Relevant models mimicking different areas of the lung in vitro are commercially available and represent the whole respiratory system from the nasal cavity to the alveolar barrier. These commercial models can be based on primary cells or cell lines with the inherent advantages and limitations of the two different approaches.

In the past most in vitro models including those representing the pulmonary system were usually based on a single cell-type and in case for the pulmonary system even cultured under submerged conditions, which is not the physiological situation for lung cells. In recent years complex models using more than one cell type in a 3D orientation were developed and have become widely applied.

Endpoints studied in these various models can range from carcinogenesis, irritation to inflammation, and more recently also respiratory sensitization. The models can be incubated in static, pressurized or cyclic systems. Overall, models should be as complex as necessary to mimic the physiological responses studied and as simple as possible to reach that goal while allowing dosimetry and measurement of relevant endpoints.

Existing in vitro models will be presented, and a special focus will be on the state-of-the-art to identify the hazard of respiratory sensitization.