MoO3, MoS2, and molybdates
Toxicity studies of molybdenum trioxide, ammonium and calcium molybdates and molybdenum disulfide were carried out with rats and guinea pigs by the U.S. Public Health Service [Fairhall, 1945] Molybdenum trioxide and the molybdates when fed in large daily doses of from 1200 to 6000 mg Mo/kg body weight invariably proved fatal. Fatalities were few at doses of 120 to 600 mg. There were no fatalities with molybdenum disulfide. The highest dose corresponds to 420g for a 70 kg human being. In inhalation experiments (5 mg Mo/cu. ft. air) molybdenum trioxide and ammonium dimolybdate were injurious but molybdenum disulfide was much less so. Fumes of molybdenum trioxide produced by arcing across molybdenum electrodes were less toxic than molybdenum trioxide powder apparently because the rate of solution of the fume is greater on account of its small particle size (1.55 um) than that of the powder (15.6 um). So the fume is rapidly dissolved in the lungs and carried away from the site of deposition. Molybdenum trioxide and the molybdates also produced toxic effects when administered orally and by intraperitoneal injection in large doses (400 to 800 mg/kg).
Fairhall, L. T., Dunn, R. C., Sharpless, N. E. and Pritchard, E. A., The Toxicity of Molybdenum, U. S. Public Health Service, Public Health Bulletin, 1945, 293.
Source: International Molybdenum Association