In everyday life human organism is subjected to ionizing radiation coming from natural and artificial sources, including medical diagnostics and therapy. However, contrary to popular beliefs the highest average exposition is due to natural sources. The artificial non-medical sources of ionizing radiation include radioactive materials used in industry and radioactive fallout. Natural exposition comes from internal sources (radioactive elements in the body) and external ones, such as primary and secondary cosmic radiation as well as terrestrial radiation. Therefore it was found that exposure to ionizing radiation coming from sources other than medical amounts to 85% of the total average exposure.