Respiratory mucus is found to be a barrier to the penetration of H⁺ ions into the surrounding tissues. These ions are captured and bound in the mucus, e.g. as H-bond cross-linkages between the molecules. They increase its viscosity, which has a bearing on the mucociliary transport, airway resistance and gas exchange. The ability to bind the H⁺ ions is dependent on the buffer capacity and pH value of the mucus. Persons with acid-saturated mucus (some asthmatics and bronchitics) constitute a risk group, for whom effects can be more easily triggered from the surrounding tissues than in persons with mucus of higher pH. The latter primarily react only by changes of the mucus viscosity, which has a minimum near pH = 7.4. This, together with the mucus buffer capacity, appears to constitute the basis of the health effects reported from experimental studies. A simple test for evaluating the expected sensitivity of persons to acidic pollution is described.