Lung Tumor Induction by Inhalation Exposure to Molybdenum Trioxide in Rats and Mice

Inhalation studies of molybdenum trioxide (MoO₃) were conductedbecause of its wide use in industry, human exposure, and lackof data on carcinogenicity. Groups of 50 male and 50 femaleF344/N rats and B6C3F1 mice were exposed to MoO₃ by inhalationat 0, 10, 30, or 100 mg/m₃, 6 h/day, 5 days/week, for 2 years.In both rats and mice, survival and mean body weights of exposedgroups of males and females were similar to those of their respectivecontrols. There were significant exposure-dependent increasesin blood molybdenum concentration in exposed rats and mice.There were no toxicological differences in bone density or curvaturebetween exposed animals and their respective controls. In rats,dose-dependent increases in incidence of hyaline degenerationin the nasal olfactory epithelium and squamous metaplasia ofthe epithelium lining the base of the epiglottis were observed.The incidence of alveolar/bronchiolar adenoma or carcinoma (combined)was marginally increased in males but not in females comparedwith controls. In mice, the incidences of squamous metaplasiaof the epithelium lining the base of the epiglottis, hyperplasiaof the laryngeal epithelium, and metaplasia of the alveolarepithelium were significantly increased in all exposed malesand females compared with controls. The incidence of alveolar/bronchiolaradenoma or carcinoma (combined) in exposed groups of males andfemales was significantly greater than that in the control groups.