Many pollution particles enter the organism via the lung. In the lung, on a surface of 140 m2, the blood is separated from the air by a tissue barrier of only 1/1000 mm. The conducting airways (trachea, bronchi, bronchioli) are a very effective aerodynamic filter for inhaled particles. The mucociliary transport system functions like a self-cleaning mechanism within the filter. Inhaled particles and particles deposited in the lungs play a crucial aetiological and therapeutic role. The discussion in health policy on the relationship between the increase in air pollution and lung damage is of great importance at the present time. Epidemiological studies of recent years have shown very clearly that there is a correlation between morbidity and mortality as a consequence of respiratory and cardiogenic problems and the concentration of PM10 particles in ambient air. So far, however, this correlation has not been explained. The intrathoracic airways are coated by a respiratory epithelium. This has an irregular coating of viscous liquid, consisting of a low viscous sol phase and a high viscous gel phase. It seems, however, that those phases are not clearly distinguishable. The gel phase is moved towards the pharynx by the metachronal ciliary beat transporting the particles out of the lungs. Furthermore, at the air-liquid interface, there exists a continuous surfactant film which reduces the surface tension as is the case in the alveoli. When particles are deposited on the airway wall, that is, on the surfactant film, they are wetted by surface forces and displaced into the liquid phases. Thus, the surfaces of the particles are probably changed by the surfactant or by surfactant components. Many of these particles are transported in the liquid (gel phase) towards the pharynx (mucociliary transport), whereas some of them remain in close association with the epithelium (sol phase). Such particles remain in the airways for days or even weeks. They are either phagocytised by macrophages and carried off via the airways or taken up by dendritic cells and transported into the tissue from where they reach the lymph nodes via lymph drainage and are presented to the T-lymphocytes. The displacement of particles into the liquid phases, caused by the surfactant, can be considered as the initial step in a complex cascade of defence processes in the lungs. The surface of the particles is probably modified by surfactant or surfactant components. These modified particles may be directed to that clearance pathway which is most beneficial for our health, that is, out of the lungs or into the lymphatic glands, where an immune reaction can be triggered. We therefore consider surfactant to be a primary immune barrier.