Radon is a natural colorless, tasteless and odorless gas produced by radioactive disintegration of radium, thorium and uranium in rocks which is released from the earth crust in various amounts. From the earth surface, the radon gas gets into the atmosphere or enters straight into buildings. Other sources of radon in building can be a construction materials of poor quality or water from local wells. In the case of an insufficient ventilation of a building, radon can be retained and accumulated excessively in the building. A long-term stay in rooms with a high radon concentration may lead to lung cancer.