This publication is based on the major scientific reports of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) published in the last 25 years and aims to expand public knowledge on levels of exposure to ionizing radiation and possible associated effects. This publication does not set, or even recommend, radiation safety standards, rather, it provides information on basic science related to radiation (origin, quantities and units), on radiation effects (on humans and the environment) and on radiation sources (natural and artificial). Helping the public understand what radiation is and how it affects life on this planet lies within the core mandate of UNEP.