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Slideshow: 10 Worst Smog Cities in America

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Smog Towns, U.S.A.

While air in the U.S. is cleaner than it was 10 years ago, about half of us are risking our health by inhaling pollution. One of the biggest dangers is from ozone -- a gas that worsens asthma and COPD and can even shorten your life. So what cities have the worst air? Here's a rundown of the 10 dirtiest cities from the American Lung Association's "State of the Air 2013" Report -- along with a few of the cleanest.

No. 10: El Centro, Calif.

This hot, dry city sits on the U.S.-Mexico border and has to deal with ozone from both sides. Pollution blows north from Mexicali, a nearby Mexican city. And burning farm waste and traffic from clogged border crossings worsen the problem. Why is smog often worse in hot climates like El Centro? Ozone smog forms when pollutants -- from factories and cars -- interact with heat and sunlight.

No. 9: Washington, D.C. -- Baltimore

Commuters spewing exhaust from their cars caused some of this area's worst ozone-pollution days during the last few years. But the problem isn't just homegrown. Dirty air also blows in from coal-fired power plants as far away as Georgia and Ohio. Smokestack controls are helping to fight the problem. The Environmental Protection Agency's proposal for cleaner gas and vehicles may also help.

No. 8: Dallas, Texas

Texas has made strides in cleaning up its air during the last decade. Replacing old SUVs and pickup trucks with cleaner ones has helped. But Dallas still has a long way to go. Its traffic, extreme heat, and frequent droughts all help generate unhealthy ozone.

No. 7: Houston, Texas

Houston is known as the Energy Capital of the World -- and all that energy hasn't helped its air quality. Ozone days have climbed as shale gas drilling has boomed and refineries and plants have expanded. The EPA now has stronger rules on new oil and gas drilling and processing that may help keep the air cleaner in the future.

No. 6: Sacramento, Calif.

Major freeways are pollution hotspots, and Sacramento has four passing through its downtown. About 90% of the area's smog-forming fumes come from cars, trucks, busses, trains, tractors, and other vehicles. The area has cut ozone pollution by nearly half since 2000. But as the number of cars keeps growing, pollution will be tough to control.

No. 5: Hanford, Calif.

Despite California's tough clean-air standards, four of the top six smog cities in the U.S. are in the San Joaquin Valley, like Hanford. Hot and dry and surrounded by mountains, the bowl-shaped valley traps pollutants. Hanford sits along the valley super-highway. Exhaust and dust and gasses from massive cattle feedlots nearby add to its pollution problems.

No. 4: Fresno, Calif.

The Fresno area had its fewest unhealthy ozone days since 2000. Rich farmland surrounds the city. That helps make it the commercial hub of the San Joaquin Valley, where the population and highway use are growing faster than anywhere else in the state. High traffic and heavy farming make ozone a big challenge in this city. Nearly 1 in 3 children in Fresno has asthma.

No. 3: Bakersfield, Calif.

This Southern California city of about 350,000 is No. 1 for both year-round and short-term spikes in particle pollution (soot). These tiny bits of solids and liquids can lodge deep in your lungs and raise the risk of heart disease, stroke, and asthma attacks.

No. 2: Visalia, Calif.

This small farm town, a gateway to Sequoia National Park, is plagued by pollution trapped against the nearby Sierra Nevada mountains. Lots of sunshine plays a role.So does dirty exhaust from diesel trucks, busses, and other vehicles, which causes 80% of the valley's smog.

No. 1: Los Angeles, Calif.

Enclosed by mountains on three sides and home to about half of California's residents -- and their many cars -- sunny Los Angeles has had a long, tough battle with smog. It’s still the nation's smoggiest city. But L.A. has made progress. It has reduced car emissions even as the city has grown. And it has cut harmful ozone days by more than one-third since 2000.

Particle Pollution: Salt Lake City, Utah

Spikes in soot give this mountain lovers' heaven the worst score in short-term particle pollution outside of California. Warm air above can trap cold air, car emissions, and industrial fumes below to surround the city in a toxic smog. Lasting a few hours or a few days, peaks in soot might seem minor. But they can trigger asthma attacks and raise the risk of heart attacks and strokes.

Smoky Surprise: Fairbanks, Alaska

Most people think of pollution as a problem in hot, urban areas -- not arctic Alaska. But it's a serious issue in Fairbanks. Wood-burning stoves churn out chemicals that can stay in the air for a week or more. Fairbanks gets wrapped in a blanket of soot held down by warmer air above on cold, clear days.

Most Improved: Pittsburgh, Pa.

Outside of California, Pittsburgh is No. 1 for year-round and No. 2 for short-term particle pollution. But the city is working hard to clear its smoky skies, cleaning up the largest U.S. plant for making coke, a fuel used to make steel. It's paying off -- Pittsburgh has enjoyed its lowest levels ever of ozone and particle pollution.

Clean Air: Rapid City, S.D.

Want to breathe easy? Head for the Black Hills of South Dakota. Rapid City is one of the top four cleanest cities in the U.S. for both ozone and soot. The flat prairies and strong winds usually mean pollution blows elsewhere.

Clean Air: Cape Coral, Fla.

On the Gulf of Mexico, this sun-splashed city has more than 400 miles of canals and some of the cleanest air in the nation. Why? Mostly flat and swept by sea winds on both sides, there's nowhere for pollution to get trapped. Most of the time, dirty air just blows away.

Clean Air: Bismarck, N.D.

Coal-fired power plants, oil fields, and diesel engines in North Dakota help create air pollution across the upper Midwest. But the second largest city in the state has some of the cleanest air in the U.S. Part of Bismarck's high scores come from its mild summers, cold and windy winters, and open, flat land.

Clean Air: Palm Bay, Fla.

Like other cities in southern Florida, Palm Bay is blessed by nature. Despite a major interstate highway and nearby port, the area's strong winds and flat landscape don't give pollution a place to settle. As Florida updates old power plants and cars get cleaner, Palm Bay may breathe in clean, fresh air for years to come.