National Toxicology Program

• Primary Uses or Exposures: X-rays, gamma rays, and materials and processes that emit X-rays and gamma rays are used in medicine, the nuclear power industry, the military, scientific research, industry, and various consumer products. Neutron radiation is used less than other types of radiation in industry, medicine, and research. The most important sources of X-radiation and gamma radiation include natural sources, medical uses, atmospheric nuclear weapons tests, nuclear accidents, and nuclear power generation. Sources of neutron radiation include the interaction of high-energy cosmic rays with the earth's atmosphere, nuclear fusion or fission of atomic nuclei in nuclear reactors or atomic explosions, and the collision of charged particles with a lithium or beryllium target. Of the estimated worldwide exposure to X-radiation and gamma radiation, about 43% is from natural sources, about 55% is from medical diagnosis and treatment with all other sources contributing less than 2%. The general population is exposed to neutrons primarily from cosmic radiation originating from outer space. The RoC listing for X-radiation and gamma radiation is based on sufficient evidence of carcinogenicity in humans and the RoC listing for neutrons is based on studies of their mechanisms of carcinogenesis. X-radiation and gamma radiation are most strongly associated with leukemia and cancer of the thyroid, breast, and lung. The risk of developing these cancers, however, depends to some extent on age at exposure with childhood exposure mainly responsible for increased leukemia and thyroid-cancer risks, reproductive-age exposure for increased breast cancer risk and exposure later in life to lung cancer risk. Associations between radiation exposure and cancer of the salivary glands, stomach, colon, bladder, ovary, central nervous system, and skin also have been reported. Neutrons cause genetic damage similar to that caused by X-radiation and gamma radiation, and they also produce gamma radiation when they interact with biological materials and thus would cause the same cancers as X-radiation and gamma radiation.
• Nominated by: NIEHS
• Basis for Nomination: IARC¹ classification of as carcinogenic to humans based on sufficient evidence in humans.
• Current Status: Listed in the RoC as known to be a human carcinogen since the 11th RoC (2004): See Ionizing Radiation

¹ International Agency For Research On Cancer (IARC)