This is a list from the American Lung Association's "State of the Air 2013" Report.
10. El Centro, CA
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.
9. Washington, D.C. -- Baltimore
Commuters' exhaust from their cars caused some of this area's worst ozone-pollution days during the last few years. Dirty air also blows in from coal-fired power plants as far away as Georgia and Ohio.
8. Dallas, Texas
Replacing old SUVs and pickup trucks with cleaner ones has helped this dirty city get a litter cleaner but its traffic, extreme heat, and frequent droughts all help generate unhealthy ozone.
7. Houston, Texas
Houston is known as the Energy Capital of the World and all that energy hasn't helped its air quality. The EPA now has stronger rules on new oil and gas drilling and processing that may help keep the air cleaner in the future.
6. Sacramento, CA
Major freeways are pollution hot spots, and Sacramento has four major through-ways making the pollution level go through the roof. About 90% of the area's smog-forming fumes come from cars, trucks, buses, trains, tractors, and other vehicles.
5. Hanford, CA
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.
4. Fresno, CA
High traffic and heavy farming make ozone a big challenge in this city. Nearly one in three children in Fresno has asthma.
3. Bakersfield, CA
Bakersfield's particle pollution (soot) is a huge problem. Soot can lodge deep in your lungs and raise the risk of heart disease, stroke, and asthma attacks.
2. Visalia, CA
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, buses, and other vehicles, which causes 80% of the valley's smog.
1. Los Angeles, CA
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.
To read more and to find out which are the cleanest cities click here.