Ionizing radiation is a form of energy emitted by atoms and propagating in the form of electromagnetic waves (gamma rays or X-rays) or particles (neutrons, beta or alpha rays). The spontaneous decay of atoms is called radioactivity, and the excess energy released is a form of ionizing radiation. Half-life is the time it takes for the radioactivity of a radionuclide to decay to half its original value. This can span fractions of a second to millions of years. Human exposure to radiation also comes from man-made sources, from nuclear power generation to the medical use of radiation for diagnosis and treatment. Medical devices, including X-ray machines, are the most common man-made sources of ionizing radiation today. External radiation stops when the source is blocked or when the person leaves the radiation field. People can be exposed to ionizing radiation in a variety of settings, including at home and in public places (public exposure), at work (occupational exposure), and in medical settings (patients, caregivers, volunteers, etc.). Exposure to ionizing radiation can be divided into three exposure situations. The second type of situation, pre-existing exposure, is when exposure already exists and decisions need to be made to control it. For example, exposure to radon at home or at work, or exposure to natural background radiation from the environment. The final type, emergency exposure situations, result from unforeseen events that require immediate response, such as nuclear accidents or malicious acts. Effective dose is used to measure the potential for harm from ionizing radiation. This is a method of measuring the potential for harm from ionizing radiation. Prenatal exposure to ionizing radiation can cause brain damage in the fetus after acute doses >100 mSv between 8 and 15 weeks of gestation and >200 mSv between 16 and 25 weeks of gestation. Human studies have shown no radiation risk to fetal brain development before the 8th week of gestation or after the 25th week of pregnancy. Epidemiological studies show no cancer risk after fetal radiation exposure has been shown to be similar to the risk after early childhood exposure. WHO has established a radiation program to protect patients, workers and the public from the health risks of radiation exposure in planned, existing and emergency exposure situations? The program focuses on radiation protection aspects in the field of public health and includes activities related to radiation risk assessment, management and communication. In line with its core functions of setting norms and standards and promoting and monitoring their implementation, WHO is working with seven other international organizations to revise and update the International Basic Safety Standards (BSS) on radiation. Radiation is energy that travels through space or matter at very high speeds. This energy can take the form of particles, such as alpha and beta particles emitted by radioactive materials, or waves, such as light, heat, radio waves, microwaves, X-rays, and gamma rays. Radioactive substances, also called radionuclides or radioisotopes, are unstable atoms. In nature, unstable atoms tend to transform into stable forms. When they change shape they emit radiation. Radiation that can produce ions when interacting with matter is called ionizing radiation. An ion is a charged particle that is created when an electron is removed from a position within an atom. Alpha particles, beta particles, X-rays, and gamma rays are types of ionizing radiation. On the other hand, radiation that cannot produce ions in matter is called non-ionizing radiation. Radio waves, microwaves, heat waves, visible light, and ultraviolet light are types of non-ionizing radiation.