Ionizing radiation is a type of energy that is released by atoms in the form of electromagnetic waves (gamma or X-rays) or particles (neutrons, beta or alpha) . It has enough energy to remove tightly bound electrons from the orbit of an atom, causing that atom to become charged or ionized. This ability to remove electrons from atoms and molecules is what makes ionizing radiation potentially harmful to living organisms, as it can alter molecules within the cells of our body and cause eventual harm, such as cancer.

Examples of ionizing radiation include x-rays, gamma rays, alpha, beta, and neutron particles. X-rays are a familiar example of ionizing radiation, as they can penetrate our body and reveal pictures of our bones. Other sources of ionizing radiation include nuclear power generation, medical devices such as x-ray machines and CT scanners, and radioactive decay.

Exposure to ionizing radiation can occur through internal or external pathways. Internal exposure occurs when a radionuclide is inhaled, ingested, or otherwise enters into the bloodstream. External exposure occurs when a person is near a source of ionizing radiation, such as a nuclear power plant or medical device.

Ionizing radiation is used in a wide variety of fields such as medicine, nuclear power, research, and industrial manufacturing, but presents a health hazard if proper measures against excessive exposure are not taken. Exposure to ionizing radiation causes cell damage to living tissue and organ damage. In high acute doses, it will result in radiation burns and radiation sickness, and lower level doses over a protracted time can cause cancer.