Poison by subcutaneous and intraperitoneal routes. Molybdenum and its compounds are highly toxic based upon animal experiments. Symptoms of acute poisoning include severe gastrointestinal irritation with diarrhea, coma, and deaths from heart failure. Experimental animals exposed to high levels (57 mg Mo/m³) of molybdenum dust for 120 days (4 hours/day) accumulated Mo in the lungs, spleen, and heart, and showed a decrease of DNA and RNA in the liver, kidneys, and spleen. Workers exposed to Mo or MoO₃ (concentrations of 1–19 mg Mo/m³) over a period of 3–7 years have suffered from pneumoconiosis. Inhalation of molybdenum dust from alloys or carbides can cause “hard-metal lung disease.” It is suggested that suitable precautions should be taken against human inhalation of significant amounts of the more soluble molybdenum compounds. MoO₃ and Na₂MoO₄ are soluble. CaMoO₄, MoO, and MoS₂ are insoluble. Hexavalent molybdenum compounds are readily absorbed through the gastrointestinal tract. Coal-fired electrical power plants can be significant sources of molybdenum. Application of some fertilizers may raise molybdenum concentrations in ground water. Molybdenum is rapidly excreted by the body. Molybdenum is an important trace element in the normal growth and development of plants. It is found also in animal tissue, although its precise function is unknown. It is a common air contaminant. See also specific compounds.