Iodine-131 is a radioactive isotope of iodine. It is created when the atom uranium undergoes nuclear fission, and it has a short half-life of only 131 days. Iodine-131 can be found in nature as part of the decay process of other radioactive elements, but it can also be produced by nuclear reactors or by bombarding uranium with neutrons. Iodine-131 is used to diagnose cancer and to monitor the health of people living near nuclear facilities.