The good news: Air has gotten cleaner in recent years. Now the bad: More than half of people in the U.S. still breathe air dirty enough to cause health problems, according to the American Lung Association.

Bakersfield, Calif.

Population: 807,407

Bakersfield and Delano, Calif., residents breathe some of the country’s most polluted air, based on several measures. The area ranks highest in annual and 24-hour peak averages in fine particulates.

Agriculture in this region of the San Joaquin Valley "whips up" a lot of dust, pesticides, and fertilizers, says Dr. Kaufman.

North of Los Angeles, the area also has the second-highest level of ozone pollution in the U.S. Mountains surround the valley on three sides, creating inversion layers that trap pollution, with little wind to carry it away.

Visalia, Calif.

Population: 429,668

Not far from Bakersfield, Visalia and neighboring Porterville also struggle with dirty air.

Fine particles emitted by vehicles (especially diesel-powered ones), coal-fired power plants, and burning wood can penetrate deep into the lungs, while car exhaust, heat, and sunlight contribute to high ozone levels.

The dual airway irritants can exacerbate asthma and chronic obstructive pulmonary disease (COPD) as well as make breathing more difficult for people with healthy lungs.

Los Angeles

Population: 17,820,893 (includes Long Beach and Riverside)

Lots of people, cars, factories, and shipping ports, as well as sunny, stagnant weather and a "bowl-like" topography, all contribute to local air pollution. (L.A. is the top spot for ozone and second for year-round particulates.)

"But there’s reason to be a little optimistic," says Jonathan M. Samet, MD, director of the Institute for Global Health at the University of Southern California, in Los Angeles. "It’s a pretty nice day and I can see downtown L.A. That would not have been the case 30 or 40 years ago."

Phoenix

Population: 4,364,094

Arizona was long known as a place of respite for people who needed fresh air, particularly those suffering from respiratory problems. Unfortunately, its capital, Phoenix, and the nearby cities of Mesa and Glendale have a topography that makes it easy for pollution to settle, and hard for it to get out, says Norman H. Edelman, MD, chief medical officer at the ALA.

People at risk—children, the elderly, and those suffering from diabetes, heart, or lung disease—should keep tabs on air quality and stay indoors (or at least not exercise outdoors) on bad air days, says Dr. Samet.

Hanford, Calif.

Population: 148,764

The Hanford and Corcoran areas, also located in California’s San Joaquin Valley, rank in the top 10 for all three measures of pollutants.

Dr. Kaufman notes that individuals have little control over their exposure, as opposed to other heart or lung risk factors like smoking or bad eating habits.

Dr. Edelman recommends that those who live in polluted regions take control of what they can: Use air conditioners with quality filters on warm days and exercise outdoors only in the early morning, when ozone is relatively low.

Fresno, Calif.

Population: 1,063,899

"Fresno shares much of the same story," says Dr. Samet, pointing particularly to the trifecta of traffic, sunshine, and agriculture.

The ALA says there is growing evidence that breathing pollution near busy roads may not only worsen diseases over time, but also increase the risk of chronic diseases such as diabetes, heart disease, asthma, and COPD.

This region of the San Joaquin Valley, which includes Madera, ranks among the worst for ozone and short-term particle pollution—fourth and second spots, respectively.

Birmingham, Ala.

Population: 1,212,848

Residents of Birmingham, and nearby Hoover and Cullman, breathe particle-ridden, hazy air that is common in many industrial cities, notes Dr. Samet.

In the late 1800s, the city was known as the Steel Capital of the South. While most of those steel mills have since closed, plenty of tailpipes and power-plant smokestacks continue to spew out pollutants.

The area is 8th for both 24-hour and annual particle pollution, and 21st on the list for ozone.

Cincinnati

Population: 2,214,954

The Cincinnati area, which includes Middletown, Ky., and Wilmington, Ind., has power plants that create smog and soot that find their way into the lungs of local residents.

Dr. Edelman’s advice: "Stay healthy, and fight for clean air nationally and locally."

A growing number of communities across the country are now banning wood-burning, and retrofitting school buses so they no longer run on dirty diesel, he says. The Ohio government is on board.

Louisville, Ky.

Population: 1,395,634

The Louisville area (Jefferson County, as well as neighboring Elizabethtown and Scottsburg, Ind.) is surrounded on all sides by hills that can hold dirty air hostage.

The heat island effect also concentrates pollutants in the urban region of Louisville, as it does in other cities such as Los Angeles and Phoenix.

The region is 10th for annual particle pollution and 22nd for 24-hour particle pollution, but didn't make the top 25 list for ozone.

Modesto, Calif.

Population: 510,385

The final spot among the dirtiest 10 is shared by yet another San Joaquin Valley city. Modesto is surrounded by farmland and suffers from agricultural pollution similar to that in Bakersfield and the surrounding areas.

But this city, too, is making efforts to clean the air, including establishing strict rules for the burning of trash and yard waste.

Pittsburgh

Population: 2,445,117

Although far less common than in the past, the processing of coal and, yes, steel continues in this part of the Ohio River Valley, which includes neighboring New Castle.

These industries aren't uncommon in areas within wind’s reach of Pittsburgh, such as West Virginia.

The region ranks 7th in annual particle pollution, 3rd for 24-hour particle pollution, and 24th for ozone.

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