The absence of the neptunium decay chain is due to the lack of sufficiently long-lived members of this chain complete decay of the parent radionuclides and their progeny has already occurred. The fact that the uranium, thorium, and actinium decay chains are found in nature is directly related to the very long half-lives of the parents of these chains. The origin of the primordial natural radionuclides of the earth is associated with the phenomenon of nucleosynthesis in stars (Fowler 1967). Variations of radon concentrations in buildings are responsible for the largest variations in doses received by the public from natural internal sources. Particularly high concentrations of radioactive minerals in soil have been reported in Brazil, India, and China. Cosmic radiation alone varies by about a factor of 2 over the range of elevation that encompasses most of the world's population (0-2,000 m) and to a much smaller degree with latitude because of the variation in the earth's magnetic field. In most places on the earth, natural radiation from external sources varies within about a factor of 4 but in some localities, the variation is greater because of abnormally high or low soil concentrations of radioactive minerals. A particular category of exposure to internal radiation, in which the bronchial epithelium is irradiated by alpha particles from the short-lived progeny of radon, constitutes a major fraction of the exposure from natural sources. The main routes of radionuclide intake are ingestion of food and water and inhalation. Humans are exposed to natural radiation from external sources, which include radionuclides in the earth and cosmic radiation, and by internal radiation from radionuclides incorporated into the body. Cosmogenic radionuclides are produced continuously by bombardment of stable nuclides by cosmic rays, primarily in the atmosphere. Primordial radionuclides have half-lives comparable with the age of the earth. The natural radionuclides include the primordial radioactive elements in the earth's crust, their radioactive decay products, and radionuclides produced by cosmic-radiation interactions. Natural radiation comprises cosmic radiation and the radiation arising from the decay of naturally occurring radionuclides. A more detailed account of natural radiation can be found in Eisenbud and Gesell (1997), which was used as a guide to prepare parts of this chapter.

Its purpose is to provide background information for discussions of the mechanisms by which exposures to natural sources can be increased by technologic activities, that is, can become exposures to TENORM. This chapter describes the behavior of selected natural radionuclides in the environment, the sources and variability of natural radiation, and the doses received by humans.

Radioactive elements