Radioactive waste arises from many sources, including medical and industrial activities, as well as power stations and fuel reprocessing. The waste can be in a gaseous, liquid or solid state, but the most important factor regarding its disposal is the level of radiation emitted by it. This has implications for the temperature of the waste materials; if the level is high, then the waste will generate heat. The other important factors are the half-lives of the isotopes forming the waste and the type of radiation emitted. A highly radioactive material with a half-life of seconds only requires a minimal delay period before it can be safely discharged into the environment whereas many radioactive elements have half-lives of several thousands of years and require isolation for long periods.