Ionizing radiation consists of subatomic particles (that is, particles that are smaller than an atom, such as protons, neutrons, and electrons) and electromagnetic waves. These particles and waves have enough energy to strip electrons from, or ionize, atoms in molecules that they strike. Ionizing radiation can arise in a number of ways, including the following:

• From the spontaneous decay (breakdown) of unstable isotopes. Unstable isotopes, which are also called radioactive isotopes, give off, or emit, ionizing radiation as part of the decay process. Radioactive isotopes occur naturally in the Earth’s crust, soil, atmosphere, and oceans. These isotopes are also produced in nuclear reactors and nuclear weapons explosions.
  
• From cosmic rays originating in the sun and other extraterrestrial sources and from technological devices ranging from dental and medical x-ray machines to the picture tubes of old-style televisions.

Everyone on Earth is exposed to low levels of ionizing radiation from natural and technological sources in varying proportions, depending on their geographic location, diet, occupation, and lifestyle.

At high doses, ionizing radiation can cause immediate damage to a person’s body, including radiation sickness and death. Ionizing radiation is also a carcinogen, even at low doses; it causes cancer primarily because it damages DNA. However, the lower the dose of ionizing radiation, the lower the chances of harm.

Children and adolescents are more sensitive to the cancer-causing effects of ionizing radiation than adults because their bodies are still growing and developing. In addition, children and adolescents usually have more years of life following radiation exposure during which cancer may develop.

Nuclear power plants use energy released by the decay of certain radioactive isotopes to produce electricity. Additional radioactive isotopes are produced during this process. In nuclear power plants, specially designed fuel rods and containment structures enclose the radioactive materials to prevent them, and the ionizing radiation they produce, from contaminating the environment. If the fuel and surrounding containment structures are severely damaged, radioactive materials and ionizing radiation may be released, potentially posing a health risk for people. The actual risk depends on several factors:

• The specific radioactive materials, or isotopes, released, and the quantities released.
• How a person comes into contact with the released radioactive materials (such as through contaminated food, water, air, or on the skin).
• The person’s age (those exposed at younger ages are generally at higher risk).
• The duration and amount of the exposure.

The radioactive isotopes released in nuclear power plant accidents include I-131 and Cs-137. In the most severe kinds of accidents, such as the Chernobyl accident in 1986, other dangerous radioactive isotopes, such as strontium-90 (Sr-90) and plutonium-239, may also be released.

Human exposure to I-131 released from nuclear power plant accidents comes mainly from consuming contaminated water, milk, or foods. People may also be exposed by breathing dust particles in the air that are contaminated with I-131.

Inside the body, I-131 accumulates in the thyroid gland, which is an organ in the neck. The thyroid gland uses iodine to produce hormones that control how quickly the body uses energy. Because the thyroid does not distinguish between I-131 and nonradioactive iodine, the thyroid gland will accumulate either form. Exposure to radioactive iodine may increase the risk of thyroid cancer many years later, especially for children and adolescents.

Exposure to Cs-137 can be external to the body or internal. External exposure comes from walking on contaminated soil or coming into contact with contaminated materials at nuclear accident sites. Internal exposure can come from breathing particles in the air that contain Cs-137, such as dust originating from contaminated soil, or ingesting contaminated water or foods. Because Cs-137 is not concentrated in a particular tissue, the ionizing radiation that it releases can expose all tissues and organs of the body.

Much of what is known about cancer caused by radiation exposures from nuclear power plant accidents comes from research on the April 1986 nuclear power plant disaster at Chernobyl, in what is now Ukraine. The radioactive isotopes released during the Chernobyl accident included I-131, Cs-137, and Sr-90.

Approximately 600 workers at the power plant during the emergency received very high doses of radiation and suffered from radiation sickness. All of those who received more than 6 grays (Gy) of radiation became very sick right away and subsequently died. Those who received less than 4 Gy had a better chance of survival. (A Gy is a measure of the amount of radiation absorbed by a person’s body.)

Hundreds of thousands of people who worked as part of the cleanup crews in the years after the accident were exposed to lower external doses of ionizing radiation, ranging from approximately 0.14 Gy in 1986 to 0.04 Gy in 1989. In this group of people, there was an increased risk of leukemia.

Approximately 6.5 million residents of the contaminated areas surrounding Chernobyl received much lower amounts of radiation. From 1986 through 2005, these people received an accumulated average dose of 0.0092 Gy from external and internal sources of radiation. Children and adolescents exposed to I-131 showed an increased risk of developing thyroid cancer.

Although the time it takes for the radiation to decrease by half (the half-life) of I-131 is only 8 days, the damage it causes can increase the risk of thyroid cancer for many years after the initial exposure.

A study led by National Cancer Institute (NCI) researchers followed more than 12,500 people who were younger than age 18 at the time they were exposed to high doses of I-131 (0.65 Gy on average) from the Chernobyl accident. A total of 65 new cases of thyroid cancer were found in this population between 1998 and 2007. Roughly half of these new cases were attributed to I-131 exposure. The researchers found that the higher a person’s dose of I-131, the more likely they were to get thyroid cancer (with each Gy of exposure associated with a doubling of risk). They also found that this risk remained high for at least 20 years.

Cancer patients who are being treated with systemic chemotherapy or radiation therapy should be evacuated from the area where a nuclear power plant accident has occurred so their medical treatment can continue without interruption. Patients should always keep a record of the treatments they have had in the past and that they may be currently receiving, including the names of any drugs and their doses. These records may be important in the aftermath not only of a nuclear power plant accident but also of other large-scale events that may disrupt medical services, when medical records may be lost.

Local or national authorities may also advise certain people (newborns, infants, children, adolescents, and women who are pregnant) in areas with high I-131 contamination to take potassium iodide (KI) to prevent the accumulation of I-131 in their thyroid. KI should not pose a danger to someone who previously received radiation therapy or chemotherapy. Patients who are actively being treated for cancer and who are advised to take KI should consult with their doctor before taking the medication, so their doctor can evaluate their treatment plan and their health status, including their nutritional status, to determine the safety of KI treatment for them.