The study graded the air quality of states, counties and cities across the country. No counties in Colorado received an "A." Counties that got a "B" were Denver, Garfield, La Plata, Mesa and Montezuma.
The lowest-graded counties in the state were Douglas, Jefferson and Larimer counties. They all received an "F."
The association examined ozone levels from 2008 to 2010. Researchers also examined traffic congestion and the presence of coal-fired power plants, which they say contribute to air pollution. The three most ozone-polluted regions are all in California.
For more information on the study, or to find where other Colorado counties rank in the study, visit: http://www.stateoftheair.org/2012/states/colorado/
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