Both natural and artificial radioactive isotopes are present in the environment (Table 13.1) in the atmosphere, hydrosphere, lithosphere, and living organisms, including the plant—animal—human food chain. The radioactive isotopes in the environment give the external radiation dose to human organisms, while the iso­topes incorporated by breathing, feeding, and so on are the source of the internal radiation dose (see Sections 13.4.1 and 13.4.3).

Radioactive isotopes, similar to stable isotopes, continuously circulate among the different parts of the environment, as illustrated in Figure 13.2. The radioactive isotopes in compartments are in direct exchange; in other words, they may pass through to other compartments. The distribution of radioactive isotopes in different compartments is mainly determined by their chemical properties. In nature, radio­active isotopes may react with different components of air (oxygen, nitrogen, car­bon dioxide, and aerosols), may dissolve in water, or may be sorbed on the solid phases of the spheres, especially the lithosphere. The chemical reactions of radioac­tive isotopes produce chemical species, which then distribute in the different com­partments in order to approach the thermodynamic equilibria. In nature, of course,

Table 13.1 The Most Important Radioactive Isotopes, Including Natural and Artificial Ones,

in the Environment

Radioactive Isotope

the thermodynamic equilibria are never reached; therefore, only the tendencies of the chemical reactions and the tendency of the distribution of the radioactive iso­topes can be discussed.

The occurrence of radioactive isotopes in the different spheres is discussed on the basis of Figure 13.2.