structure particularly their height, and the quality of the mucus produced that needs to be maintained at a correct humidity, temperature, and acidity. The cilia need to be able to move freely in the periciliary liquid layer and when this is impaired through damage to the cilia or by imbalances in the moisture or pH of the PCL, the mucus is unable to be cleared properly from the airways. Cystic fibrosis is a consequence of imbalances in the PCL. Accumulated mucus, apart from causing varying degrees of airflow obstruction, makes a breeding ground for bacteria that cause many respiratory infections that can seriously worsen existing lung disorders. Obstructive lung diseases often result from impaired mucociliary clearance that can be associated with mucus hypersecretion and these are sometimes referred to as mucoobstructive lung diseases. Studies have shown that the dehydration of airway surface liquid is enough to produce mucus obstruction even when there is no evidence of mucus hypersecretion. Humidity High humidity enhances mucociliary clearance. One study in dogs found that mucus transport was lower at an absolute humidity of 9 g water/m3 than at 30 g water/m3. Two methods of supporting this, particularly in mechanical ventilation, are provided by active and passive respiratory gas humidifiers. See also Airway clearance therapy References External links Genetic disorders of mucociliary clearance consortium Respiratory system Histology