Time interval in which half of the nuclei of a radioactive nuclide have decayed. Short half-lives lead to a high radiation activity and long half-lives lead have a low radiation activity. The biological half-life is the time in which a biological system, e. g. a human being or an animal, excretes in a natural way half of the amount of a certain substance taken up from the body or a certain organ. The effective half-life is the time in which in a biological system the amount of a radionuclide reduces by half, namely in coaction of radioactive decay and excretion as a result of natural processes.