Radiation is the emission of a particular type of energy from any source. It is typically associated with radioactive materials or nuclear reactions. Most forms of radiation have not been linked to cancer, but high-frequency radiation such as ionizing radiation or ultraviolet radiation can cause the type of genetic damage that leads to cancer.

Ionizing radiation consists of high-energy waves that penetrate cells and cause ionization, the development of a positive charge in a molecule (group of atoms) that is normally neutral (without a charge). Ionized molecules are unstable and undergo chemical changes that lead to the formation of free radicals, which can damage the molecule or nearby molecules. DNA is a molecule potentially affected by this process, and a mutation in a DNA molecule can contribute to cancer.

Exposure to ionizing radiation comes from natural background radiation (such as the cosmic rays of the solar system or the radioactive gas, radon) non-medical synthetic radiation (such as from nuclear weapons testing and occupational and commercial sources) or medical radiation (such as from diagnostic x-rays or radiation therapy). Radon, an invisible gas released when uranium breaks down, can accumulate indoors at dangerous levels. Houses are now usually tested for radon when they are sold, especially in areas with high levels of uranium in the soil.

Ionizing radiation is one of the few scientifically proven human carcinogens. Its effects are related to dose exposure – the higher the dosage level, the more likely it is to potentially trigger cancer. The thyroid gland and bone marrow are most susceptible to radiation, and the kidneys, bladder and ovaries are the least affected. Leukemias are the most common cancers associated with ionizing radiation. Other cancers associated with this form of radiation include breast cancer and lung cancer.  Although medical treatments with radiation therapy present risks for cancer, the benefits of the therapy usually far outweigh the risks.

The sun is the greatest source of ultraviolet radiation, which causes basal cell and squamous cell cancers, the most common types of skin cancer, and melanoma, the most dangerous form.