What is ionizing radiation?
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 several ways, including
- from the spontaneous decay (breakdown) of unstable isotopes. Unstable isotopes, which are also called radioactive isotopes, give off (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.
What are the health hazards of exposure to ionizing radiation?
At high doses, ionizing radiation can cause immediate damage to a person’s body, including, at very high doses, radiation sickness and death. At lower doses, ionizing radiation can cause health effects such as cardiovascular disease and cataracts, as well as cancer. It causes cancer primarily because it damages DNA, which can lead to cancer-causing gene mutations.
Children and adolescents can be more sensitive to the cancer-causing effects of ionizing radiation than adults because their bodies are still growing and developing. Also, children and adolescents usually have more years of life following radiation exposure during which cancer may develop.
More information about the health effects of ionizing radiation exposure is available from the Centers for Disease Control and Prevention (CDC)and the Environmental Protection Agency.
How are people exposed to ionizing radiation after a nuclear power plant accident?
Nuclear power plants use energy released by the decay of certain radioactive isotopesto 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
- the specific types and quantities of radioactive materials, or isotopes, released
- how much radiation someone is exposed to and for how long
- how a person comes in contact with the released radioactive materials (such as through contaminated food, water, air, or on the skin)
- the person’s age (with those exposed at younger ages generally at higher risk of cancer)
The radioactive isotopes released in nuclear power plant accidents include iodine-131 (I-131), cesium-134 (Cs-134), 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 for many years, especially for children and adolescents.
Exposure to Cs-134 and 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-134 and Cs-137, such as dust originating from contaminated soil, or ingesting contaminated water or foods. Because Cs-134 and Cs-137 do not become concentrated in a particular tissue, the ionizing radiation that it releases can expose all tissues and organs of the body.
What have researchers learned about cancer risks from nuclear power plant accidents?
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 Ukraine (Chornobyl in Ukrainian) (1, 2). The radioactive isotopesreleased during the Chernobyl accident included I-131, Cs-134, Cs-137, and Sr-90.
Power plant workers on-site at the time of the accident. 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.)
Cleanup workers. Hundreds of thousands of people who worked as part of the cleanup crews in the years after the accident were exposed to average external doses of ionizing radiation that ranged from approximately 0.14 Gy in 1986 to 0.04 Gy in 1989. Studies conducted in this group of people have found an increased risk of leukemia (3–5).
Residents near Chernobyl. From 1986 through 2005, approximately 5 million residents of the contaminated areas surrounding Chernobyl received an accumulated whole-body average dose of around 0.01 Gy (6). Studies that have followed children and adolescents exposed to I-131 from the Chernobyl accident showed an increased risk of developing thyroid cancer (7–9).
Recent studies have used genomic analysisof people affected by the Chernobyl accident to better understand how radiation exposure leads to cancer. In a 2021 study, investigators found that thyroid tumors in children who were exposed to fallout from the Chernobyl accident had higher levels of a particular kind of DNA damage that involves breaks in both DNA strands than tumors in unexposed individuals born more than 9 months after the accident (10). The more radiation the children had been exposed to, the more of this type of DNA damage was seen. This association was stronger the younger the children were at the time of exposure.
Another way in which radiation exposure could lead to cancer is through transgenerational effects, in which people exposed to ionizing radiation develop new genetic changes in their gametes (sperm or eggs) that are passed on to their future offspring, increasing cancer risk in those offspring. Transgenerational effects have been observed in some animal studies. However, genomic analysis of children born to people exposed to radiation at Chernobyl indicates that this exposure did not lead to an increase in new genetic changes in the children of exposed parents (11).
How long after exposure to I-131 is the risk of thyroid cancer increased?
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 NCI researchers followed more than 12,500 people who were younger than age 18 at the time they were exposed to a range of doses of I-131 (0.65 Gy on average) from the Chernobyl accident (7). A total of 65 new cases of thyroid cancer were found in this population between 1998 and 2007. 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 30 years (9).
What can people do to protect themselves from health risks associated with exposure to contamination from a nuclear power plant accident?
Information on this topic is available from the CDC and other federal agencies.
What should cancer patients do if they live in an area that may be contaminated due to a nuclear power plant accident?
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 after 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.
What research is NCI currently supporting on ionizing radiation and cancer risk?
Researchers at NCI and elsewhere continue to learn about the cancer risks from ionizing radiation by studying various groups of people, including those who were exposed as a result of the Chernobyl accident, survivors of the atomic bomb explosions in Japan during World War II, and people who were exposed to radiation during medical diagnostic procedures or as part of their job.
- NCI conducts much of this research through the Radiation Epidemiology Branch of the Division of Cancer Epidemiology and Genetics (DCEG).
- DCEG researchers are carrying out a long-term study of Chernobyl survivors.
- Through DCEG and the Division of Cancer Biology, NCI supports theChernobyl Tissue Bank, which contains samples from the Chernobyl survivors. These are being used to investigate the effects of radioactive exposure from nuclear power plant accidents.
- NCI collaborates with researchers from Japan’s Radiation Effects Research Foundation to learn about the health effects from the 1945 atomic bomb exposures in that country. This ongoing project is called the Life Span Study.
- NCI works closely with the National Institute of Allergy and Infectious Diseases to support the federal government’s Radiation and Nuclear Countermeasures Program.
- Health professionals can also find information about the medical management of exposed persons during radiation emergencies at the USDepartment of Health and Human Services'sRadiation Emergency Medical Management website.
