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Get the Facts about Exposure to I-131 Radiation

Use the Thyroid Dose/Risk Calculator
This calculator estimates radiation dose and risk of developing thyroid cancer from fallout exposure from nuclear tests.


During the Cold War in the mid-1940s through early 1960s, the U.S. government conducted about 100 nuclear weapons (atomic bomb) tests in the atmosphere at a test site in Nevada, more than 100 in the Pacific, and one—the first ever—in New Mexico. The radioactive substances released by these tests are known as "fallout." They were carried thousands of miles away from the test site by winds. As a result, people living in the United States at the time of the testing were exposed to varying levels of radiation.

Among the numerous radioactive substances released in fallout, there has been a great deal of concern about and study of one radioactive form of iodine--called iodine-131, or I-131. I-131 collects in the thyroid gland. People exposed to I-131, especially during childhood, may have an increased risk of thyroid disease, including thyroid cancer. Thyroid cancer is uncommon and is usually curable. Typically, it is a slow-growing cancer that is highly treatable. About 98 out of 100 people who are diagnosed with thyroid cancer survive the disease for at least five years after diagnosis.

The thyroid controls many body processes, including heart rate, blood pressure, and body temperature, as well as childhood growth and development. It is located in the front of the neck, just above the top of the breastbone and overlying the windpipe.

Although the potential of developing thyroid cancer from exposure to I-131 from nuclear weapons testing is small, it is important for Americans who grew up during the atomic bomb testing between 1945 and 1963 to be aware of risks.

How Americans Were Exposed to I-131

Because of wind and rainfall patterns, the distribution of I-131 fallout varied widely after each test. Therefore, although all areas of the United States received fallout from at least one nuclear weapons test, certain areas of North America received more fallout than others.

Scientists estimate that the larger amounts of I-131 from the Nevada test site fell over some parts of Utah, Colorado, Idaho, Nevada, and Montana. But I-131 traveled to all states, particularly those in the Midwestern, Eastern, and Northeastern United States. Some of the I-131 collected on pastures and on grasses. Depending on the location, grazing cows and goats sometimes consumed contaminated grasses resulting in I-131 collecting in the animals' milk. Much of the health risk associated with I-131 occurred among milk-drinkers--usually children. From what is known about thyroid cancer and radiation, scientists think that people who were children during the period of atomic bomb testing are at higher risk for developing thyroid cancer.

In addition to nuclear testing in Nevada, the Pacific, and New Mexico, Americans were potentially exposed to I-131 from a number of events, including:

  • Nuclear testing by other nations elsewhere in the world (mainly in the 1950s and 1960s)
  • Nuclear power plant accidents (such as the Chernobyl accident in 1986 and the Fukushima accident in 2011 (primarily Americans in Japan)
  • Releases from atomic weapons production plants (such as the Hanford facility in Washington state from 1944 to 1957)

Scientists are working to find out more about ways to measure and address potential I-131 exposure. They are also working to find out more about other radioactive substances released by fallout and their possible effects on human health.

I-131's Rapid Breakdown

The "active" in "radioactive" means that unstable substances produced in nuclear reactions break down and change, so that they eventually become stable and no longer release radiation. The rate of breakdown can occur quickly in some radioactive substances, often within a few days. Half of the I-131 released during each atomic bomb test was gone in about 8 days. Almost all of it was gone (less than 1 percent remained) 80 days after the test.

Like all radioactive substances, I-131 releases radiation as it breaks down. It is this radiation that can injure human tissues. But I-131's steady breakdown means that the amount of I-131 present in the environment after a bomb test steadily decreased. Therefore, farm animals that grazed in fields within a few days after a test would have consumed higher levels of I-131 than animals grazing later.

The Milk Connection

People younger than 15 at the time of aboveground testing (between 1945 and 1963) who drank milk, and who lived in the Mountain West, Midwestern, Eastern, and Northeastern United States, probably have a higher thyroid cancer risk from exposure to I-131 in fallout than people who lived in other parts of the United States, who were over the age of 15 in the 1940s, or who did not drink milk. Their thyroid glands were still developing during the testing period. And they were more likely to have consumed milk contaminated with I-131. The amount of I-131 people absorbed depends on:

  • Their age during the testing period (between 1945 and 1963)
  • The amount and source of milk they drank in those years
  • Where they lived during the testing period

Age and residence during those years are usually known. But few people can recall the exact amounts or sources of the milk they drank as children. While the amount of milk consumed is important in determining exposure to I-131, it is also important to know the source of the milk. Fresh milk from backyard or farm cows and goats usually contained more I-131 than store-bought milk. This is because processing and shipping milk allowed more time for the I-131 to break down.

About Thyroid Disease

There are two main types of thyroid diseases:

Noncancerous Thyroid Disease

Some thyroid diseases are caused by changes in the amount of thyroid hormones that enter the body from the thyroid gland. Doctors can screen for these with a simple blood test.

Noncancerous thyroid disease also includes lumps, or nodules, in the thyroid gland that are benign and not cancerous.

Thyroid Cancer

Thyroid cancer occurs when a lump, or nodule, in the thyroid gland is cancerous.

Thyroid Cancer and I-131

Thyroid cancer accounts for a little less than 4 percent of all cancers diagnosed in the United States. Incidence has been going up in recent years, in part due to increased detection. Researchers suspect that rising rates of obesity are also influencing rates. However, these two factors do not fully explain the increases. Typically, thyroid cancer is slow-growing, highly treatable, and usually curable. About 98 out of 100 people who are diagnosed with thyroid cancer survive the disease for at least five years, and about 92 out of 100 people survive the disease for at least 20 years after diagnosis.

The cause of most cases of thyroid cancer is not known. Exposure to I-131 can increase the risk of thyroid cancer. It is thought that risk is higher for people who have had multiple exposures and for people exposed at a younger age. But even among people who have documented exposures to I-131, few develop this cancer. It is known that children have a higher-than-average risk of developing thyroid cancer many years later if they were exposed to radiation. This knowledge comes from studies of people exposed to x-ray treatments for childhood cancer or noncancerous head and neck conditions, or as a result of direct radiation from the atomic bombings of Hiroshima and Nagasaki.

The thyroid gland in adults, however, appears to be more resistant to the effects of radiation. There appears to be little risk of developing thyroid cancer from exposure to I-131 or other radiation sources as an adult.

For more on thyroid cancer, see NCI’s Thyroid Cancer page.

Who's at Risk?

How can people reach a sound decision about their risk of thyroid cancer? When is it time to visit a doctor?

A "personal risk profile" includes four key points that may influence a person's decision to visit a doctor or other health professional for evaluation:

  • Age—People who were born between 1936 and 1963 and were children at the time of testing are at higher risk.
  • Milk drinking—Childhood milk drinkers, particularly those who drank large quantities of milk or those who drank unprocessed milk from farm or backyard cows and goats, have increased risk.
  • Childhood residence—The Mountain West, Midwest, East, and Northeast areas of the United States generally were more affected by I-131 fallout from nuclear testing.
  • Medical signs—A lump or nodule that an individual can see or feel in the area of the thyroid gland requires attention. If you can see or feel a lump or nodule, it is important that you see a doctor.

Key Facts

Scientists know that:

  • I-131 breaks down rapidly in the atmosphere and environment
  • Exposure was highest in the first few days after each nuclear test explosion
  • Most exposure occurred through drinking fresh milk
  • People received little exposure from eating fruits and leafy vegetables as compared to drinking fresh milk because although I-131 was deposited on fruits and leafy vegetables, the I-131 in fallout was deposited only on the surface; people generally wash or peel fruits and leafy vegetables
  • Thyroid cancer is uncommon, usually curable, and approximately 2 to 3 times more common in women

Reliable information about I-131's impact on human health has been difficult to collect, but scientists think that:

  • Risk for thyroid cancer increases with exposure, but even among people exposed to I-131, few develop this cancer
  • People exposed as children have a higher risk than people exposed as adults

Taking Care of Yourself

Key steps to estimating personal risk of thyroid cancer, and taking charge of personal thyroid health include:

  • Using the "personal risk profile" described above (see Who's at Risk?)
  • Using the thyroid dose and risk calculator to estimate radiation dose and risk of developing thyroid cancer from fallout exposure from nuclear tests
  • Taking this material to a health care professional to discuss dose estimates and steps—if any—required for further evaluation
  • Getting more information by calling NCI's Cancer Information Service at 1-800-4-CANCER