Exposure to ionizing radiation, a type of energy released by atoms in the form of electromagnetic waves, such X-ray and gamma waves, or particles, such as electrons or protons, is a regular part of life on earth. Natural sources of ionizing radiation include radioactive material in soil, water and vegetation, and human-made sources include X-rays and medical devices. Another source is cosmic radiation, which comes from the sun (solar) and outer space (galactic) and, according to the International Commission on Radiological Protection (ICRP), comprises about one-sixth of our exposure to natural radiation.