Energy in the form of waves or particles. Radiation is mainly electromagnetic and is broadly classified as ionising and non-ionising. The former can propel ions from an atom; these have an electrical charge and can combine chemically with each other. Ionisation occurring in molecules that have a key function in living tissue can cause biological damage which may affect existing tissue or cause mutations in the GENES of germ-cell nuclei (see GAMETE; CELLS). Non-ionising radiation agitates the constituent atoms of nuclei but is insufficiently powerful to produce ions.

Ionising radiation comprises X-RAYS, GAMMA RAYS and particle radiation. X-rays have a very short wavelength and very high frequency, and their ability to penetrate matter depends upon the electrical energy generating them. X-rays that are generated by 100,000 volts can pass through body tissue and are used to produce images – popularly known as X-rays or radiographs. X-rays, generated at several million volts can destroy tissue and are used in RADIOTHERAPY for killing cancer cells. Gamma rays are similar to X-rays but are produced by the decay of radioactive materials. Particle radiation, which can be produced electrically or by radioactive decay, comprises parts of atoms which have mass as well as (usually) an electrical charge.

Non-ionising radiation includes ultraviolet light, radio waves, magnetic fields and ULTRASOUND. Magnetic fields are used in magnetic resonance imaging (MRI) and ultrasound, which is inaudible high-frequency sound waves, and is used for both diagnoses and treatment in medicine.