Radioactive contamination
Some organs concentrate certain elements and hence radionuclide variants of those elements. This action may lead to much lower removal rates. For instance, the thyroid gland takes up a large percentage of any iodine that enters the body. Large quantities of inhaled or ingested radioactive iodine may impair or destroy the thyroid, while other tissues are affected to a lesser extent. Radioactive iodine-131 is a common fission product; it was a major component of the radioactivity released from the Chernobyl disaster, leading to nine fatal cases of pediatric thyroid cancer and hypothyroidism. On the other hand, radioactive iodine is used in the diagnosis and treatment of many diseases of the thyroid precisely because of the thyroid's selective uptake of iodine.
Iodine-129
I is one of the seven long-lived fission products that are produced in significant amounts. Its yield is 0.706% per fission (U-235). Larger proportions of other iodine isotopes such as I are produced, but because these all have short half-lives, iodine in cooled spent nuclear fuel consists of about  I and the only stable iodine isotope, I. 
Because I is long-lived and relatively mobile in the environment, it is of particular importance in long-term management of spent nuclear fuel. In a deep geological repository for unreprocessed used fuel, I is likely to be the radionuclide of most potential impact at long times.