3. Physical phenomena of radon in soils

The main source of radon is soil, where it is formed by the decay of radium atoms present in the minerals that make up rocks. Other less important sources are groundwater, the oceans and building materials.

The terms "emanation" and "exhalation" both express the action of releasing. Radon must first be released from the solid phase containing the radium (emanation), then released into the atmosphere on a macroscopic scale (exhalation). The transport of radon atoms present in the air or water contained in the pores of the material act as an intermediary between emanation and exhalation. Four successive stages can be distinguished, as illustrated in figure 1 .