Ionizing Radiation

Ionizing radiation is electromagnetic radiation that has sufficient energy to remove electrons from atoms. Ionization results in the production of negatively charged free electrons and positively charged ionized atoms. Ionizing radiation can be classified into two categories: photons (X-radiation and gamma radiation) and particles (alpha and beta particles and neutrons). Five types or sources of ionizing radiation are listed in the Report on Carcinogens as known to be human carcinogens, in four separate listings: • X-radiation and gamma radiation (included in one listing) were first listed in the Eleventh Report on Carcinogens (2004). • Neutrons were first listed in the Eleventh Report on Carcinogens (2004). • Radon and its isotopic forms radon-220 and radon-222, which emit primarily alpha particles, were first listed in the Seventh Annual Report on Carcinogens (1994). • Thorium dioxide, which decays by emission of alpha particles, was first listed in the Second Annual Report on Carcinogens (1981).