Natural radionuclides
Natural radionuclides are not the product of human activity and can be found throughout the environment. Natural radionuclides are divided into five main categories:
- Radionuclides induced in Earth's atmosphere by cosmic rays (e.g. H-3, Be-10, C-14).
- Nonchain primordial radionuclides (e.g. K-40, Rb-87). Activity per unit mass of K-40 is measured in building materials for radiation protection purposes.
- Uranium-238 chain radionuclides (e.g. Ra-226, Rn-222, Pb-214, Bi-214, Pb-210). Activity per unit mass of Ra-226 is measured in building materials for radiation protection purposes. Concentration of Rn-222 is measured in soil and indoor air for radiation protection purposes.
- Thorium-232 chain radionuclides (e.g. Th-228, Rn-220). Activity per unit mass of Th-228 is measured in building materials for radiation protection purposes. Concentration of Rn-220 is measured in indoor air as a complement to Rn-222 concentration.
- Uranium-235 chain radionuclides (e.g. Th-231, Pa-231).