Agency for Toxic Substances and Disease Registry
Radiation Exposure from Iodine 131
Who is at Risk

Who is at Risk

• Incidence of thyroid cancer depends on many factors, including thyroid dose and age at the time of exposure.

During 1945-1962, when they were young children (and more vulnerable than adults to radiation exposure), many persons in the United States were exposed to radiation fallout from I-131 from multiple sources. Those exposures put those persons at risk for thyroid and parathyroid disease and cancer of the thyroid.

Any person who was a child under the age of 10 between 1945 and 1962 in the United States and who drank milk should be considered potentially exposed to I-131. People who lived near or around weapons production facilities, especially downwinders, are at risk for having received higher levels of exposure to I-131.

More detailed information on how Americans were exposed to I-131 is available from the National Cancer Institute (NCI). The institute provides free print materials and maintains a Web site on I-131.

National Cancer Institute materials on I-131

Web site: http://www.cancer.gov/i131

Phone: 1-800-4-CANCER (1-800-422-6237)

Epidemiological studies on thyroid cancer and I-131 at Chernobyl confirmed that the risk for thyroid cancer is dependent on the absorbed dose, the age and location of persons at time of exposure, and the absence of immediate iodine prophylaxes for I-131 exposure. A strong relationship exists between the incidences of thyroid neoplasia, hypothyroidism, and autoimmune thyroiditis and the received dose. In the most contaminated area after the nuclear release at Chernobyl, thyroid cancer incidence was significantly higher compared with other regions. There are no earlier studies comparable to those for Chernobyl because no studies were conducted around U.S. weapon production facilities when I-131 was released.

• Children are the most sensitive group for exposure to I-131.

Age is a factor for exposure to I-131 because of the differences between thyroid doses for children and adults. The dose to children is much higher than that to adults because the thyroid mass in children is smaller, and because milk, as the main route of contamination, is consumed in higher quantities during childhood. For an equivalent uptake of I-131, a child's thyroid receives a higher radiation dose because the same amount of energy is deposited in a smaller tissue mass (more energy per gram = higher dose). For newborns, the thyroid dose is about 16 times higher than that for adults for the same ingested radioactivity; similarly, the absorbed dose is about 8 times higher for children under 1 year old and 4 times higher for children 5 years old.

Most of the immigrants from the former Soviet Union who came to the United States during the 1990s came from the Ukraine. Many of them came from areas that had been contaminated with I-131 during and after the Chernobyl explosion. Therefore, many of them-some of whom were children at the time-have been exposed to I-131.

• Special considerations exist for pregnant women and nursing mothers.

During pregnancy, the maternal thyroid has an increased rate of I-131 uptake, especially during the first trimester. I-131 crosses the placental barrier. During the second and third trimesters, the fetal thyroid takes up and stores iodine in increasing amounts. During the first postpartum week, thyroid activity increases up to fourfold. This critical period lasts for a couple of days. Infants and children are at high risk from radioiodine exposure because their thyroids are small. This risk decreases as children age, although it continues until they are about 20 years old. About one-quarter of the iodine ingested by the mother is secreted in breast milk, which adds an additional risk factor for the breast-feeding infant.

• Drinking fresh versus pasteurized milk leads to a higher dose of
  I-131.

The geographic distribution of persons exposed to I-131 is important for three reasons. First, the risk is higher for those in rural areas because fresh milk is often consumed. This is important when the milk is produced from a contaminated pasture. The delay between the production and consumption of milk contributes to decreasing radioactivity for urban populations. Second, the risk is higher for populations with endemic deficiency of iodine. I-131 absorption is higher for these populations. Third, different types of milk and dairy products are consumed in some rural areas. In goat's milk and sheep's milk, I-131 concentrations are up to 10 times higher than in cow's milk for the same concentration of I-131 in the pasture.

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