[en] Ionizing radiation occurs naturally and ubiquitously, mainly as a result of decay of atoms with too few or too many neutrons in relation to their number of protons. The most common decay modes are alpha radiation (emission of helium nuclei to increase the proportion of neutrons), beta radiation (emission of electrons to decrease the proportion of neutrons), and gamma radiation (electromagnetic waves). X rays (also electromagnetic waves) are usually artificially induced. The activity of a source is measured in becquerel (1/s), the effective dose to a person is measured in sievert (J/kg). The average annual effective dose of, say, 5 mSv to a typical European consists of 1 mSv or so from natural background radiation, 1 or a few mSv from naturally occurring radon in dwellings, 1 mSv or 2 from medical uses of radiation, and a minute fraction of 1 mSv from other artificial uses of radiation, such as a nuclear power. (author). 10 refs., 7 figs., 1 tab