arsenic (As) /är″sənik/ [Gk, arsen, strong] , an element that occurs throughout the earth’s crust in metal arsenides, arsenious sulfides, and arsenious oxides. Its atomic number is 33; its atomic mass is 74.92. This element has been used for centuries as a therapeutic agent and as a poison and continues to have limited use in some trypanocidal drugs. The introduction of nonarsenic trypanocides with less dangerous side effects in the treatment of trypanosomiasis has greatly reduced its use. Exposure to arsenic can occur occupationally in mining, in the pesticide and pharmaceutical industries, and in glass and microelectronics manufacturing, as well as environmentally from both industrial and natural sources. The average concentration in the human adult is about 20 mg, which is stored mainly in the liver, kidney, GI tract, and lungs. The mechanisms for the biotransformation of arsenics in humans are not well understood. Most arsenics are slowly excreted in the urine and feces, which accounts for the toxicity of the element. −arsenic, adj.
