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.

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 up one-sixth of the entire U.S. naval fleet. San Diego's geography is great for diesel tankers, sailboats, and beachgoers, but it also contributes to the pollution problem. Ocean breezes blow car emissions and other pollutants inland, where they are blocked by mountains. Smog also blows south from Los Angeles and its suburbs.

No. 6: Hanford, Calif.

This San Joaquin Valley farming town, is equidistant from San Francisco and Los Angeles and is home to only 53,000 people. Yet, it ranks high for both ozone and particle pollution (soot), another measure of dirty air. In fact, Hanford's short-term and year-round particle pollution worsened since last year's State of the Air report. Hot, stagnant air in the summertime contributes to the pollution problem.

No. 5: Sacramento, Calif.

As one of the sunniest cities in the country, Sacramento has all the ingredients to create smog. Sunlight triggers the chemical reaction between nitrogen oxides and VOCs that creates ozone. Inside the lungs, this invisible gas irritates body tissues and can lead to coughing, chest pain, and acute asthma attacks. At times, wildfires have contributed to the air quality problems around Sacramento.

No. 4: Fresno, Calif.

Rich farmland surrounds Fresno and helps make it the commercial hub of the San Joaquin Valley. Despite some progress in reducing nitrogen oxide emissionss, it remains one of the top cities for ozone. Cars and trucks are the main culprit. Truckers are now being offered incentives to upgrade or retrofit their trucks for cleaner emissions. The San Joaquin Valley also has limited the burning of agricultural waste.

No. 3: Visalia, Calif.

Visalia is a small, picturesque, farm town in the shadow of the magnificent Sierra Nevada mountains. It's a gateway to Sequoia National Park, home of giant trees that reach heights of 300 feet. But along with Visalia's easy access to these natural attributes comes considerable smog, trapped in the valley surrounding the town. Visalia also ranks second for year-round particle pollution.

No. 2: Bakersfield, Calif.

At the southern end of the San Joaquin Valley, Bakersfield vies for the title of the dirtiest air in America. This city of about 340,000 is No. 2 for ozone and No. 1 for both year-round particle pollution and short-term soot levels. It has ample sunshine - 330 days a year - and nearby mountains that trap smog. Heavy-duty trucks passing through play a role. The best protection is to avoid outdoor exercise when pollution levels are high.

No. 1: Los Angeles, Calif.

Los Angeles has battled its smog problem as far back as the 1940s, when the city was just a fraction of its size today. Now the nation's second-largest city, Los Angeles has worked hard to combat smog, reducing its car emissions even as the population swelled. Since 1980, peak smog levels have declined by 70 percent. Yet the city still reigns as the nation's smoggiest, according to the State of the Air report.

Particle Pollution: Phoenix, Ariz.

People came to this city in the desert for refuge from allergies or asthma, but population growth also brought pollution. Car emissions, construction equipment, mowers, and even desert dust creates a "brown cloud." Phoenix tied for second with Los Angeles and Visalia, CA, for worst year-round particle pollution. Smog and particle pollution pose the greatest health risks for children and older people.

Particle Pollution: Pittsburgh, Pa.

This one-time steel town worked hard to clean up the smoky skies over the city. But now car and truck emissions, power plants, and other industry sources contribute to Pittsburgh's No. 3 ranking for daily particle pollution and No. 7 ranking for year-round particle pollution.

Smoggy Surprise: Provo, Utah

Smog is usually a summertime problem because it forms in sunlight and heat. But Provo, in the Utah Valley near the Sundance ski resort, winter air pollution is a serious concern. Cars and wood-burning stoves and fireplaces contribute to the pollution. Provo ranked sixth in the nation for the average daily levels of particle pollution.

Fresh Air: Honolulu, Hawaii

Want to breathe easy? Honolulu was one of the two cleanest cities in the country for ozone and particle pollution, according to the American Lung Association. Trade winds clear away the car emissions from this city of 900,000, and there's little heavy industry. Perhaps that's one reason why Hawaii has the longest life expectancy in the country.

Fresh Air: Santa Fe, N.M.

This city in the foothills of the Rocky Mountains has crisp, cool evenings, even in the summer. With low humidity and mountain breezes, Santa Fe is known for its blue skies and clean air. The only particulate pollution you're likely to see in Santa Fe is dust from the desert sand.

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