The human nose not only provides the main portal through which air passes to and from our respiratory system, it also plays an important role in maintaining airway health. This function takes the form of heating and humidifying inhaled air to prevent airway drying, as well as trapping and disposing of inhaled particles and pathogens within the mucus lining. The ability of the nose to perform these duties appears compromised during the nasal breathing of pressurised air, with many users reporting symptoms associated with nasal airway drying. There are many situations where nasal breathing of pressurised air occurs. The cause of airway drying is currently not known, but is typically relieved through the use of supplementary humidification. This investigation examines the role air-pressure has on nasal airway drying by the development of a nasal air-conditioning model to predict inter-nasal airway hydration throughout the breathing cycle during ambient and pressurised breathing. Furthermore, the effect air pressure has on mucosal water supply and airway geometry is investigated in tissue experiments as well as in-vivo MRI clinical trials. Together the latter two investigations demonstrate that augmentation in air pressure adversely effects water supply and cause an opposing response in airway geometry between airways. Implementation of these previously unknown behaviours into the nasal air-conditioning model provides additional new insight into the actual function of the nose and its pathophysiology during pressurised air breathing. For the first time, a different airway hydration status during ambient air breathing is identified between the airways. During inhalation, the patent airway experiences drying that likely disables mucocilary transport, while the patent airway remains fully hydrated. This work sheds new light as to the purpose of the nasal cycle. Modelling pressurised nasal breathing demonstrates drying occurring within both airways and that supplementary humidification prevents this from occurring by effectively relieving the nose of its air-conditioning function. Modelling also demonstrates how supplementary humidification assists in the removal of entrapped particles and pathogens by providing ideal conditions for mucus transport to occur. An alternative method to relieve nasal airway drying is proposed. This method maintains normal airway hydration conditions during pressurised air breathing and avoids the problems of perceived stuffiness and congestion commonly encountered with supplementary humidification.