The major functions of nasal breathing are:
- to heat and moisten the air
- to remove particulate matter
About 10 000 L of air are inhaled daily. The relatively low flow rates and turbulence of inspired air are ideal for particle deposition, and few particles greater than 10 microns pass through the nose. Deposited particles are removed from the nasal mucosa within 15 minutes, compared with 60-120 days from the alveolus. Nasal secretion contains many protective proteins in the form of IgA antibodies, lysozyme and interferon. In addition, the cilia of the nasal epithelium move the mucous gel layer rapidly back to the oropharynx where it is swallowed. Bacteria have little chance of settling in the nose. Mucociliary protection against viral infections is more difficult because viruses bind to receptors on epithelial cells. The majority of rhinoviruses bind to an adhesion molecule, intercellular adhesion molecule 1 (ICAM-1), shared by neutrophils and eosinophils. Many noxious gases, such as SO2, are almost completely removed by nasal breathing.