There are many factors that affect air quality in an area, and these factors are always changing with time, weather, and other circumstances. Therefore, EPA prefers not to rank cities as "best" or "worst." We do, however, have a tool that allows you to compare the number of unhealthy air quality index (AQI) days in one place versus another. The tool is called AirCompare, and it is located at http://www.epa.gov/aircompare.
Which cities have the best (or worst) air quality?
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