The radioactive isotope, Iodine-131, has been in the news recently because of the disaster at the Fukushima power plant in Japan. One term that keeps arising is ‘half-life’. At its most basic, the half-life of an isotope refers to the time it will take for half of that element to have decayed into its daughter element.
Atomic half-life is the specific form of decay that radioactive elements undergo. According to the Merriam-Webster Dictionary, radioactivity is “the property possessed by some elements or isotopes of spontaneously emitting energetic particles by the disintegration of their atomic nuclei.” This emission is due to the unstable configuration of the element or isotope. Iodine-131 is unstable due to an excess of neutrons; Iodine-131 has 78 neutrons compared to the basic element’s 75. This excess of neutrons indicates that the isotope’s form of decay would be β-(beta minus) decay and Wolfram|Alpha confirms this. According to the Lawrence Berkeley National Laboratory, in β- decay, a neutron becomes a proton and an electron is emitted. This activity transmutes the radioactive element to a more stable element, one whose atomic number is higher; while an emitted electron, antineutrino and gamma rays maintain the conservation of energy(mass). The result is that half of Iodine-131 will become Xenon-131 in an eight day period, which is Iodine-131’s half-life.
With an understanding of what Iodine-131’s half-life is, it is possible to understand why this topic is important. The Agency for Toxic Substances and Disease Registry reports in Radiation Exposure from Iodine 131 Exposure Pathways that “I-131 is produced during nuclear fission, which occurs during the operation of nuclear reactors or detonation of a nuclear bomb. When uranium or plutonium atoms undergo fission, about 1.5% – 2.0% of the fission products become I-131.” They also note that “Only I-131 and I-135 are associated with medical administration.” This indicates that the two ways that a person could come in contact with I-131 would be the fallout from a nuclear incident or through its use in medical testing. The minimal amount used in medical testing means that its risks are considerably less than those from fallout. Wolfram|Alpha reports that the biological half-life of Iodine-131 is 138 days, and half-life, in the biological sense, is the time it would take for the substance involved to be decayed in half by natural processes. Obviously, this process would not be important to Iodine-131 since it would have decayed away to Xenon-131 far before any biological process would break it down. Although Iodine-131 has a short half-life it could still be dangerous, since the process of β- decay releases gamma rays, which are a form of ionizing radiation which can penetrate into cells. Upon penetration, this radiation can damage the cells which can ultimately destroy the cells or lead to unhealthy DNA modification; for example, those leading to cancer. The specific danger from Iodine-131 is Thyroid cancer, because as World Book states, the thyroid requires Iodine for the production of Thyroxine; which is essential for healthy growth and the prevention of gout. As Iodine-131 is a form of Iodine, the body will use it as happily as a non-radioactive type. Due to its short half-life, problems would generally occur as a result of long-term exposure over short-term.
With this basic understanding of Iodine-131’s half-life, and why it is important to know about it, you may wish to learn more.You can begin with this link to Wikipedia as a general source on Iodine-131.
 Busch, Marianna A. “Iodine.” World Book Online InfoFinder. World Book, 2011. Web. 9 Dec. 2011.