CNN Newsblog

Despite some gains, more than half of the United States' population lives in cities where pollution levels make the air unhealthy to breathe, an annual report says.

Cities in California dominated the most-polluted list in the American Lung Association's "State of the Air 2010" report released Wednesday.

The organization rates communities on three criteria: ozone, short-term particle spikes and long-term particle averages.

Los Angeles, Fresno and Bakersfield, all in California, had the dubious distinction of being in the top-10 list of all three categories.

The organization found that the worst places to breathe were California cities Los Angeles, Bakersfield and Visalia for ozone pollution; and Bakersfield, Los Angeles and Phoenix, Arizona in terms of average particulates.

Bakersfield and Fresno in California, and Pittsburgh, Pennsylvania, recorded the highest number of particle pollution spikes, which are jumps in the number of particles in the air that can last for many hours or even days.

At least 20 of the most polluted cities had improved from last year's report, the organization said.

Some of those cities were Atlanta, Georgia; Cleveland, Ohio and Detroit, Michigan. But several cities in California had gotten worse.

"Americans can take steps today and every day that will improve air quality immediately and ultimately impact climate change as well," said Charles D. Connor, American Lung Association President and CEO. "Drive less.

Don't burn wood or trash. Use less electricity, and make sure your local school system
requires clean school buses."