Introduction: Silica nanoparticles have received more attraction in medical and industrial applications due to their unique properties such as small size, the possibility of surface functionalization, ease of production, and low cost. So, it is necessary to study the respiratory toxicity of occupational exposure due to the production and increasing use of silica nanoparticles, especially in the size of 20-40 nm. 
Materials and Methods: In vitro, cytotoxicity of silica nanoparticles (SiO2 NPs) in 0.6 -3 mg/ml concentrations were evaluated on human lung epithelial (A549) and human fetal lung fibroblast (MRC5) cells by MTT assay and scanning electron microscopy (SEM) images AFTER 72 hours.
Results: The MTT assay showed that the cell viability was reduced by increasing the concentration of SiO2 NPs from 0.6 to 3 mg/ml in both types of cells. In 1.6 mg/ml concentration, the half-maximal inhibitory concentration (IC50) of SiO2 NPs on A549 cancer cells, the chance of survival is as such: %46 for A549 cancer and %70 for MRC5 normal cells. The SEM images of normal lung cells (MRC5), which have been exposed to Sio2-NP, showed that the morphology of MRCS cells has transformed from a long shape to a circular one.
Conclusion: The cytotoxicity of SiO2 NP depends on the concentration of SiO2 NP and the type of cells. By increasing SiO2 NP concentration, the cell viability decreases for both types of cells. Moreover, the cytotoxicity of SiO2 NP against cancer cells (A549) is higher than normal lung cells (MRC5).

Keywords: SiO2 NP, A549 Cells, MRC5 Cells

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