ABSTRACT
Every human being is exposed to ionising radiation from natural sources. By natural sources we mean cosmic rays and terrestrial sources that originate from radioactive elements present in the Earth's crust. Naturally occurring radionuclides of terrestrial origin (also called primordial radionuclides or NOR) are present in various amounts in the soil and rocks, in water and in the air: radionuclides with half-lives comparable to the age of the Earth, and their decay products, exist in significant quantities in these media. Many examples of exposure to natural radiation sources are modified by human practices: This is the case with NORM activities, which will be dealt with in Section 14.5.
