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发帖数: 17190 | 1 https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation/i-131
Get the Facts about Exposure to I-131 Radiation
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This calculator estimates radiation dose and risk of developing thyroid
cancer from fallout exposure from nuclear tests.
ON THIS PAGE
Introduction
How Americans Were Exposed to I-131
The Search for Answers
I-131's Rapid Breakdown
The Milk Connection
About Thyroid Disease
Thyroid Cancer and I-131
Who's at Risk?
Key Facts
Taking Care of Yourself
Introduction
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.
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. A number of
reports have been published documenting what they have learned (1997, 1999)
. This information was put together to educate the American people about the
potential health risks from exposure to I-131 from nuclear weapons testing.
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 are now 50 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.
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
Related Resources
Estimated Exposure and Thyroid Doses Report pdf file (8 MB)
State and County Exposure Levels to I-131
Fallout From Nuclear Weapons Tests and Cancer Risks pdf file (4 MB)
Recent and Current NCI Studies of Radioactive Fallout
Exposure of the American People to Iodine-131 from Nevada Nuclear-Bomb Tests
Updated: August 3, 2015 |
