Get the facts

about exposureto

I-131 Radiation

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

National Institutes of Health

National Cancer Institute
Table of Contents

How Americans Were 

Exposed to I-131 .............................................. 4

The Search for Answers .................................. 6

I-131’s Rapid Breakdown ................................ 7

The Milk Connection ........................................ 8

About Thyroid Disease .................................... 9

Thyroid Cancer and I-131 .............................. 10

Who’s at Risk ..................................................12

How Do Doctors Diagnose

and Treat Thyroid Cancer? ..............................14

Key Facts ..........................................................16

Taking Care of Yourself ..................................17
2

During the Cold War in the 1950s and early 1960s,

the U.S. government conducted about one

hundred nuclear weapons (atomic bomb) tests in

the atmosphere at a test site in Nevada. 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 95 out of 

100 people who are diagnosed with thyroid 

cancer survive the disease for at least five 

years after diagnosis.
3

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.

This brochure is designed to provide information

about I-131 and its possible effects 

on the thyroid gland. A companion brochure 

offers a decision-making aid to help 

determine personal risk.

Although the potential of

developing thyroid cancer

from exposure to I-131 is

small, it is important for Americans

who grew up during the atomic

bomb testing between 1951 and

1963 to be aware of risks.
How Americans Were 3

Exposed to I-131

During the Cold War, the United States developed and

tested nuclear weapons in an effort to deter and to be fully

prepared for nuclear attacks from other nations. Most of

the aboveground U.S. nuclear tests were conducted in

Nevada from 1951 to 1963. As a result of these tests,

potentially health-harming radioactive materials were

released into the atmosphere and produced fallout.

I-131 was among the radioactive materials released by 

the atomic bomb tests. It was carried thousands of miles

away from the test areas on the winds. Because of wind

and rainfall patterns, the distribution of fallout varied widely 

after each test. Therefore, although all areas of the U.S.

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 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, where it was consumed by cows and goats.

When consumed by cows or goats, I-131 collects in the

animals’ milk. Eating beef from cows exposed to I-131

carried little risk. Much of the health risk associated with 

4
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, Americans were

exposed to I-131 through:

• Nuclear testing elsewhere in the world (mainly

in the 1950s and 1960s)

• Nuclear power plant accidents (such as 

the Chornobyl accident in 1986, also 

known as Chernobyl) 

• 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 from other

sources. Scientists are also working to find out more about

other radioactive substances released by fallout and about

their possible effects on human health.

5
5

The Search for Answers

Congress directed government health agencies to

investigate the I-131 problem many years ago, and to

make recommendations to Americans who might have

related health risks. Gathering information turned out to be

very complicated. Record-keeping was incomplete at the

time of the bomb testing. Much of the information needed

to calculate an individual’s dose of I-131 and associated

risk is either unreliable or unavailable.

Despite such challenges, government agencies organized

expert scientific teams that have devoted many years to

learning more about I-131. Reports were published in

1997 and 1999. This brochure continues the effort to

educate the American people about the potential health

risks from exposure to I-131 from the Nevada Test Site

during the Cold War years.

6
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.

7
7

The Milk Connection

People younger than 15 at the time of aboveground 

testing (between 1951 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 other

people. 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 1951 and 1963)

• The amount and source of milk they 

drank in those years

• Where they lived during the testing period

Age and residence during the Cold War 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.

8
8 9

About Thyroid Disease

There are two main types of thyroid diseases:

noncancerous thyroid disease and thyroid cancer.

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.
9

Thyroid Cancer and I-131

Exposure to I-131 may increase a person’s risk of

developing thyroid cancer. It is thought that risk is higher

for people who have had multiple exposures and for

people exposed at a younger age. Thyroid cancer

accounts for less than 2 percent of all cancers diagnosed

in the United States. Typically, it is a slow-growing cancer

that is highly treatable and usually curable. About 95 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.

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.

10
10

There is no single or specific symptom of thyroid cancer.

Doctors screen for thyroid cancer by feeling the gland, to

check for a lump or nodule. If a doctor feels a nodule, it

does not mean cancer is present. Most thyroid nodules

found during a medical exam are not cancer.

If thyroid cancer is found, it is treated by removing the

thyroid gland. People who undergo surgery will need to

take thyroid hormone replacement pills for the rest of their

lives. Although this is inconvenient and expensive, cancer

survival rates are excellent. In fact, the cause of death

among people who once had thyroid cancer is rarely the

result of the return or spread of the same cancer.

Living with a serious disease like thyroid cancer isn’t easy.

A cancer diagnosis can be devastating. Some people find

they need help coping with the emotional and practical

aspects of their disease. Doctors and other health

professionals can help with concerns about treatment and

managing side effects. Support groups can help also. The

National Cancer Institute’s Cancer Information Service can

help put you in touch with support groups in your

community. Call 1-800-4-CANCER for more information.

11
11

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?

Scientists estimate that about 25 percent of the radioactive

materials released during atomic bomb testing in Nevada

reached the ground somewhere in the United States. But

information about where the wind carried these materials

is not precise. In addition, most adults cannot remember

exact details of their milk-drinking habits in childhood.

Still, scientists and doctors think that I-131 exposure is a

potential risk factor for thyroid cancer, and that some

Americans have a higher risk than others. 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 are now 40 years of age

or older, particularly those born between

1936 and 1963 and who 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.

12
• 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.
How Do Doctors Diagnose 13

and Treat Thyroid Cancer?

There are two methods of investigating a thyroid lump 

or nodule:

1. Ultrasound—to locate and describe the lump, and

2. Biopsy—to determine if the lump is cancerous.

Thyroid ultrasound creates pictures by bouncing sound

waves off the gland. This technique is painless and quick.

But it cannot determine whether a lump is cancerous. The

ultrasound device uses sound waves that people cannot

hear. A computer uses the echoes to create a picture

called a sonogram. From the picture, the doctor can see:

• How many nodules are present 

• How big they are 

• Whether they are solid or filled with fluid

Confirmation of cancer requires biopsy, usually using 

fine needle aspiration. Cells removed from a nodule 

during biopsy are directly examined in the laboratory with

a microscope.

Fine needle aspiration biopsy—in which a few cells are

withdrawn from a nodule in a thin, hollow needle—is fast

and carries minimal risk. Most people with a thyroid nodule

who have a biopsy turn out not to have thyroid cancer. But

even noncancerous nodules require medical follow-up. If a

diagnosis cannot be made from the biopsy, the doctor may

14
operate to remove the nodule. A pathologist then checks

the tissue for cancer cells.

If thyroid cancer is found, it is treated by removing the

thyroid gland. People who undergo surgery will need 

to take thyroid hormone replacement pills for the rest 

of their lives.

Unlike many other far more common and threatening

cancers, thyroid cancer is generally cured by surgery, often

along with postoperative radioiodine treatment.

People who think they may be at risk for thyroid cancer

should discuss this concern with their doctor. The doctor

may suggest a schedule for checkups.
16

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 

in this brochure (see “Who’s at Risk?”)

• Using the dose estimator on the National 

Cancer Institute’s Web site www.cancer.gov

(search keyword: I-131)

• Taking this brochure to a health care 

professional to discuss dose estimates 

and steps—if any—required for 

further evaluation

• Getting more information by calling the 

National Cancer Institute’s Cancer Information

Service at 1-800-4-CANCER

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NIH Publication No. 02-5111

Printed September 2002

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