Good question. Radon is a naturally occurring gas and can be anywhere. Please visit
http://www.epa.gov/radon/pubs/citguide.html for more information. If your purchase contract calls for a radon test, the seller may be responsible for the remediation depending upon the structure of your contract. After the remediation unit is installed, they should retest to verify. Read the information from the EPA and you should be more comfortable. There are no radon free zones that I am aware of nor would I ever attest to one as radon levels could increase at any point.