Ionizing radiation includes gamma rays, X-rays and particulate radiation. The first two are short wavelength electromagnetic waves composed of high-energy photons and lower-energy photons, respectively.

Particulate radiation is generated from the spontaneous disintegration of radioactive compounds, resulting in emission of alpha particles (helium nuclei), beta particles (electrons) and other forms of energy.

X-rays and gamma rays penetrate tissues deeply, but generate ions sparsely along their path. Therefore, they are considered as having low linear energy transfer (LET) as opposed to particulate radiation.

Ionizing radiation can be expressed in units of exposure (roentgen-R), its absorbance into biologic tissue (rad, gray-Gy) and as biological effectiveness of absorbed radiation (roentgen equivalent man-rem, sievert-Sv). For radiation in soft tissue, rad and rem are often used interchangeably. One hundred rad or rem are 1 Gy or 1 Sv, respectively.

Ionizing radiation can damage biological cells by two mechanisms. The first involves addition of sufficient energy to incite electron shells to free an electron from its atomic orbit, thereby producing charged or ionized biological molecules. The second mechanism involves radiolysis of water to form reactive compounds (e.g., OH-, H+, H2O2) which can attack and disrupt neighbouring molecules.

The insult from a single, random modification of a cell component (e.g., DNA) is termed a stochastic effect and it may still allow the cell to proliferate. Therefore, dose-response curves for effects such as mutagenesis and carcinogenesis may not include a threshold below which no adverse effects occur. It is assumed that this effect may be associated even with very low doses of ionizing radiation and there is great uncertainty as to how to best predict unavoidable injurious effects from such an exposure.