Gamma rays are uncharged and pass through humans at the speed of light. As gamma rays pass through the body, they may damage cells. Cobalt-60 is a source of gamma radiation. This is the amount of time it takes for a radioactive substance to lose one-half of its radioactivity. When an atom decays and becomes stable, it is no longer radioactive.
Some radioactive substances decay quickly into non-radioactive materials.
Others decay over long periods of time into other radioactive materials which, in turn, undergo radioactive decay. Ionizing radiation may damage human cells. Non-ionizing radiation includes visible, ultraviolet, and infrared light, as well as radio waves. A single element may have many isotopes. For example, stable iodine is iodine-127.