Radon is a noble gas and occurs in nature in the form of 3 isotopes: Radon-222, Radon-220 also known as Thoron and Actinon (Radon-219).
The respective half-life (t1/2) is essential to occurrence and spreading of these 3 Radon isotopes. To enable the gas to enter into the atmosphere and this way to gain access to human lungs the respective half-life must be long enough to allow the gas – before further decay – to migrate by diffusion or convection from the source rock into the atmosphere. Therefore, Thoron (t1/2: 55s) and Actinon (t1/2: 4 s) due to their short half-life are rarely found in the atmosphere. Radon-222 (t1/2: 3,8 d) and its progeny products are therefore the major originators of radiation exposition by Radon.
Within the Uranium exploration, when mining and treating thorium consisting ore and minerals as well as with certain “earthier” types of housing where thorium containing construction material is used, separate evaluation of Radon and Thoron is requested.