The degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure (close contact, inhalation or ingestion), and the biochemical properties of the element; with increased risk of cancer the most usual consequence.However, radionuclides with suitable properties are used in nuclear medicine for both diagnosis and treatment.Elements heavier than lead, and the elements technetium and promethium, exist only as radionuclides.
The range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude.Radionuclides occur naturally or are artificially produced in nuclear reactors, cyclotrons, particle accelerators or radionuclide generators.At least another 60 radionuclides are detectable in nature, either as daughters of primordial radionuclides or as radionuclides produced through natural production on Earth by cosmic radiation.More than 2400 radionuclides have half-lives less than 60 minutes.The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay.
Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay.) for that collection can be calculated from their measured decay constants.
Radionuclides that find their way into the environment may cause harmful effects as radioactive contamination.
They can also cause damage if they are excessively used during treatment or in other ways exposed to living beings, by radiation poisoning.
The process of nuclear fission creates a wide range of fission products, most of which are radionuclides.
Further radionuclides can be created from irradiation of the nuclear fuel (creating a range of actinides) and of the surrounding structures, yielding activation products.
This complex mixture of radionuclides with different chemistries and radioactivity makes handling nuclear waste and dealing with nuclear fallout particularly problematic.