Sources of Exposure
Humans and other organisms are exposed continually to ionising radiations and radionuclides from a variety of sources. These include secondary radiations originating from the interactions of cosmic rays with the atoms and molecules present in the Earth’s atmosphere, radionuclides such a carbon-14 induced by those same cosmic ray interactions, radioactive materials present in the crust of the Earth and in other environmental media, such as soils, sediments and water bodies, radionuclides released to the environment as a result of the atmospheric testing of nuclear weapons between 1945 and 1980, and radionuclides released as a consequence of power production from nuclear reactors, as well as from other industrial sources. In addition, humans are exposed to ionising radiations as a result of diagnostic and therapeutic medical procedures involving either external irradiation (x-ray examinations, CT computed tomography scanning) or the administration of radionuclides (nuclear medicine).
In general, the most important sources of radiation exposure of humans are those present in the natural environment (natural background), but medical exposures are of comparable importance in the more highly developed countries and there is a continuing increasing trend in average annual exposures due to medical diagnostic procedures. Annual exposures due to the atmospheric testing of nuclear weapons peaked in 1962/63, but have decreased substantially since then. In comparison with exposures due to natural background and diagnostic medical procedures, exposures due to routine releases of radionuclides in the course of nuclear power production and other industrial processes are very small.
Major Nuclear Accidents involving releases of radionuclides to the environment are addressed separately under the heading ‘Major Nuclear Accidents and their Consequences’.