An acronym for Naturally Occurring Radioactive Material. Naturally occurring radioactive materials are common in virtually all rocks, minerals, and soils. They naturally contain small amounts of uranium, thorium, and a radioactive isotope of potassium. Plants and animals are also naturally radioactive; they contain small (but measurable) levels of radioactive potassium as well as radioactive carbon (C-14) and hydrogen (tritium, or H-3) that are formed by cosmic ray interactions in the atmosphere.