Exposure to airborne contaminants is significantly associated with human health risks, ranging from bronchial reactivity to morbidity and mortality due to acute intense or long term low level repeated exposures. However, the precise mechanisms that derive such effects are not always understood. Although inhalation studies are technologically complicated, correct hazard characterisation is essential for comparable risk assessment of inhaled materials. The aim of this study was to investigate the comparative in vitro cytotoxicity of selected gaseous contaminants in human lung cells. The cytotoxicity of nitrogen dioxide (NO(2)), sulphur dioxide (SO(2)) and ammonia (NH(3)) was investigated in A549- human pulmonary type II-like epithelial cell lines cultured on porous membranes in Snapwell inserts. A dynamic direct exposure method was established by utilizing the horizontal diffusion chamber system (Harvard Apparatus Inc, USA) for delivery of test atmospheres. Test atmospheres were generated using a dynamic direct dilution method and the concentration monitored by appropriate analytical methods. A diversified battery of in vitro assays including the MTS (tetrazolium salt; Promega), NRU (neutral red uptake; Sigma) and ATP (adenosine triphosphate; Promega) assays was implemented. Airborne IC(50) (50% inhibitory concentration) values were calculated based on the most sensitive assay for each test gas including NO(2) (IC(50)=11+/-3.54 ppm; NRU)>SO(2) (IC(50)=48+/-2.83 ppm; ATP)> and NH(3) (IC(50)=199+/-1.41 ppm; MTS). However, all in vitro assays revealed similar toxicity ranking for selected gaseous contaminants. Identical toxicity ranking was achieved using both in vitro and published in vivo data. This comparison suggests that results of in vitro methods are comparable to in vivo data and may provide greater sensitivity for respiratory toxicity studies of gaseous contaminants.