I, with a half-life of 8 days, is a hazard from nuclear fallout because iodine concentrates in the thyroid gland. See also Radiation effects from Fukushima Daiichi nuclear disaster#Iodine-131 and
Downwinders#Nevada.
In common with Sr, I is used for the treatment of cancer. A small dose of I can be used in a thyroid function test while a large dose can be used to destroy the thyroid cancer. This treatment will also normally seek out and destroy any secondary tumor which arose from a thyroid cancer. Much of the energy from the beta emission from the I will be absorbed in the thyroid, while the gamma rays are likely to be able to escape from the thyroid to irradiate other parts of the body.
Large amounts of I was released during an experiment named the Green Run in which fuel which had only been allowed to cool for a short time after irradiation was reprocessed in a plant which had no iodine scrubber in operation.
I, with a half-life almost a billion times as long, is a long-lived fission product. It is among the most troublesome because it accumulates in a relatively small organ (the thyroid) where even its comparatively low radiation dose can cause great damage as it has a long biological half life. For this reason, Iodine is often considered for transmutation despite the presence of stable  in spent fuel. In the thermal neutron spectrum, more Iodine-129 is destroyed than newly created since Iodine-128 is short lived and the isotope ratio is in favor of . Depending on the design of the transmutation apparatus, care must be taken as Xenon, the product of Iodine's beta decay, is both a strong neutron poison and a gas that is nigh impossible to chemically "fix" in solid compounds, so it will either escape to the outside air or put pressure on the vessel containing the transmutation target.
I is stable, the only one of the isotopes of iodine that is nonradioactive. It makes up only about  of the iodine in spent fuel, with I-129 about .