Every day people are exposed to ionizing radiation from natural sources as well from human-made sources. However, the major contribution comes from naturally-occurring radioactive materials (NORM) found in soil, water and air.
Radon is a naturally occurring gas that comes from uranium that has been present in the Earth's crust since it was formed. The rate of emanation of radon is variable since the amounts of uranium in the soil are significantly different.
Radon when released from the ground can penetrate buildings. Although radon levels can be considered very low in the outdoors, in the interior of the buildings their concentration can reach very high values.

Though radon is chemically inert it is a radioactive gas which means that it can spontaneously transform into other elements. When these elements, called radon progeny, form, they are electrically charged so that they can be added to dust particles present in the ambient air. These dust particles can be inhaled and adhere to lung tissue. The atoms that are deposited there, decay emitting a type of radiation, called alpha radiation, which has the capacity to damage the cells of the lung. Alpha radiation can break the DNA strand of lung cells and this damage can develop a chain of mutations that can lead to lung cancer. Recent studies indicate that alpha particles can be deposited in deeper areas of the respiratory tree and enter the bloodstream. In this case they can radiate cells from organs other than the lung.
Radon can also be dissolved and accumulated in groundwater, being present, for example, in springs and wells. When used in domestic activities, the radon contained in these waters, can free itself inland and contribute to an increase in its concentration inside the dwelling. At the same time, this water can be used for human consumption and, in this case, in addition to the risk of lung cancer there may additionally be the risk of stomach cancer.