Ionizing Radiation

Introduction Radiation that has sufficient energy to remove electrons from atoms is called “ionizing radiation.” Ionization results in the production of negatively charged free electrons and positively charged ionized atoms. Ionizing radiation can be classified into two groups: photons (includes X-radiation and gamma radiation) and particles (includes alpha and beta particles and neutrons). The radioactive compound thorium dioxide, which decays by emission of alpha particles, was first listed in the Second Annual Report on Carcinogens (1981), and radon and its most common isotopic forms (radon-220 and radon-222), which also emit primarily alpha particles, were first listed in the Seventh Annual Report on Carcinogens (1994). Three types of ionizing radiation (X-radiation, gamma radiation, and neutrons) were first listed in the Eleventh Report on Carcinogens (2004). X-radiation and gamma radiation are included in one listing. The profiles for X-radiation and gamma radiation, neutrons, radon, and thorium dioxide, which are listed as known to be human carcinogens, follow this introduction.