Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley.

It also plays a major role as a radioactive isotope present in nuclear fission products, and was a significant contributor to the health hazards from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a large fraction of the contamination hazard in the first weeks in the Fukushima nuclear crisis.

The isotope 131I is still occasionally used for purely diagnostic (i.e., imaging) work, due to its low expense compared to other iodine radioisotopes.

The low-cost availability of 131I, in turn, is due to the relative ease of creating 131I by neutron bombardment of natural tellurium in a nuclear reactor, then separating 131I out by various simple methods (i.e., heating to drive off the volatile iodine).

By contrast, other iodine radioisotopes are usually created by far more expensive techniques, starting with cyclotron radiation of capsules of pressurized xenon gas.

[7] Much smaller incidental doses of iodine-131 than those used in medical therapeutic procedures, are supposed by some studies to be the major cause of increased thyroid cancers after accidental nuclear contamination.

These studies suppose that cancers happen from residual tissue radiation damage caused by the 131I, and should appear mostly years after exposure, long after the 131I has decayed.

A tellurium compound can be irradiated while bound as an oxide to an ion exchange column, with evolved 131I then eluted into an alkaline solution.

[12] More commonly, powdered elemental tellurium is irradiated and then 131I separated from it by dry distillation of the iodine, which has a far higher vapor pressure.

The electrons, due to their high mean energy (190 keV, with typical beta-decay spectra present) have a tissue penetration of 0.6 to 2 mm.

When adults are exposed, it has been difficult for epidemiologists to detect a statistically significant difference in the rates of thyroid disease above that of a similar but otherwise-unexposed group.

Within the US, the highest 131I fallout doses occurred during the 1950s and early 1960s to children having consumed fresh milk from sources contaminated as the result of above-ground testing of nuclear weapons.

[8] The National Cancer Institute provides additional information on the health effects from exposure to 131I in fallout,[21] as well as individualized estimates, for those born before 1971, for each of the 3070 counties in the USA.

The levels were expected to drop relatively quickly[24] A common treatment method for preventing iodine-131 exposure is by saturating the thyroid with regular, stable iodine-127, as an iodide or iodate salt.

European guidelines recommend administration of a capsule, due to "greater ease to the patient and the superior radiation protection for caregivers".

[32] Ablation doses are usually administered on an inpatient basis, and IAEA International Basic Safety Standards recommend that patients are not discharged until the activity falls below 1100 MBq.

For this reason, it is advised to regularly clean toilets, sinks, bed sheets and clothing used by the person who received the treatment.

Patient is advised if possible to stay in a room with a bathroom connected to it to limit unintended exposure to family members.

Because of the carcinogenicity of its beta radiation in the thyroid in small doses, I-131 is rarely used primarily or solely for diagnosis (although in the past this was more common due to this isotope's relative ease of production and low expense).

[40] Such material can enter the sewers directly from the medical facilities, or by being excreted by patients after a treatment Used for the first time in 1951 to localize leaks in a drinking water supply system of Munich, Germany, iodine-131 became one of the most commonly used gamma-emitting industrial radioactive tracers, with applications in isotope hydrology and leak detection.

Tagged at the surface, water is then tracked downhole, using the appropriated gamma detector, to determine flows and detect underground leaks.