The emission of energy (as electromagnetic waves) or matter (as particles) from unstable atoms, which turns them into a more stable form. Some types of radiation are harmful to life; other types are essential (for example, light and heat energy radiated from the sun). Even harmful radiation may be used for beneficial purposes; for example, in treatment by radiotherapy, the biologically-damaging effects of radiation are used to destroy cancerous cells. Four significant types of harmful radiation are gamma radiation, X-rays, alpha particles, and beta particles. Gamma radiation and X-rays are types of electromagnetic waves, and are similar to more energetic forms of light. All 4 types cause damage by ionization – the waves or particles knock out electrons from atoms in the matter that they pass through, turning them into highly reactive ions. In the case of living tissue, the ions formed cause biological damage. Radioactive substances that emit any of these types of radiation constitute a health hazard. However, alpha particles cannot penetrate the skin, so sources of alpha radiation are only dangerous if ingested or inhaled. Gamma radiation can travel large distances through many substances, and even distant gamma sources can pose a risk to humans. Most sources of radiation are natural. Natural sources of ionizing radiation include cosmic rays from space, and radioactive minerals. In some areas, the gas radon, found in soil, rocks, or building materials, is a major source. Artificial sources of ionizing radiation include Xray machines, radioactive isotopes used in diagnosis and treatment (see radionuclide scanning), and nuclear reactors. Less energetic types of radiation, such as ultraviolet light, may also cause biological damage by mechanisms other than ionization. Ultraviolet radiation from the sun does not penetrate the body deeply, but can damage genetic material in cells and may lead to skin cancer. Other types of nonionizing radiation to which people are subjected are ultrasound, used in medicine for diagnosis and treatment, and radio waves that are generated during MRI. These techniques are not thought to have any adverse side effects. (See also radiation hazards; radiation sickness; radiation units.)


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