Radiation of sufficiently high energy to cause ionization in the medium through which it passes. It may consist of a stream of high-energy particles (e.g. electrons, protons, alpha-particles) or short-wavelength electromagnetic radiation (ultraviolet, X-rays, gamma-rays). This type of radiation can cause extensive damage to the molecular structure of a substance either as a result of the direct transfer of energy to its atoms or molecules or as a result of the secondary electrons released by ionization. In biological tissue the effect of ionizing radiation can be very serious, usually as a consequence of the ejection of an electron from a water molecule and the oxidizing or reducing effects of the resulting highly reactive species:
where the dot before a radical indicates an unpaired electron and denotes an excited species. Organisms are constantly exposed to background radiation from naturally occurring radioactive elements in soil, water, and air, such as radon gas and cosmic rays. Human-derived sources include nuclear power stations and medical uses, such as X-ray sources in radiography. Radiation damage depends on the dose received.