A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively.

Other sources include X-rays from medical radiography examinations and muons, mesons, positrons, neutrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth's atmosphere.

The word radiation arises from the phenomenon of waves radiating (i.e., traveling outward in all directions) from a source.

This aspect leads to a system of measurements and physical units that are applicable to all types of radiation.

A wild boar has been shot and killed after being found with radiation levels more than ten times the safe limit.

The boar was found in central Sweden and is thought to have high radiation due to living in an area still largely affected by nuclear fallout from the Chernobyl disaster, 31 years ago.

People, however, who consume meat with these radiation levels face an increased risk of developing cancer.

Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter.Gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum.The lower-energy, longer-wavelength part of the spectrum including visible light, infrared light, microwaves, and radio waves is non-ionizing; its main effect when interacting with tissue is heating.Like any ideal law, the inverse-square law approximates a measured radiation intensity to the extent that the source approximates a geometric point.Radiation with sufficiently high energy can ionize atoms; that is to say it can knock electrons off atoms and create ions.The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, and is a function of the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the irradiated organism or tissue (effective dose).