Radon Gas is considered the second leading cause of lung cancer in the US. It seems that there is a lot of misinformation with regards to what causes radon as well as the testing protocols. Radon is also very prevalent in southern Wisconsin.
Radon gas is caused by the breakdown of Uranium in the soil. Mostly given off by rock and soil. It is found in indoor air and can be contained in water as well. The risk with radon in water is not drinking it but rater that the radon is released into the air from faucets, showers and other plumbing fixtures. Radon is measured in Picocuries per liter of air (pCi/L). Acceptable levels measure between 0.0 pCi/L to 3.9 pCi/L. Homes that test 4.0 pCi/L and higher should have Radon reduction systems installed
Very often I hear people say “Well, I don’t use my basement so I don’t need to test”, or “I know it has to be winter time to test”. Both of these statements are incorrect. The area that is tested is determined by the use of the area. The lowest lived in level of the home is where testing should be conducted. Closed building conditions must be kept for 12 hours prior-to and for the duration of the testing period. So, in this sense winter is a great time to test but testing can be conducted at any time of year as long as closed building conditions are kept.
Generally speaking each upper floor would measure about half of the floor below it, so if your basement measured at 9 pCi/L your first floor would exceed the allowable level.
More information can be found in the EPA Buyers and Sellers Guide to Radon
Testing for radon is a simple process and is conducted over a period of 48 hours for 
real estate transactions. A longer-term test can also be conducted to provide more accurate results.
If you haven’t had your home tested for Radon within the last two years now is the time to have a test conducted.
-Aaron
Aaron M. Zuehlke is the owner and lead inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving southern Wisconsin. Specializing in Home inspection, Radon Testing and residential Thermal Imaging.