The airway surface liquid (ASL) lining the upper respiratory tract has an essential role in heat and moisture exchange, as well as having an important role in airway defense. Continuous positive air pressure (CPAP) users frequently report troublesome symptoms of airway dryness and nasal congestion. Clinical investigations have demonstrated that supplementary humidification reduces these symptoms but the reason for their occurrence remains unexplained. Previously, symptoms of nasal drying have been attributed to unidirectional airflow created by mouth leaks; however these still occur when leaks are absent. Tidal breathing stresses have previously been shown to regulate epithelial cell ionic fluid secretion and reabsorption into the ASL [1]. The purpose of this study was to determine whether augmented air pressures change overall mucosal ASL water supply and, if so, the extent of this effect. It is hypothesized that the low-level positive airway pressures used in CPAP therapy could reduce the ability of respiratory mucosa to humidify inhaled air as a result of reduced ASL supply from the airway mucosa.