National Cancer Institute's Responses

Questions Related To Research

When did NCI begin to look at I-131 fallout exposure?

In 1982 Congress passed legislation calling for the Department of Health and Human Services to develop methods to estimate I-131 exposure, to assess thyroid doses of I-131 received by individuals across the country from the Nevada tests, and to assess risks from thyroid cancer from these exposures. This research request followed a series of Congressional hearings held in the late 1950s and early 1960s to assess the extent of fallout exposure to the public from these tests. The published scientific literature, which was introduced into the public record at that time, reported estimated I-131 exposure for children ranging from 4 to 120 rad. The NCI’s I-131 fallout report results are remarkably consistent with those early reports; we place ranges of exposures to children between 0 and 100 rad. Our study provides, we believe, both important new methodology and much more detailed exposure information than has been previously available. (On website see "Questions and Answers on the NCI Fallout Report")

How was the study conducted?

To assess the thyroid doses of I-131 received by persons residing in different areas of the country, researchers had to estimate the amount of I-131 deposited on soil and vegetation after each bomb test, the amount of contaminated vegetation consumed by dairy cows, the amount and source of milk (and other foods) that people consumed, and the proportion of I-131 accumulating in the thyroid. For most of the bomb tests, passive fallout collection devices were placed in up to 100 locations across the U.S. in an attempt to measure the fallout. But these devices measured only overall radioactivity, not specific radionuclides such as I-131. To estimate the amount of I-131 that fell in each county following each test, the NCI researchers used a re-analysis of the original data collected at the time of the tests, along with meteorological records on wind and rainfall patterns at the time, plus mathematical models. (On website see "Executive Summary" and "Technical Summary")

Was the frequency or potency of individual devices factored into the exposure estimates?

Yes. The timing of individual tests, and test series, was accounted for in the calculations. The amount of I-131 released in each of the ninety tests was considered, as well. (On website see "Full Report")

What exactly is a rad? How is it measured?

A "rad" means "radiation absorbed dose." It is a physical unit of energy deposition. To put this amount of exposure into perspective, routine medical use of x-rays for treating disease during the 1940s and 1950s exposed children to anywhere from 5 to several hundred rad, and all persons received doses from natural background radiation of about 0.1 rad per year. The NCI I-131 fallout report estimates that the average dose of I-131 to Americans from atmospheric nuclear bomb tests at the Nevada Test Site was 2 rad. (On website see "Congressional Testimony By NCI Director")

How strong is the link between exposure to I-131 and thyroid cancer?

Scientists do not know what causes most cases of thyroid cancer. What is known about radiation and thyroid cancer comes from studies of external radiation sources. Children who received x-ray treatments for noncancerous conditions such as enlarged tonsils, acne and ringworm have a higher-than-average risk of developing thyroid cancer many years later. Since it is not known if the potency of ingested I-131 is equal to external radiation, children exposed to radioactive fallout from nuclear explosions in the 1950s may also be at risk. However, at this time there is no reliable way to estimate the risk of thyroid cancer from radioactive fallout. Thus far, studies of internal exposure to I-131 for medical purposes or from fallout in areas close to the site of atomic bomb tests during the 1950s have produced no conclusive evidence that such exposure is linked to cancer. The level of increased risk can be estimated but it is highly uncertain. This question continues to be studied. (On website see "Fact Sheet on Thyroid Cancer")

When will it be determined if this exposure to I-131 has resulted in greater numbers of thyroid cancer cases?

In 1993, researchers at the University of Utah published results of an NCI-sponsored study of children living in parts of Utah and Nevada that had high I-131 fallout levels, and a comparable number living in parts of Arizona that had relatively little fallout. The researchers found a statistically significant association between I-131 exposure and all thyroid neoplasms combined (including cancerous and benign tumors). They found some evidence of an association between estimated dose and thyroid cancer, but this could have been due to chance. To provide more accurate information on the risk of thyroid cancer from radioactive fallout, NCI investigators are collaborating with other U.S. government agencies, international organizations, and governments and scientists in Belarus and Ukraine to study thyroid cancer among persons in those countries who were exposed during childhood to fallout from the Chernobyl nuclear accident in 1986.

Researchers sponsored by the Centers for Disease Control and Prevention are studying the health effects of the radioactive iodine releases from the Hanford nuclear weapons plant in the 1940s and 1950s. (On website see "Questions and Answers on the NCI Fallout Report")

Why has cow’s milk been the focus of the study?

Children and persons who drank large quantities of milk, or who drank milk from goats or family-owned cows, are estimated to have received higher doses of I-131. Goat’s milk concentrates I-131 more than cow’s milk, but most milk consumed by Americans is cow’s milk. Most children aged 3 months to 5 years probably received about three to seven times the average dose of I-131 for each nuclear test, because in general they drank more milk than did adults. Also, children’s thyroids are smaller and concentrate more I-131. (On website see "Questions and Answers on the NCI Fallout Report")

Why is raw milk more dangerous than store-bought milk?

I-131 has a radioactive half-life of about eight days, meaning that its radioactivity decreases by half every eight days. This means that nearly all the exposures took place primarily within two months following each test. Estimates of I-131 doses in milk consumed soon after it was taken from a family-owned cow are usually higher than for store-bought milk because store-bought milk takes time to process and ship, allowing more time for the radioactivity in I-131 to diminish. (On website see "Questions and Answers on the NCI Fallout Report")

How much research has been done on things other than cows’ milk, for example, goat milk; soil; eggs; air; water--drinking or physical contact; gardens, vegetation; meat from cows and other animals, including wild game; dairy products made from cow milk, such as ice cream, cottage cheese, etc?

There is extensive information in the scientific literature on the fallout-pasture-cows' milk exposure route for I-131, and that cows' milk is the primary route of human exposure to I-131. Less information is available on goats' milk, eggs, leafy vegetables and cottage cheese; however, these foods were considered in the NCI study. In mammals, the small amount of iodine which is not excreted after exposure is concentrated in the thyroid, with some going to lactating female mammary glands and a small percentage removed and re-circulated by the salivary glands and gastric mucosa. Because very little I-131 would be found in muscle tissue -- eaten as beef, chicken, fish, game -- this is not considered to be a food source of concern and has not been a subject of much investigation. Other pathways of potential exposure, such as the consumption of soil by small children or the consumption of water, rarely have been considered a concern because they result in such small doses of exposure. (On website see"Full Report")

Why did it take so long for the public to be made aware of the danger?

It has been widely known since the late 1950s that I-131 fallout occurred across the U.S., and published information available in the mid-1960s indicated that I-131 might have resulted in thyroid doses ranging from 4 to 120 rads in some locations during 1951-1955. Unfortunately, scientists do not know what causes most cases of thyroid cancer. The cancer is uncommon, accounting for just one percent of all cancers in this country. Studies to determine the relative risk of I-131 exposure to thyroid cancer have been inconclusive; at this time, there is no scientific basis for believing that anyone has been harmed by the length of this study.

The NCI’s I-131 fallout report results are remarkably consistent with these early reports made part of the public record during Congressional hearings held in the late 1950s and early 1960s. While our study provides, we believe, both important new methodology and much more detailed exposure information than has been previously available, the results ought not to be characterized as unexpected. (On website see "Congressional Testimony by NCI Director" and "Questions and Answers on the NCI Fallout Report")

Are there studies other than NCI’s being done on I-131 exposure from the Nevada Test site? Can those who have had thyroid cancer possibly caused by exposure to I-131 be of any assistance in the study?

We are not aware of any studies being carried out on I-131 exposure from the Nevada Test Site. Prior to the NCI study, Department of Energy studies estimated radiation exposure of persons in regions and states near the Nevada Test Site. In 1993, the University of Utah (under a contract from the NCI with support from the Departments of Defense and Energy) published results from a study of thyroid disease among a cohort of children exposed to I-131 from nuclear weapons tests at the Nevada Test Site. They showed a suggestive relationship between thyroid cancer and thyroid radiation doses due to I-131, although that result could have been due to chance. There was a significant relationship between I-131 doses to the thyroid and all thyroid neoplasms (both benign and malignant tumors) grouped together, however.

Questions Related to Testing

When exactly were these tests conducted?

The NCI’s I-131 fallout report estimated exposures to atmospheric nuclear bomb tests that were carried out at the Nevada Test Site in the 1950s and 1960s. Ninety nuclear tests released almost 99 percent of the total I-131 entering the atmosphere in the years 1952, 1953, 1955, and 1957.

(On website see "Full Report")

Is there danger of exposure now?

No. The I-131 that was released from the tests is no longer present in the environment. I-131 has a radioactive half-life of about eight days, meaning that its radioactivity decreases by half every eight days. This means that nearly all the exposures took place primarily within two months following each test. (On website "Questions and Answers on the NCI Fallout Report")

Is there still any radiation in the soil and water?

The I-131 released from the tests is no longer present in the environment. Iodine-131 has a radioactive half-life of about 8 days -- meaning that its radioactivity decreases by half every 8 days -- so nearly all of the exposures from I-131 took place within the two months after each test. Isotopes from fallout with longer half-lives, such as strontium-90 and cesium-137, are still in the soil; however, they contribute radiation doses that are very small compared to those from I-131 or to those naturally-occurring radionuclides with very long radioactive half-lives, such as uranium-238 and potassium-40. Naturally-occurring radionuclides have been in soil and water since the formation of the earth and have been irradiating human populations at a relatively constant rate. In fact, the radiation dose from the naturally-occurring radionuclides and from cosmic rays is much greater than are doses from residual fallout. (On website see "Full Report")

Why is Vermont one of the "hot spots"? What areas of the country are not?

The report shows that everyone living in the contiguous 48 states during the test period was exposed to I-131 at some level. Doses varied widely according to geographic area. In general, persons living in Western states to the north and east of the test site had higher doses than those living in West Coast, Southern, and Eastern states. The deposition of airborne radioactive particles in an area of the country depended on wind and rainfall patterns. Estimated exposures are a result of estimates of I-131 deposition in each country following each test, the environmental transport of the I-131 from pasture grasses to cows’ milk, the production and distribution pattern of cows’ milk throughout the U.S., and milk consumption patterns as a function of age and region of the country. NCI urges caution in interpreting the report results, particularly because the study does not directly address the question of cancer risk from the fallout. (On website see "U.S. Map with County-by-County Exposures" and "Full Report")

Questions Related to Health

Are there other causes of thyroid cancer?

Scientists do not know what causes most cases of thyroid cancer. One known risk factor is exposure to radiation during childhood, such as exposure from childhood treatments for noncancerous conditions such as enlarged tonsils and ringworm. Children exposed to radioactive fallout from nuclear explosions in the 1950s may also be at risk; however, scientists do not yet have a reliable way to estimate that risk. (On website see "Fact Sheet on Thyroid Cancer" and "Questions About Thyroid Cancer")

What are the warning signs of thyroid cancer? How terrible is the disease?

Each year, more than 12,000 Americans find out they have thyroid cancer; this disease accounts for about 1 percent of all cancers. Thyroid cancer usually responds well to treatment, and the 5-year survival rate is about 95 percent. The most common symptom of thyroid cancer is a lump, or nodule, that can be felt in the neck. Other symptoms are rare. Pain is seldom an early warning sign of thyroid cancer. However, a few patients have a tight or full feeling in the neck, difficulty breathing or swallowing, hoarseness, or swollen lymph nodes. These symptoms can be caused by thyroid cancer or by other, less serious problems. Only a doctor can determine the cause of a persons’ symptoms. (On website see "Questions About Thyroid Cancer")

What should people who are concerned about their possible exposure to I-131 do?

Anyone who is concerned about cancer risk from fallout should request a thyroid examination as part of their next visit to a physician. To ensure that you have the most up to date information, please call the Cancer Information Service (CIS). The CIS, a program of the National Cancer Institute, is a nationwide telephone service for cancer patients and their families, the public, and health care professionals. CIS information offices serve specific geographic areas and have information about cancer-related services and resources in their region. The toll-free number is 1-800-4-CANCER (1-800-422-6237).

Additionally, the NCI web site provides an interactive format which allows an individual to estimate his or her own exposure. By designating a state and county, and date of birth, users will receive a table of the estimated doses to the thyroid after each nuclear test. Dosages are also calculated for 4 different milk-drinking scenarios. (On website see "Questions About Thyroid Cancer" and "Full Report" Report Contents Full Data Set Annexes and Subannexes Data Indexed by State and County")

Are people in some age groups more likely to have been affected than others?

Yes. Children and persons who drank large quantities of milk, or who drank milk from goats or family-owned cows, are estimated to have received higher doses. Most children ages 3 months to 5 years during the test periods probably received about three to seven times the average dose for each test, because in general they drank more milk than adults did, and because their thyroids were smaller. (On website see "Questions and Answers on the NCI Fallout Report")

Were pregnant women more susceptible because they drank more milk?

In general, the dose to the thyroid is related to the amount of milk consumed. Therefore, if a pregnant woman consumes more milk than does a non-pregnant woman, and the I-31 concentration in the milk is the same, the thyroid radiation dose to the pregnant woman would be higher. In addition, the thyroid in a pregnant woman tends to absorb more I-131, and may be larger, than that in the not-pregnant woman. Since these two changes in the thyroid of a pregnant woman offset each other, the radiation dose to the thyroid remains strongly dependent upon the amount of milk consumed. (On website see "Questions and Answers on the NCI Fallout Report")

Could exposure to I-131 cause any thyroid conditions other than cancer? Could this exposure cause cancer in parts of the body other than the thyroid? Is there any connection between I-131 exposure and Vermont’s high rate of breast cancer? Is the exposure related to other medical conditions, such as hair loss and premature births? What effects may exposure have on future generations (offspring of those exposed)?

Researchers know that external I-131 may increase a person’s risk for thyroid cancer, and limited data on persons exposed as children to I-131 from the nuclear test fallout have provided suggestive but not conclusive evidence of an association. The thyroid is the only human organ currently known to be affected by external I-131 exposure. There is no scientific evidence in the literature to link I-131 with cancer at other sites. Studies of offspring of A-bomb survivors show no evidence of radiation-related damage to DNA.

In 1993, researchers at the University of Utah published results of an NCI-sponsored study of children living in parts of Utah and Nevada that had high I-131 fallout levels, and a comparable number living in parts of Arizona that had relatively little fallout. The researchers found a statistically significant association between I-131 exposure and all thyroid neoplasms combined (including cancer and non-cancer tumors). The relationship between thyroid cancer and I-131 dose to the thyroid was suggestive, but not conclusive. Researchers sponsored by the Centers for Disease Control and Prevention are studying the health effects of the radioactive iodine released from the Hanford, Washington, nuclear weapons plant in the 1940s and 1950s; results are expected in 1998. For additional information about non-malignant thyroid conditions, contact the American Thyroid Association at (718) 882-6047. (On website see "Questions and Answers on the NCI Fallout Report")

Other Questions

What is the U.S. government doing in light of this report?

NCI and the Department of Health and Human Services have enlisted the help of the foremost radiation experts in the country to fully evaluate the risk and develop an appropriate public health response. The Department has requested that the National Academy of Sciences’ Institute of Medicine review the data to assess whether risks can be determined, and to recommend to physicians how to identify, evaluate, and treat persons who might be at risk of disease because of their exposure to radioactive iodine. (On website see "Press Release")

Have similar studies been conducted on other environmental hazards, such as Three Mile Island, Chernobyl, Vermont Yankee?

No studies of the scope of the NCI’s I-131 fallout report have ever been done before. The NCI study was specific to tests performed at the Nevada Test Site during the 1950s and 1960s.

There has been extensive environmental monitoring of the affected atolls that make up the Marshall Islands, and the U.S. has spent millions of dollars to clean up the area, and build homes and communities for the returnees. The U.S. also has monitored the health of those Marshallese known to have been exposed to fallout and has provided treatment for those persons who developed thyroid problems. Populations around weapons production facilities were exposed to both I-131 fallout and facility releases, such as at Hanford Washington, Oak Ridge Reservation (Tennessee), and Savannah River Site (South Carolina); health effects of exposure are currently being studied at each of these sites. NCI investigators are collaborating with other U.S. government agencies, international organizations, and governments and scientists in Belarus and Ukraine to study thyroid cancer and its relationship to exposure to I-131 among children exposed to fallout from the Chernobyl nuclear accident in 1986; a clear increase in thyroid cancer has been seen in this population.

In 1979 a Presidential Commission reported that after the Three Mile Island accident, monitoring of milk did not reveal any I-131 in cows’ milk (though I-131 was detected in milk from goats grazing on property adjacent to the TMI facility), and the average dose from the accidental release to a person living within 5 miles of TMI was calculated to be about 10 percent of the annual background (natural) radiation or less. In 1991 an NCI study of 62 nuclear facilities -- which included the Vermont Yankee facility in Massachusetts -- showed no general increased risk of death from cancer for people living in the counties containing or closely adjacent to these facilities.

Will those who are exposed be eligible for any form of compensation?

At this time, the only program that provides compensation for non-military individuals who were exposed to radiation released during above-ground nuclear tests is authorized by the Radiation Exposure Compensation Act, which was passed by Congress in 1990. This act provides for compensation for very specific groups: "downwinders," on-site participants at the tests, and uranium miners. The U.S. Department of Justice handles claims made pursuant to this Act. Information about this program is available from the claims department at the agency at 1-800-729-7327.

Is there anything that a Vermont endocrinologist specializing in thyroid cancer can do to help?

The NCI offers technical assistance on interpreting the report for health professionals either by telephone (1-800-273-7092) or by email (CISOCC@nih.gov). Patients, their families, and physicians can find information about thyroid cancer and its treatment through NCI’s Cancer Information Service, toll-free at 1-800-4-CANCER. The Vermont State Health Department has received both the full report and a briefing on the I-131 fallout report; the contact person there is Dr. Jan Carney, who can be reached at (802) 863-7280. Interested physicians may also wish to contact the American Thyroid Association at (718) 882-6047.

For additional details, please refer to the website of the
National Cancer Institute AND National Academy of Sciences.