This is an old thread but one aspect of radon is completely absent. Radon is also found in well water, particularly from drilled wells. Once the water enters your house it outgasses and enters the air. Radon in drinking water has a much smaller chance of causing stomach cancer than airborne radon causing lung cancer so it is generally given less attention.
In the U. S. each state can modify the EPA radon guidelines so you need to check locally to see what the rules and recommendations are. Here in Maine the state government is more conservative than the federal government and recommends the testing of water for radon in every home. The federal guideline is to only test for radon in water if the radon air levels are high. There are treatment systems to remove radon from domestic water if your levels are high.
You can get radon in air test kits from retail stores such as hardware stores and take the sample yourself, unless you are selling your home. At the time of real estate transaction Americans are required to hire a trained, registered person to take the sample or set out a testing device which gives the results directly.
Jim