I hear it all the time:

"Why do I need to test for radon if I don't have a basement?"

The answer is simple.  The presence or absence of radon in a home has nothing to do with the presence or absence of a basement.  Indoor radon levels are affected by the soil composition under and around the house, and the ease with which radon can enter the house.  In fact, slab foundations can make a home more vulnerable to radon gas because the radon can radiate directly into the home when there is no ventilated crawlspace to separate the living space from the ground.  Another way to think of it is to remember that radon gas naturally moves into the building from the ground through openings and pores in concrete - whether that concrete is the slab under the first level of the home or is the floor of a basement.