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Los Angeles, California

If you don't really care about breathing, Los Angeles is a great place. The metro area that stretches from Long Beach to Riverside has the worst ozone pollution in the country, according to the American Lung Association's State of the Air report for 2010. Along with being tops in ozone pollution, L.A. is ranked third in year-round particle pollution, and fourth in short-term particle pollution.

Ozone is the byproduct of pollutants released by cars, chemical plants, refineries, and other sources. It exists naturally in the upper atmosphere of the Earth, but when emitted at ground level, it's considered a harmful outdoor pollutant. Inhaling ozone can cause wheezing, coughing, chest pain, throat irritation, congestion, and can make people more susceptible to respiratory illnesses such as bronchitis and pneumonia, according to the U.S. Environmental Protection Agency. Think about that next time you drive in Los Angeles, which also lays claim the worst traffic in the country.