The EPA recommends that all residences below the third floor level be tested for radon. In addition, the EPA also recommends testing all rooms in contact with the ground or over crawlspaces in schools. If you have tested your home, you should retest every two years since radon levels can change with structural changes in the home. If you decide to use a lower floor of your home, such as a basement, you should test this level before occupancy. In addition, you should always test prior to the purchase of a home.