There are two broad classes of radiation : ionizing radiation which comes from radioactive materials and x-ray machines and non-ionizing radiation ( usually electromagnetic radiation ) which comes from other sources . There are two distinct types of radiation ; ionizing and non-ionizing . The word radiation is commonly used in reference to ionizing radiation only ( i.e. , having sufficient energy to ionize an atom ) , but it may also refer to non-ionizing radiation ( e.g. , radio waves or visible light ) .