accident or terrorist activity. Exposure to 131I (and other radioisotopes of iodine) by inhalation of contaminated air or ingestion of contaminated food or milk can lead to radiation injury to the thyroid, including increased risk of thyroid cancer and other thyroid diseases, because the thyroid gland concentrates and stores iodine from the blood. For example, the 1986 nuclear accident in Chornobyl exposed many people to 131I, and reports of radiation epidemiology studies indicate that that exposure caused excess cases of thyroid cancer years later in the exposed susceptible population. The Chornobyl experience is discussed in this report because it shows the consequences of exposure in a qualitative sense, even though it is not as relevant for determining quantitatively the risk of such an event in the United States in light of substantial safety and other facility design features in US reactors. Nevertheless, nuclear power plants in the United States contain a source of radioactive iodine that in the event of a very severe incident might impose risks of exposure, which could lead to thyroid cancers. Given that KI is effective in protecting against potential thyroid cancer, KI distribution plans should be considered.

The detonation of a nuclear weapon would lead to release of radioiodine, but radioiodine would not be a primary concern compared to the principal thermal and blast effects and the large amount of radiation and non-iodine radioactive materials that would be released.

Radiation incidents may be unintentional, as in NPP accidents, or intentional, as in terrorist attacks that damage NPPs or explode "dirty bombs" or nuclear weapons. In the event of an accidental or intentional release of radioactive iodine into the environment, radiation doses1 to the thyroid from radioiodine can be limited by


To eliminate confusion over the use of the word dose which is often used to describe both the radiation absorbed dose, expressed in Gray, and the medically administered dose, often expressed in grams or milligrams—the committee has chosen to use the term dose when indicating the radiation absorbed dose and the term dosage when referring to the amount of a drug administered. To eliminate confusion due to the various ways that radiation doses are expressed in the literature, the committee decided to limit its usage to the SI unit for radiation absorbed dose, the Gray (Gy) when possible. For informational purposes SI units are followed in brackets by their equivalent in terms of the older English units. For units presented in the literature in terms of dose equivalent and effective dose equivalent, Sievert (Sv) and rem, the committee converted these doses to the absorbed dose units of Gray and rad when possible. To eliminate confusion over the use of the term “dose” and “exposure” for the radiation absorbed dose to the thyroid gland, the committee chose to use only the term thyroid dose.

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