Thorium (Th) is a radioactive chemical element; serial number 90 in the periodic table of elements of DI Mendeleev . The most significant is the natural isotope of thorium with a mass number of 232. Th 232 is an alpha emitter (see Alpha radiation) with a half-life of 1.4 · 10 10 years; used in obtaining atomic energy. Mining of thorium ores, their processing to produce metallic thorium and its use in industry are associated with occupational hazards , since along with thorium its decay products are always present in larger or smaller quantities. The maximum permissible concentration ( MPC ) of Th 232 in the air of working premises is 2 · 10 -15 curie / l. The toxicity of thorium is relatively low, but if it is ingested inside the body, malignant neoplasms may develop in the long term.