Iodine

Iodine (symbol: I) is a chemical element with the atomic number 53.

Iodine-131
It has a half-life of 8.02 days and is a beta emitter, while gamma radiation only makes some 10% of the total emission. I-131's beta particles have a range of up to 2 mm in tissues.

Iodine-131 is a volatile radionuclide which would be quickly spread widely by air and can be breathed in by human beings. It also is easily water-soluble. As stored in plants and collected by animals it quickly gets concentrated in milk, meat and fish and thus reaches the human food chain, too. The thyroid absorbs iodine until its capacity is saturated - usually the thyroid is not saturated. Thus an exposition with radioactive iodine after a nuclear accident leads to its accumulation in the thyroid. Processing of accumulated iodine in the thyroid takes quite long. An exposition to radioactive iodine has been verified to cause a high risk of thyroid cancer, especially for children. Low doses can also lead to dysfunctions of the thyroid, for instance hypothyroidism.

The biological half-life of iodine-131 is different by organ. For instance it is 100 days in the thyroid, 14 days in bones and 7 days in kidneys, spleen and gonad.

Iodine-129
It has a half-life of 15.7 million years. Compared to I-131 it is a less strong beta and gamma emitter.

Iodine-129 mainly appears in nuclear power plants. If detected in the environment it usually origins from above-ground nuclear weapons tests.

I-129 acts similar to I-131 and is harmful for the thyroid, too. Its biological half-life in the human body is some 120 days.