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.

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

• 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.

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 isotopes released 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 analysis of 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). 

Information on this topic is available from the CDC and other federal agencies.