Potassium iodide (chemical formula KI) is a salt that is similar in structure and physical character to common table salt (sodium chloride; NaCl). Indeed, potassium iodide is a common commercial additive to table salt, to produce "iodized" salt.
Potassium iodide is noteworthy in security because of its ability to block the uptake of radioactive iodine by the body's thyroid gland. Located in the neck, the sole task of the thyroid gland is the production of a hormone that is one of the body's principle metabolic regulators. Thus, the disruption of the thyroid gland—such as occurs when the uptake of radioactive iodine triggers the development of thyroid cancer—threatens health and can even led to death.
If taken in time following an accidental or deliberate release of radioactive iodine, such as would occur with a leak from a nuclear power plant or the detonation of a bomb containing a radioactive payload, potassium iodide saturates the thyroid with a form of iodine that persists in the gland. The radioactive form of iodine cannot out-compete this stable form of iodine, and so is excreted from the body.
Ingestion of KI has long been a precaution for workers in nuclear power plants and for military personnel engaged in a conflict where the use of nuclear weapons is considered to be a possibility. Much of what is known of the protective effects of potassium iodide has come from the measurements of radiation accumulation in the thyroid glands of hundreds of thousands of people in the weeks following the Chernobyl reactor disaster of April 1986, and the therapeutic effects KI achieved in Poland during that time.
Since the terrorist attacks on the United States in the latter months of 2001, the need for a distribution of KI to civilians has become recognized. This has become especially evident with the exposed vulnerability of nuclear power plants to terrorist attack, and to the conceivable use of "dirty" bombs by terrorists. The latter, essentially a conventional explosive charge that spews out radioactive substances including iodine, could contaminate many people in a crowded urban area.
The protective effects of potassium iodide last about 24 hours from the time it is ingested. Thus, a civilian or military protective strategy requires daily doses of KI. Longer term or more permanent use of the salt is not recommended yet, as prolonged use has been linked to thyroid malfunction, especially in those with Grave's disease or autoimmune inflammation of the thyroid gland.
█ FURTHER READING:
Harrison, J. R., W. Paile, and K. Baverstock. "Public Health Implications of Iodine Prophylaxis in Radiological Emergencies" in: Thomas, G., A. Karaoglou, and E. D. Williams, eds. Radiation and Thyroid Cancer. Singapore: World Scientific, 1999.
Astakhova, L. N., L. R. Anspaugh, G. W. Beebe, et al. "Chernobyl-Related Thyroid Cancer in Children in Belarus." Radiation Research no. 150 (1998): 349–356.
Robbins, J., and A. B. Schneider. "Thyroid Cancer following Exposure to Radioactive Iodine." Reviews in Endocrine and Metabolic Disorders no. 1 (2000): 197–203.
U.S. Food and Drug Administration. "Guidance: Potassium Iodide as a Thyroid Blocking Agent in radiation Emergencies." Center for Drug Evaluation and Research. December 10, 2001. < http://www.fda.gov/cder/guidance/4825fnl.htm > (April 9, 2003).
U.S. Nuclear Regulatory Commission. "Frequently Asked Questions About Potassium Iodide." National Research Council. April 2, 2003. < http://www.nrc.gov/what-wedo/regulatory/emer-resp/emer-prep/ki-faq.html > (April 12, 2003).