July 28, 1997

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The National Cancer Institute (NCI) has completed a study to assess Americans' exposures to radioactive iodine-131 fallout from atmospheric nuclear bomb tests carried out at the Nevada Test Site (NTS) in the 1950s.

Depending on their age at the time of the tests, where they lived, and what foods they consumed, particularly milk, Americans were exposed to varying levels of I-131. Because of the radioactive decay of I-131, such exposures did not exceed two months following each test. Because I-131 accumulates in the thyroid gland, concerns have been raised that the fallout could cause thyroid cancer in people who were exposed to it as children.

In 1982, Congress passed legislation calling for the Department of Health and Human Services to develop methods to estimate I-131 exposure, to assess I-131 exposure levels across the country from the Nevada tests, and to assess risks for thyroid cancer from these exposures.

The fallout report fulfills the first two of these three requirements. The complete report includes estimates of average I-131 dose to the thyroid for representative persons in each county during 1951 to 1958, when the nuclear tests were carried out in Nevada. Estimates of thyroid doses have been made for persons by age, sex, and source and quantity of milk consumption. (For most people, the bulk of I-131 exposure came from drinking milk. Smaller amounts came from eating other dairy products, eggs, and leafy vegetables, and through inhalation.) Although the study was carried out with as much care as possible, a large degree of uncertainty is associated with these dose estimates, as they are based on a small number of radiation measurements made at that time and relied heavily on the use of mathematical models.

The average cumulative thyroid dose to the approximately 160 million people in the country was about 2 rads-less than the dose received from diagnostic thyroid scans in the 1950s. Persons living in heavy fallout areas, children, and persons who drank large quantities of milk received higher doses. In general, persons living in Western states to the north and east of the test site had the highest doses, averaging 5 to 16 rads. Children aged 3 months to 5 years averaged about 10 times the dose of adults, because in general they drank more milk than did adults and because their thyroids were smaller.

A summary of the fallout report will be published in September, and the complete report, including exposure estimates for each of the 3,071 counties in the 48 contiguous U.S. states, will be available later this year. The complete report is about 100,000 pages. Although the fallout report was not intended to fulfill the third requirement, the results will be linked with findings from relevant epidemiological studies, including some currently in progress, to estimate thyroid cancer risk.

The small number of studies on persons exposed to I-131 from NTS fallout have produced suggestive but not conclusive evidence that it is linked to cancer. The effects of radiation from other sources, such as medical X-rays, have been studied much more extensively. This larger body of data suggests that the fallout radiation doses received by the vast majority of U.S. children in the 1950s are unlikely to be an important cancer risk factor. However, the radiation doses among children who lived in areas with high fallout levels, and particularly those who drank a great deal of milk, or who drank milk from goats or family cows, may well have an increased risk of thyroid cancer, the level of which is highly uncertain.

In the 1960s, the U.S. Public Health Service conducted a study of children living in parts of Utah and Nevada that had high fallout to determine whether thyroid disease was associated with fallout exposure. This early study found no evidence of harmful effects.

In 1993, researchers at the University of Utah, Salt Lake City, published a report on thyroid disease among members of the same group who were still residing in the area. They found some evidence of an association between estimated dose and thyroid cancer, but this was not statistically significant. The level of uncertainty was high because the number of cancer cases involved was very small. Reflecting this uncertainty, the researchers estimated that between zero and six cases of the eight observed thyroid cancers might have been caused by the fallout.

To provide more accurate information on the risk of thyroid cancer from radioactive fallout, NCI investigators, in collaboration with other U.S. government agencies and international organizations, are working with the governments of and scientists in Belarus and Ukraine to study thyroid cancer among children in those countries who were exposed to fallout of various radioisotopes of iodine, mainly I-131, from the Chernobyl nuclear accident in 1986. This population had a wide range of exposures. The tens of thousands of children exposed to the fallout received radiation doses to the thyroid that ranged from comparatively little to those that are ten times higher than U.S. residents received from the Nevada tests. Because of the wide range of exposures and the larger numbers of persons exposed, information from the Chernobyl studies is expected to be relevant to the assessment of the impact of the U.S. exposures.

Researchers sponsored by the Centers for Disease Control and Prevention are studying the health effects of the radioactive iodine released from the Hanford, Wash., nuclear weapons plant from in the 1940s and 1950s. Results are expected next year.

The new fallout data from NCI could be used to correlate fallout levels with thyroid cancer rates in specific geographic areas; however, such studies will encounter difficulties. Thyroid cancer is a rare disease, particularly among children. The small number of cases in any one geographic area-particularly in sparsely populated areas-makes it difficult to determine with confidence whether rates are significantly elevated. And while some states have registries with historical data on all cancer cases in the state, most do not. In addition, many children in the 1950s were exposed to medical X-rays at levels known to increase risk for thyroid cancer, and it is difficult to disentangle these effects from those of fallout exposure occurring during the testing program.

In 1997, an estimated 16,100 Americans will be diagnosed with thyroid cancer and 1,230 will die from the disease. The incidence rate for women is more than twice as high as that for men. Thyroid cancer is highly curable: The 5-year survival rate is about 95 percent.