America's Most Polluted Cities

This time of year, many Americans are concerned with sunburns. In some areas, they should pay more attention to smog.

The reason? Though it's often invisible, air pollution is a threat to 186 million Americans, according to a new report released by the American Lung Association.

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The annual report--State of the Air 2009--found that six in 10 Americans live in counties where ozone or particle pollution has reached dangerous levels. Both types of pollution can be deadly and have been linked to worsening respiratory conditions like asthma, emphysema and bronchitis, and there is evidence that particle pollution increases risk of heart attacks and strokes.

In Depth: Worst Cities For Short-Term Particle Pollution

In Depth: Worst Cities For Year-Round Particle Pollution

In Depth: Worst Cities For Ozone Pollution

In compiling this list, the American Lung Association ranked U.S. metropolitan statistical areas (MSAs)--geographic entities defined by the U.S. Office of Management and Budget for use by federal agencies in collecting, tabulating and publishing federal statistics--using the highest weighted average for any county within that MSA. Grading was based on the Environmental Protection Agency's determination of violations of the national ambient air quality standard.

Though overall air pollution is down compared to previous years, the problem is still widespread. Visalia and Fresno, two mid-size towns in central California ranked high for short-term and year-round particle pollution. Birmingham, Ala., and Cincinnati were listed in the top 10 of metro areas with unhealthy levels of year-round particle pollution. In the Southwest, the Houston, Dallas and Phoenix metro areas had high ozone levels. link...