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Questions and Answers on theNCI Fallout Report
The National Cancer Institute (NCI) report contains an assessment ofradioactive Iodine-131 (I-131) fallout exposure from the nuclear bomb teststhat were carried out at the Nevada Test Site in the 1950s and 1960s. In1982, Congress passed legislation calling for the Department of Health andHuman Services to develop methods to estimate I-131 exposure to the Americanpeople, to assess thyroid doses from I-131 received by individuals acrossthe country from the Nevada tests, and to assess risks for thyroid cancerfrom these exposures. The fallout report fulfills the first two of thesethree requirements. The report includes county-by-county estimates of average I-131 dosesto the thyroid for persons living or born in the contiguous 48 states duringthe period when the bomb tests were carried out, mostly in the 1950s. Estimatesof thyroid dose have been made for people by age category, sex, and amountand source of milk consumed (see Question 11). Doses have been estimatedfor each of the 90 tests that together released nearly 99 percent of theI-131 produced during the bomb testing program, and summed over test seriesand for all the tests combined. 2. What does the report show? The report shows that everyone living in the contiguous 48 states wasexposed to I-131 at some level. The average cumulative thyroid dose to theapproximately 160 million people in the country at the time was about 2rads. (By comparison, a routine I-131 diagnostic thyroid scan of a childin the 1950s gave approximately 200 to 300 rads to the thyroid. Today, athyroid scan would give about 0.4 to 4 rads to the thyroid, depending onthe radionuclide used. A rad is a unit of measurement that stands for radiationabsorbed dose, the amount of radiation that is absorbed by the tissues inthe body. Scientists now measure radiation dosage in grays: One gray equals100 rads.) Children and persons who drank large quantities of milk, or who drankmilk from goats or family-owned cows, are estimated to have received higherdoses. (Goat's milk concentrates I-131 more than cow's milk. Estimates ofI-131 doses in milk consumed soon after it was taken from a family-ownedcow are usually higher than for store-bought milk because store-bought milktakes time to process and ship, allowing more time for the radioactivityin I-131 to diminish.) Most children aged 3 months to 5 years probably receivedabout three to seven times the average dose for each test, because in generalthey drank more milk than adults did, and because their thyroids were smaller.Most adults, by contrast, probably received doses two to four times lowerthan the average estimated dose. Doses varied widely according to geographic area. In general, personsliving in Western states to the north and east of the test site had higherdoses than those living in West Coast, Southern, and Eastern states. Inthe 24 counties with the highest exposures, estimated average cumulativedoses ranged from 9 to 16 rads. 3. Does radiation cause thyroid cancer? Does I-131 cause cancer? The risk of thyroid cancer from childhood exposure to external radiationsuch as X-rays has been known for some time. Increased risk of thyroid cancerhas continued for many years after the atomic bombings in Hiroshima andNagasaki, and after childhood X-ray exposures to treat conditions of thehead and neck, such as acne and tonsillitis. A combined analysis of sevenstudies of persons exposed to a wide variety of external radiation sources-- including atomic bombs and treatment for cancer, tinea capitis (ringworm),and enlarged tonsils or thymus glands -- found that on average, personsexposed as young children to 100 rads (1 gray) had a 7.7 times higher riskof thyroid cancer compared with persons not exposed to radiation. But theoverwhelming majority of thyroid cancers are not related to X-ray treatments. The limited data on persons exposed as children to I-131 from the nucleartest fallout have provided suggestive but not conclusive evidence that itis linked to thyroid cancer. I-131 is thought to be no more likely to causethyroid cancer than the same dose of external radiation, and it may be aslittle as one-fifth as hazardous. However, the radiation doses receivedby young children, especially those who lived in areas with high falloutlevels, and particularly those who drank a great deal of milk, may wellhave increased the risk of thyroid cancer. The level of increased risk canbe estimated but it is highly uncertain. 4. What has NCI done to assess the risk of thyroid cancer from thefallout? What further studies are planned? In 1993, researchers at the University of Utah, Salt Lake City, publishedresults of an NCI-sponsored study of children living in parts of Utah andNevada that had high I-131 fallout levels, and a comparable number livingin parts of Arizona that had relatively little fallout. The average dosefor children in the study was estimated at about 10 rads. The researchersfound a statistically significant association between I-131 exposure andall thyroid neoplasms combined (including cancer and benign tumors). Theyfound some evidence of an association between estimated dose and thyroidcancer, but this was not statistically significant. The level of uncertaintywas high because the number of cancer cases involved was very small. Reflectingthis uncertainty, the researchers estimated that between zero and six ofthe eight observed thyroid cancers might have been caused by the fallout.The risks observed in the study were consistent with reported risks fromexternal radiation (X-rays). To provide more accurate information on the risk of thyroid cancer fromradioactive fallout, NCI investigators are collaborating with other U.S.government agencies, international organizations, and governments and scientistsin Belarus and Ukraine to study thyroid cancer among persons in those countrieswho were exposed during childhood to fallout from the Chernobyl nuclearaccident in 1986. The tens of thousands of children exposed to the falloutreceived radiation doses to the thyroid that ranged from comparatively smalldoses to doses ten times higher than U.S. residents received from the Nevadatests, and a clear increase in thyroid cancer has been seen in this population.The dose to each person in the study group will be reconstructed based onthyroid measurements made within about two months of the accident, residentialand dietary histories, and other environmental measurements. Because ofthe wide range of exposures, the large numbers of persons exposed, and thelarge number of thyroid cancer cases observed, information from the Chernobylstudies is expected to be relevant to the assessment of the impact of theU.S. exposures. Researchers sponsored by the Centers for Disease Control and Preventionare studying the health effects of the radioactive iodine released fromthe Hanford, Wash., nuclear weapons plant in the 1940s and 1950s. Resultsare expected in 1998. The new fallout data from NCI could be used to correlate fallout levelswith thyroid cancer rates in specific geographic areas; however, such studieswill encounter difficulties. Thyroid cancer is an uncommon disease, particularlyamong children. The small number of cases in any one geographic area --particularly in sparsely populated areas -- makes it difficult to determinewith confidence whether rates are significantly elevated. And while somestates 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 medicalX-rays at levels known to increase risk for thyroid cancer, and it is difficultto disentangle these effects from those of fallout exposure. 5. What is the U.S. government doing in light of this report? The Department of Health and Human Services has requested that the NationalAcademy of Sciences' Institute of Medicine (IOM) review the data to assessthe risks to individuals, and to recommend to physicians how to treat personswho might be at risk of disease because of their exposure to radioactiveiodine. This review will be comprehensive and public as are all IOM studies.IOM is expected to establish a diverse expert panel including epidemiologists,public health authorities, risk communication experts, specialists in thyroiddisease and its treatment, family physicians, public interest groups, andcitizen representatives. In the meantime, persons concerned about falloutexposure should consult a health professional. The administration will establish an interdepartmental group to lookat broader policy issues. 6. Is there danger of exposure to I-131 fallout now? No. The I-131 that was released from the tests is no longer present inthe environment. I-131 has a radioactive half-life of about eight days,meaning that its radioactivity decreases by half every eight days. Thismeans that nearly all the exposures took place primarily within two monthsfollowing each test. 7. Why did the study take 14 years to complete? The study involved the identification, collection, synthesis, and analysisof an enormous amount of data from many sources, including the Departmentof Energy, the National Oceanic and Atmospheric Administration, the Departmentof Agriculture, the Department of Commerce, agencies in each state, countyagencies, the American Dairy Herd Improvement Association, the dairy industry,and numerous private individuals knowledgeable in some aspect of the study. Early in the study an advisory committee was established to guide andassist in the study's conduct. From 1985 until 1993, it provided guidanceregarding what data existed and where, which data might be useful and how,the retrieval and compilation of data (much of which was not in computerizedform), the analysis of data, the methodologies to be used in reconstructingfallout deposition levels, milk concentration of I-131, and in the calculationof dose estimates by test, county, age, and dietary assumptions. During1994 and 1995, the study report of approximately 1,000 pages was redraftedseveral times. Since that time, efforts have been made to review, organize,and summarize the results (about 100,000 pages) in an understandable format,and to revise and improve the clarity of the text. 8. When will the report be released? The study's executive and technical summaries, along with a list of the24 counties that were most highly exposed to the fallout, were releasedAug. 1, along with a list of all U.S. counties with their estimated averagethyroid dose levels. The complete report, about 1,000 pages long, is scheduledto be published by Oct. 1, as well as the full data set with annexes andsubannexes. The complete report will include a description of the study methods,the most important measurement results, and summary information on estimatedI-131 doses to individuals. The full data set will include tabulated resultsof all the dose information by county, by age, and by milk consumption.It will include the amount of I-131 fallout in each county after each test,and the amount of I-131 that got into the food supply. Users of the dataset will be able to group and present data by county, by test, by test series,and for all tests. Maps displaying the tabulated data will also be available. 9. What is radioactive fallout? What is iodine-131? Radioactive fallout refers to a variety of airborne radioactive particlesthat fall to the ground during and following nuclear weapons tests. Peopleand animals may ingest these particles in their food or inhale them fromthe air. Iodine-131 is a radioactive isotope of iodine -- an altered formof the element that is chemically the same as the naturally occurring elementbut is radioactive. I-131 is found in fallout from atmospheric nuclear bombexplosions along with a variety of other radionuclides such as strontium-90.I-131 has been the focus of most concern because it concentrates in thethyroid, particularly in children, and may increase the risk for thyroidcancer. There appears to be little risk of thyroid cancer for persons exposedto radiation as adults. Because iodine concentrates in the thyroid, highdoses of I-131 are used to treat some types of benign thyroid disease andthyroid cancer. 10. How were people exposed to I-131? For most people, the greatest I-131 exposure came from drinking contaminatedmilk. I-131 fell on pasture lands, and the vegetation was eaten by cows,contaminating their milk. Smaller amounts came from breathing contaminatedair and eating other contaminated dairy products, eggs, and leafy vegetables. 11. How was the study conducted? To assess the thyroid doses of I-131 received by persons residing indifferent areas of the country, researchers had to estimate the amount ofI-131 deposited on soil and vegetation after each bomb test, the amountof contaminated vegetation consumed by dairy cows, the amount and sourceof milk (and to a lesser extent, other foods) that people consumed, andthe proportion of I-131 accumulating in the thyroid. For most of the bomb tests, passive fallout collection devices were placedin up to 100 locations across the United States in an attempt to measurethe fallout. But these devices measured only overall radioactivity, notspecific radionuclides such as I-131. To estimate the amount of I-131 thatfell in each county following each test, the NCI researchers used a reanalysisof the original data collected at the time of the tests, along with meteorologicalrecords on wind and rainfall patterns at the time, plus mathematical models.(In Western states most I-131 fell to the ground in dry form, while in Easternstates most was associated with rainfall.) To determine the amount of I-131 consumed by cows, the researchers usedother mathematical models to estimate what proportion of I-131 fell on vegetation,and combined these estimates with data on whether cows were on pasture innumerous regions of the country at the time of the tests, how much freshpasture grass was eaten by cows in each state, and how much of the I-131ingested by cows gets into their milk. The researchers then used recordsof milk production and distribution within each state and in the entireUnited States during the time of the tests to estimate how much I-131 wasin the milk consumed by the populations of each county. Finally, data onaverage milk consumption levels for persons of different ages and in differentregions of the country were combined with data from the analyses describedabove to derive estimates of average I-131 thyroid doses following eachtest for persons residing in each U.S. county. The data will be reportedfor 13 age categories (including four fetal age groups, four age groupsfor infants, four age groups for children, plus adults as a single group,with men and women reported separately). Within each of these age categories,four scenarios for milk consumption are being calculated: persons who drankaverage quantities of milk with average I-131 contamination levels for thecounty, persons who drank higher-than-average levels of the most highlycontaminated milk available in the county, persons who drank milk from family-owned"backyard" cows (see Question 2), and persons who drank no milk.Much less detailed analyses are being done for exposures from foods otherthan milk, and for exposure through inhalation, because these exposure pathwaysare much less important than drinking milk. 12. How reliable are the study's estimates of I-131 exposure? There are large uncertainties in the estimated thyroid doses that willbe given in the report because it is impossible to know all the informationneeded to determine exact doses. To evaluate the validity of the mathematicalmodels used in the study, results obtained from the model were comparedwith the limited data collected at the time of the tests. The comparisonsalso provide an estimate of the uncertainty attached to the calculated doses. These comparisons show a relatively good agreement between actual dataand predictions made by the mathematical models. For example, independentanalysis of urine samples volunteered by soldiers at Army bases throughoutthe United States following one of the test series showed iodine-131 doselevels consistent with or lower than the doses predicted. However, it shouldbe noted that the comparison between measured and predicted values requiredthe use of several assumptions, and there is no assurance that the samplesmeasured were representative of county averages. 13. Can an individual person calculate his or her own thyroid radiationdose? Because of the large number of variables involved in calculating individualdose estimates, it would likely be difficult for an individual to estimatehis or her own thyroid dose. However, detailed information will be providedin the full report so that individual cumulative doses can be estimated.This information is expected to be most useful to state or county healthdepartments and epidemiologists who wish to assess the impact of the fallouton specific populations. In general, the uncertainty of the thyroid dose from iodine-131 falloutfor a representative individual included in the study is about a factorof 3. For example, if the thyroid dose estimate for an individual is 3 rads-- based on county of residence, age and assumed milk consumption at thetime of the tests -- the true dose will likely lie between 1 and 9 rads. However, the uncertainty attached to the thyroid dose for an actual,specific person is larger, because of uncertainties about the dietary habitsand the metabolism of any individual. 14. How common is thyroid cancer? How treatable is it? Thyroid cancer is relatively uncommon compared with other forms of cancer,accounting for about 1 percent of all cancers diagnosed in the United States.An estimated 16,100 U.S. cases will be diagnosed in 1997. The number ofnew cases is about 10 times higher than the number of deaths, estimatedat 1,230 in the United States in 1997. Thyroid cancer occurs more than twiceas often in women as in men. Incidence rates for 1994 -- the most recentyear available -- were 7.6 per 100,000 U.S. women of all ages combined,and 2.8 per 100,000 U.S. men. Thyroid cancer mortality rates for that yearwere 0.3 per 100,000 for both men and women. Lifetime risk for thyroid canceris estimated at 0.66 percent for women (about 1 in 152) and 0.27 percentfor men (about 1 in 370). Thyroid cancer is highly curable: The 5-year survivalrate is about 95 percent. 15. What should people do who are concerned about cancer risk fromfallout exposure? Anyone who is concerned about cancer risk from fallout should requesta thyroid examination as part of their next visit to a physician.
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