Report: Half of Americans breathing bad air

Worst 3: Los Angeles, Bakersfield and Fresno, California

NEW YORK (CNN) -- About half of all Americans live in areas where levels of smog are unhealthy, the American Lung Association said in a report released Wednesday.

The annual "state of the air" study, which crunches government data on ozone, smog's main ingredient, finds more than 142 million Americans live in areas where ozone levels could endanger people's health.

"We have found there actually has been an increase of 18 percent in the areas that are rated "F" -- which is unhealthy, and that's a result of failure to take action over the last several years," said John Kirkwood, a spokesman for the group.

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According to the report, the smoggiest metropolitan areas are:

1. Los Angeles, California

2. Bakersfield, California

3. Fresno, California

4. Visalia, California

5. Houston, Texas

6. Atlanta, Georgia

7. Merced, California

8. Knoxville, Tennessee

9. Charlotte, North Carolina

10. Sacramento, California

Cities that earned an "A" for ozone pollution -- meaning they had no smog alerts from 1998 through 2000 -- are Bellingham, Washington; Colorado Springs, Colorado; Duluth, Minnesota; Fargo, North Dakota; Flagstaff, Arizona; Honolulu; Laredo, Texas; Lincoln, Nebraska; McAllen, Texas; Salinas, California; and Spokane, Washington.

Space shuttle astronauts photographed this layer of smog over upstate New York's Finger Lakes on October 21, 2000.

Ozone, the main ingredient in smog, is formed when heat and sunlight warm nitrogen oxides and hydrocarbons from tailpipes and smokestacks.

According to the association, ozone is not just a nuisance, it's a health hazard, and people with asthma and other respiratory problems, children and the elderly are especially vulnerable.

"Ozone is an irritant. It's like getting a sunburn on your airways," Kirkwood said.