are the primary sources of radon in soil, with 222 Rn being dominant in rocks, soil, building materials, underground and surface waters [2] and set to be the most hazardous radionuclide. Radon (222 Rn) and its short-lived progeny (218 Po, 214 Po, 214 Bi, 214 Pb) are attached in dust and in water droplets creating radioactive aerosols, that inhaled via breathing and enter human lungs. Radon enters buildings through gaps around pipes or cables and through cracks in floors [3]. Primary studies have shown that radon is the second most dangerous cause of lung cancer after smoking. This happens when alpha particles emitted from radon progeny damage pulmonary epithelium [4-7]. Many studies have been made for the measurement of indoor radon concentrations in several countries [8-19]. Over the years, in Greece, indoor radon concentrations measurements, to our knowledge, are as follows: several small-scale [20-24], two middle-scale [3,25] and one large-scale [26]. Under the National Strategic Reference Framework (NSRF) "Thales" project of the Technological Education Institute of Piraeus and extending the aforementioned large scale radon survey; this paper addresses the grade of severity with witch factors influence indoor radon concentration levels. Similar studies in other countries have shown that indoor radon concentrations are higher at lower floor levels [27-30]. Moreover, recent studies indicate that radon emanation from building materials contributes significantly in indoor radon concentration in dwellings [31-33]. Results of current work may strengthen these considerations and additionally provide evidence for lack of correlation of other factors such as the "building walls contact" and "construction year" with radon concentrations.