When used in Agency publications, the term radiation normally refers only to ionizing radiation. The IAEA has no statutory responsibilities in relation to non-ionizing radiation. Ionizing radiation can be divided into low LET radiation and high LET radiation (as a guide to its relative biological effectiveness), or into strongly penetrating radiation and weakly penetrating radiation (as an indication of its ability to penetrate shielding or the human body). High Linear Energy Transfer Radiation: Radiation with high linear energy transfer, normally assumed to comprise protons, neutrons and alpha particles (or other particles of similar or greater mass). These are the types of radiation for which ICRP recommends a radiation weighting factor greater than 1. Contrasting term: low LET radiation. Ionizing Radiation: For the purposes of radiation protection, radiation capable of producing ion pairs in biological material(s). Low Linear Energy Transfer Radiation: Radiation with low linear energy transfer, normally assumed to comprise photons (including X rays and gamma radiation), electrons, positrons and muons. These are the types of radiation for which ICRP recommends a radiation weighting factor of 1. Strongly Penetrating Radiation: Radiation for which limits on effective dose are normally more restrictive than limits on equivalent dose to any tissue or organ, i.e. the fraction of the relevant dose limit received will, for a given exposure, be greater for effective dose than for equivalent dose to any tissue or organ. If the reverse is true, the radiation is termed weakly penetrating radiation. For most practical purposes, it may be assumed that strongly penetrating radiation includes photons of energy above about 20-30 keV, high energy electrons (more than about 1-2 MeV) and neutrons, and that weakly penetrating radiation includes photons of energy below about 20-30 keV, beta particles and other electrons of less than about 1-2 MeV, and massive charged particles such as protons. Contrasting term: Weakly penetrating radiation. Weakly Penetrating Radiation: See: strongly penetrating radiation