Most of us can breathe easier thanks to clean air laws and improved fuels and power plants. Yet 4 in 10 Americans live in places with high amounts of ground-level ozone (a.k.a. smog), which forms when factory and auto gases mix with sunlight. It can trigger asthma attacks and other health problems. Here's a look at the American Lung Association's 10 smoggiest cities for 2015.
No. 10: Phoenix-Mesa-Scottsdale, AZ
America's sixth largest city gets an "F" for the number of high ozone days. Triple-digit summer temps and more than a million residents fuel the smog problem here. The problem gets worse in warm, dry weather -- and this desert city has plenty of that. It's worst from May through September. You can help keep the air clean by fueling your car after dark or in the evening, when it's cooler.
No. 9: Las Vegas-Henderson, NV
Let's hope the air in Vegas stays in Vegas. The average number of high ozone days went up this year in the Sin City metro area, causing it to tumble a spot from 2014. The town boasts more than 300 days of sunshine yearly. That means more smog, which thrives on sunny, warm days. The state even has a smog spotter program, where you can turn in cars and trucks spewing smoke.
No. 8: Modesto-Merced, CA
The San Joaquin Valley area has been hovering in or around the top 10 cities on this list for the last few years. Its bowl shape and wind patterns hold in unhealthy vehicle emissions and other things that gunk up the air. Rising temperatures across the area have helped create even more smog.
No. 7: Dallas-Fort Worth, TX
Plenty of traffic, refineries, power plants, and warm temperatures help fuel smog in this town. The cowboy mecca has long been known as a top ozone offender. It's teetered back and forth in the middle of this dirty cities list for years. That's bad news for the estimated 840,000 residents who have asthma or chronic obstructive pulmonary disease (COPD).
No. 6: Houston-The Woodlands, TX
The Energy Capital of the world retains its ranking from last year. The area has many refineries and power plants that continue to zap air quality and leave behind a hazy blanket of smog. Some oil and gas companies have challenged the city's ability to enforce the Clean Air Act, which calls for tighter rules on smokestack pollution.
No. 5: Sacramento-Roseville, CA
California's capital area logged its fewest unhealthy ozone days for the first time since the American Lung Association started issuing its report 16 years ago. Still, it remains a hotspot for air pollution. Hot, dry summers, four major highways, plenty of smog-producing vehicles, and its valley geography are to blame. The Northern California metro area keeps the same ranking as last year.
No. 4: Fresno-Madera, CA
The landscape of San Joaquin Valley is breathtaking -- sometimes literally. The valley traps pollution from cars and industrial equipment. That leads to unhealthy air. This central California area's average number of high ozone days jumped this year, once again cementing its place as one of the nation's top smog cities.
No. 3: Bakersfield, CA
This Southern California metro area remains the third-worst city for smog in the nation. It’s located at the edge of the state’s central valley, which traps output from commuters, truckers, and nearby oil companies. Despite the dirty air, only about 12% of the city's 864,000 residents are reported to have asthma or COPD.
No. 2: Visalia, CA
The gateway to Sequoia National Park saw a drop in the average number of unhealthy high-ozone days this past year. Yet it remains the second-smoggiest city in America thanks to warm weather and exhaust from diesel trucks and other vehicles.
No. 1: Los Angeles, CA
The City of Angels doesn't often budge from the top spot as the nation's smoggiest town. With more than 10 million people -- and their cars -- it continues its long, tough battle with dirty air. Mountains on three sides don't help. But sunny L.A. is making progress. This year marks the fewest high ozone days recorded since the American Lung Association started giving out report cards.
Particle Pollution: Fresno, CA
The American Lung Association’s report looks at more than just smog. That means more bad news for Fresno, which leads the nation when it comes to other other stuff in the air called particulates. These are tiny bits of smoke, dirt, dust, and soot. When you take a breath, they can get stuck deep in your lungs. This type of pollution raises your risk of heart attacks, lung cancer, and asthma attacks.
Clean Air: Bismarck, ND
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: Cape Coral-Fort Myers-Naples, FL
On the Gulf of Mexico, this sun-splashed area 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. The dirty air just blows away.
Clean Air: Elmira-Corning, NY
Perched on the edge of New York’s Finger Lakes region, this area is known for natural wonders and wineries in addition to clean air. Glass and ceramics giant Corning Inc., headquartered here, has a nearby plant that makes air-cleaning products used in China.
Clean Air: Fargo, ND
The state’s largest city, which has gained fame in recent years as a quirky setting for movies and TV shows, is also one of its cleanest. It shares the wind and wide-open spaces that keep the air breathable in Bismarck, which is located about 200 miles west.
Clean Air: Rapid City-Spearfish, SD
Want to breathe easy? Head for the Black Hills of South Dakota. Rapid City is one of the 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: Salinas, CA
Located at the head of a valley and close to the Pacific, this California town boasts some of the state’s best air. Unlike cities to its west and south, Salinas benefits from a fresh wind and a largely rural economy without much traffic.
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional medical advice, diagnosis or treatment and should not be relied on to make decisions about your health. Never ignore professional medical advice in seeking treatment because of something you have read on the WebMD Site. If you think you may have a medical emergency, immediately call your doctor or dial 911.