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-125
Long irradiations are disadvantageous. Iodine-125 itself has a neutron capture cross section of 900 barns, and consequently during a long irradiation, part of the I formed will be converted to I, a beta-emitter and positron-emitter with a half-life of 13.1 days, which is not medically useful. In practice, the most useful irradiation time in the reactor amounts to a few days. Thereafter, the irradiated gas is allowed to decay for three or four days to dispose of short-lived unwanted isotopes, and to allow the newly created xenon-125 (half-life 17 hours) to decay to iodine-125.