In a radiological emergency, effects caused by radiation may be a significant concern. Throughout this course, the term radiation refers to ionizing or nuclear radiation.

Nuclear radiation is energy given off by matter in the form of tiny fast-moving particles or pulsating electromagnetic rays or waves emitted from the nuclei of unstable radioactive atoms. All matter is composed of atoms, which are made up of various positively and negatively charged parts. The forces between these parts work toward a strong, stable balance by getting rid of excess atomic energy (radioactivity). In that process, unstable radioactive atomic nuclei may emit energy, and this spontaneous emission is called nuclear radiation.

All types of nuclear radiation are also ionizing radiation, but the reverse is not necessarily true. For example, x-rays are a type of ionizing radiation, but they are not nuclear radiation because they do not originate from atomic nuclei. In addition, some elements are naturally radioactive, as their nuclei emit nuclear radiation as a result of radioactive decay, but others are induced to become radioactive by being irradiated in a reactor. Naturally occurring nuclear radiation is indistinguishable from induced radiation.

Ionizing radiation cannot be seen by the human eye, nor can it be smelled, heard, or otherwise detected by our normal senses. Radiation can only be detected by radiation detection instruments. When ionizing radiation comes into contact with or passes through material, it may deposits enough energy to break molecular bonds and remove electrons from atoms. This electron removal creates two electrically charged particles (called ions), which may cause changes in living cells of plants, animals, and people.

This characteristic makes radiological emergencies different from other types of emergencies such as floods, hurricanes, or explosions.