Well i know that radon is the second cause of cancer to people in any buildings that are not protected from those ground gas . Although , The EPA ( United State environmental protection agency) reports multiple problems of health coming from Radon. From my point of view, any inside soil, in buildings, should be protected by the installation of a polyethylene .It should cover the entire sanitary space to block those nocive gas. Radon have different concentration depending of the type of soil or district. I think it is important to know the concentrations of Radon gas where problems have been reported in your city. In that case ,it is important to advise your client on this problem for his safety, in your report. (sorry for my spelling , i'm french)