The airflow that passes through the nasal fossae are subject to spontaneous changes during the day: this is called the nasal cycle.

Humidification: The fossae reheat the air inspired and humidify it. 

The humidification of the inspired air is fundamental to ciliary activity throughout the respiratory tree and to alveolar epithelium functions, since gaseous exchange would not be possible if it were not coated by a liquid film which comes from the vaporisation of the water of the mucus covering.

Purification  of inspired air and defence of the nasal mucosa function: The nasal fossae act as a filter that opposes the entrance of inert or microbial inhaled particles and clean the inspired air to protect the pulmonary alveoli from the deposition of these particles in suspension. The filtered particles are deposited on the the nasal mucosal coating. They are then eliminated from the mucosa by means of the mucociliary function, which acts as a mechanical transport agent, pushing  the attached particles towards the rhinopharynx, preventing them from penetrating into the chorion. The bactericidal function of the mucus is made possible by the action of lysozymes, IgA and interferons which are capable of neutralising infectious agents. If the mucociliary function is unable to prevent an infectious element from entering the chorion, there is a second barrier, namely the inflammation mechanisms. With inflammation, the polynuclear cells and macrophages reach the chorion, which will phagocytize and kill the foreign elements, while also mobilising the T-cells.