The cytotoxicity of MoO3 -NPs was assessed on the alveolar carcinoma epithelial cells (A549) and normal bronchial epithelium cells (BEAS-2B) after short and long-term time of exposure. Cytotoxicity studies included the effect of MoO3 -NPs on cell viability, cell membrane integrity (NRU assay), mitochondrial metabolic activity (MTT assay) and the ability of the cells to proliferation (Clonogenic assay).