The study of the action of ionizing and nonionizing radiation on biological systems. Ionizing radiation includes highly energetic electromagnetic radiation (x-rays, gamma rays, or cosmic rays) and particulate radiation (alpha particles, beta particles, neutrons, or heavy charged ions). Nonionizing radiation includes ultraviolet radiation, microwaves, and extralow-frequency (ELF) electromagnetic radiation. These two types of radiation have different modes of action on biological material: ionizing radiation is sufficiently energetic to cause ionizations, whereas nonionizing radiation causes molecular excitations. In both cases, the result is that chemical bonds of molecules may be altered, causing mutations, cell death, or other biological changes.