Slideshow Pictures: Respiratory Health -- 10 Worst Smog Cities in America

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 American Lung Association's "State of the Air 2012" report. Here are the dirtiest, the cleanest, and a few smoggy surprises.

No. 10: Merced, Calif.

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

No. 9: San Luis Obispo, Calif.

Pollution blowing in from elsewhere in California -- from the Bay Area to Los Angeles -- has made this coastal city No. 9 for dirty air. The smog here has actually improved, thanks partly to cleanup of airborne-pollution sources in other areas. But tailpipe exhaust and emissions from agriculture, offshore oil, and burning continue to be a problem.

No. 8: Houston, Texas

Houston's air has been gradually clearing since it ranked No. 4 in ozone pollution in 2008. Petrochemical refineries on the city's 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: 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. 5: Hanford, Calif.

This San Joaquin Valley farming town halfway between San Francisco and Los Angeles is home to only 53,000 people. Yet it ranks high for both ozone and particle pollution (soot), another measure of dirty air. Though its ozone pollution slightly improved since last year's "State of the Air" report, its short-term and year-round particle pollution has grown worse. Hot, stagnant air in the summertime contributes to the pollution problem.

No. 4: Fresno, Calif.

Rich farmland surrounds Fresno and helps make it the commercial hub of the San Joaquin Valley. Despite some progress over the years in reducing nitrogen oxide emissions, it remains one of the top cities for ozone. Cars and trucks are the main culprit marring this pastoral vision. 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: Bakersfield, Calif.

At the southern end of the San Joaquin Valley, Bakersfield improved its ozone pollution during the last year. But this city of about 340,000 overtook Los Angeles to become No. 1 for both year-round and short-term particle pollution (soot). These tiny particles can increase the risk of heart disease, stroke, and asthma attacks. Bakersfield has ample sunshine -- 330 days a year -- and nearby mountains that trap pollution. Heavy trucks passing through also play a role.

No. 2: 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. Still, last year the town had fewer unhealthy ozone days and its fewest spikes in particle pollution in 13 years.

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% and are now the lowest since the "State of the Air" reports began in 2000. Yet the city still reigns as the nation's smoggiest.

Particle Pollution: Pittsburgh, Pa.

Outside of California, Pittsburgh ranks highest in U.S. cities for daily and year-round particle pollution: No. 6 for both. Car and truck emissions, power plants, and other industry sources contribute to the problem. But this one-time steel town has worked hard to clean up its smoky skies and had its cleanest year since 2000 for daily particle pollution.

Smoggy Surprise: Fairbanks, Alaska

If you associate smog with summer heat, you probably associate Alaska with clean, fresh air. But in just two years, Fairbanks has moved from No. 44 to No. 9 in daily soot and ranks high in year-round particle pollution, too. Part of the problem is wood-burning stoves, which emit particles that can stay in the air for a week or more.

Most Improved: Birmingham, Ala.

Two years ago, this longtime industrial city ranked in the five worst for fine particle pollution. But with cleaner diesel fuels and reduced emissions from nearby coal-fired plants, Birmingham is no longer even among the 25 worst cities for daily particle pollution spikes. As with other cities, its stellar improvements are due in part to the federally mandated pollution controls of the Clean Air Act.

Fresh Air: Rapid City, S.D.

Want to breathe easy? Head for the Black Hills of South Dakota. Set against their eastern slope, Rapid City can now lay claim to being the second cleanest city in the U.S. for both ozone and soot, despite pollution threats from agriculture and wildfires. Rapid City edged out Bismarck, N.D., and Honolulu for the honors because it has less year-round particle pollution.

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. Ranked the cleanest in the U.S. for the second year in a row, Santa Fe had the best score in all three categories: ozone, year-round, and daily particulate pollution. The only particulate pollution you're likely to see in Santa Fe is dust from the desert sand.

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