AirCompare is an EPA-developed air quality tool which provides local air quality information to help people make informed decisions about moving, vacationing or living in different areas of the country.
Here are some other resources that provide data on this topic:
Another useful site is the EPA for Citizen Action Resources web page, which focuses on environmental issues and potential environmental and human health risks caused by pollution.
You may also be interested in the State of the Air report from the American Lung Association. It lists information regarding air quality and provides a chart for each county of every state with a letter grade of A-F according to various air quality measurements See: State of the Air Exit