The aim of this study was to establish a dynamic in vitro model for direct exposure of human cells to gaseous contaminants to investigate the cellular responses to airborne chemical exposures. Nitrogen dioxide (NO2) was selected as a model gas compound. Standard test atmospheres were generated (2.5-10 ppm), using a dynamic direct dilution method. Human cells including: A549 pulmonary type II-like epithelial cell lines and skin fibroblasts were grown on porous membranes. Human cells on snapwell inserts were placed in horizontal diffusion chambers and exposed to various airborne concentrations of NO2 directly at the air/liquid interface for 1 h at 37 degrees C. Cytotoxicity of the test gas was investigated using the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium), NRU (neutral red uptake) and ATP (Adenosine triphosphate) assays. Dose-dependent effects of NO2 were observed in human cells tested which resulted in a significant reduction of cell viability at concentrations normally encountered in workplace environments (p<0.05). Our findings suggest that the dynamic direct exposure method can be used for in vitro inhalational and dermal toxicity studies and potentially as an advanced technology for biomonitoring of airborne contaminants in future occupational and environmental toxicity assessments.