities for ozone the air can be dangerous to breathe, according to the "State of the Air 2011" report. Here are the dirtiest, the cleanest, and a few smoggy surprises.

No. 10: Charlotte, NC

This fast-growing city once again ranks 10th on the American Lung Association's list of smoggiest cities. Its emergence as a major financial center brought new residents - and more cars. Located on the Carolina piedmont midway between the coast and the Appalachian Mountains, Charlotte's vista has gradually become hazier as its population grows. Coal-fired power plants also play a role.

Coal-fired power plants also play a role.

No. 9: Merced, Calif.

Eight of the nation's 10 smoggiest cities are in California, despite tough vehicle emission requirements and pollution control efforts. In the bowl-shaped San Joaquin Valley, where Merced is located, nearby mountains trap pollutants from cars, trucks, mowers, and other sources. Merced lies one of the nation's top farming regions and is a gateway to Yosemite National Park.

No. 8: Houston, Texas

Houston's rank has gradually improved since it hit No. 4 in 2008. Petrochemical refineries on the outskirts are a major source of the nitrogen oxides and VOCs (volatile organic compounds) that form Houston's ground-level ozone. The Texas Commission on Environmental Quality now monitors emissions with infrared cameras and enforces limits on ozone-forming chemicals.

No. 7: San Diego, Calif.

California's second-largest city is homeport of the Pacific Fleet, which makes