Radioisotope Iodine-131 (I-131)

Chemical properties
I-131 can change directly from a solid into a gas, skipping the liquid phase, in a process called sublimation. I-131 dissolves easily in water or alcohol. I-131 readily combines with other elements and does not stay in its pure form once released into the environment.

Mode of decay
Beta particles and gamma radiation

Half life
8.06 days

Uses of Iodine-131
I-131 is used in medicine to diagnose and treat cancers of the thyroid gland.

Sources of Iodine-131
I-131 is produced commercially for medical and industrial uses through nuclear fission. It also is a byproduct of nuclear fission processes in nuclear reactors and weapons testing.

Exposure to Iodine-131
I-131 is gaseous fission product that form within fuel rods as they fission. Unless reactor chemistry is carefully controlled, they can build up too fast, increasing pressure and causing corrosion in the rods. As the rods age, cracks or wholes may breach the rods.
Cracked rods can release radioactive iodine into the water that surrounds and cools the fuel rods. There, it circulates with the cooling water throughout the system, ending up in the airborne, liquid, and solid wastes from the reactor. From time to time, reactor gas capture systems release gases, including iodine, to the environment under applicable regulations.
Anywhere spent nuclear fuel is handled, there is a chance that I-131 will escape into the environment. Nuclear fuel reprocessing plants dissolve the spent fuel rods in strong acids to recover plutonium and other valuable materials. In the process, they also release I-131 into the airborne, liquid, and solid waste processing systems.

Health effects of Iodine-131
Radioactive iodine can cause thyroid problems, and help diagnose and treat thyroid problems. Long-term (chronic) exposure to radioactive iodine can cause nodules, or cancer of the thyroid. However, once thyroid cancer occurs, treatment with high doses of I-131 may be used to treat it. Doctors also use lower doses of I-131 to treat overactive thyroids.
Low doses can reduce activity of the thyroid gland, lowering hormone production in the gland. Doctors must maintain the fine balance between the risks and benefits of using radioactive iodine. On one hand, this small, additional exposure may tip the balance in favor of cancer formation. On the other, this small additional exposure can restore health by slowing an overactive thyroid and improve health conditions.

Source: Centers for Disease Control and Emergency, U.S. Environmental Protection Agency