1. What is radioactive fallout?

   Radioactive fallout refers to a variety of airborne radioactive particles that fall to the ground during and following aboveground nuclear weapons tests. People and animals may ingest these particles in their food or inhale them from the air.

2. What is iodine 131 (I–131)?

   Iodine 131 is a radioactive isotope of iodine—an altered form of the element that is chemically the same as the naturally occurring element but is radioactive. I–131 is found in fallout from atmospheric nuclear bomb explosions, along with a variety of other radioactive substances such as strontium–90. I–131 has been of greatest concern because it concentrates in the thyroid, particularly in children, and may increase the risk for thyroid cancer. There appears to be little risk of thyroid cancer for people exposed to radiation as adults. Because iodine concentrates in the thyroid, high doses of I–131 are used to treat some types of benign thyroid disease as well as thyroid cancer.

3. How were people exposed to I–131?

   For most people, the greatest I–131 exposure came from drinking contaminated milk. I–131 fell on pastures, and cows ate the vegetation, contaminating their milk. People received smaller levels of exposure from breathing contaminated air and eating other contaminated dairy products, eggs, and leafy vegetables.

4. Am I still being exposed?

   Because I–131 has a short half-life, less than 1 percent remained 80 days after most nuclear tests.

5. How common is thyroid cancer and how treatable is it?

   Thyroid cancer is relatively uncommon compared with other forms of cancer, accounting for 1.6 percent of all cancers diagnosed in the United States. An estimated 20,700 cases will be diagnosed in 2002. The number of new cases is about 16 times the number of deaths, which is estimated to be 1,300 for 2002. Thyroid cancer occurs more than twice as often in women as in men. The rate of new cancers (invasive) for 1999—the most recent year available—was 10.4 per 100,000 women and 3.8 per 100,000 men. Thyroid cancer death rates for 1999 were 0.5 deaths per 100,000 women and 0.4 deaths per 100,000 men. The lifetime risk of invasive thyroid cancer is estimated at 0.82 percent (1 in 122) for women and 0.30 percent (1 in 333) for men. Thyroid cancer is also highly curable. The five-year relative survival rate is 96 percent.

6. What should people do who are concerned about cancer risk from fallout exposure?

   Anyone who is concerned about cancer risk from fallout should request a thyroid exam as part of his or her next visit to a physician.

# # #

Related NCI materials and Web pages:

• National Cancer Institute Fact Sheet 3.11, No Excess Mortality Risk Found in Counties with Nuclear Facilities ¹
  (http://www.cancer.gov/cancertopics/factsheet/Risk/nuclear-facilities)
• Radioactive I–131 from Fallout Web Page ²
  (http://www.cancer.gov/i131)
• What You Need To Know About™ Thyroid Cancer ³
  (http://www.cancer.gov/cancertopics/wyntk/thyroid)

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Glossary Terms

A change resulting in something that is different from the original.

Not cancerous. Benign tumors may grow larger but do not spread to other parts of the body. Also called nonmalignant.

A term for diseases in which abnormal cells divide without control and can invade nearby tissues. Cancer cells can also spread to other parts of the body through the blood and lymph systems. There are several main types of cancer. Carcinoma is a cancer that begins in the skin or in tissues that line or cover internal organs. Sarcoma is a cancer that begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue. Leukemia is a cancer that starts in blood-forming tissue such as the bone marrow, and causes large numbers of abnormal blood cells to be produced and enter the blood. Lymphoma and multiple myeloma are cancers that begin in the cells of the immune system. Central nervous system cancers are cancers that begin in the tissues of the brain and spinal cord. Also called malignancy.

To heal or restore health; a treatment to restore health.

The process of identifying a disease, such as cancer, from its signs and symptoms.

The amount of medicine taken, or radiation given, at one time.

An element that is necessary for the body to make thyroid hormone. It is found in shellfish and iodized salt.

Medical doctor.

Energy released in the form of particle or electromagnetic waves. Common sources of radiation include radon gas, cosmic rays from outer space, medical x-rays, and energy given off by a radioisotope (unstable form of a chemical element that releases radiation as it breaks down and becomes more stable).

Giving off radiation.

Airborne radioactive particles that fall to the ground during and after an atomic bombing, nuclear weapons test, or nuclear plant accident.

A way of comparing survival of people who have a specific disease with those who don’t. The percentage of survivors is usually determined at specific times, such as 2 years and 5 years after diagnosis or treatment. The relative survival rate shows whether the disease shortens life.

A metal often used in a radioactive form for imaging tests and in the treatment of cancer.

Cancer that forms in the thyroid gland (an organ at the base of the throat that makes hormones that help control heart rate, blood pressure, body temperature, and weight). Four main types of thyroid cancer are papillary, follicular, medullary, and anaplastic thyroid cancer. The four types are based on how the cancer cells look under a microscope.