Thorium (Th) - radioactive chemical element; serial number 90 in the periodic system of elements of D. I. Mendeleyev. The most important natural isotopes of thorium with the mass number 232. Th232 - alpha emitter (see alpha-radiation) with a half-life is 1.4·1010 years; used in obtaining nuclear energy. The thorium ores mining, processing them for production of the metallic thorium and its use in the industry are connected with occupational hazards, as with thorium are present in greater or lesser quantities of the products of its decay. Maximum permissible concentration (MPC) Th232 in the air of working premises 2·10-15 Curie/L. Toxicity thorium is relatively small, but when you hit it inside the body may develop long-term malignant neoplasms.