<P> The results of experimental studies suggest that the natural ventilation - rate of a sinus with a single sinus ostium (opening), is extremely slow . Such limited ventilation may be protective for the sinus, as it would help prevent drying of its mucosal surface and maintain a near - sterile environment with high carbon dioxide concentrations and minimal pathogen access . Thus composition of gas content in the maxillary sinus is similar to venous blood, with high carbon dioxide and lower oxygen levels compared to breathing air . </P> <P> The paranasal sinuses are joined to the nasal cavity via small orifices called ostia . These become blocked easily by allergic inflammation, or by swelling in the nasal lining that occurs with a cold . If this happens, normal drainage of mucus within the sinuses is disrupted, and sinusitis may occur . Because the maxillary posterior teeth are close to the maxillary sinus, this can also cause clinical problems if any disease processes are present, such as an infection in any of these teeth . These clinical problems can include secondary sinusitis, the inflammation of the sinuses from another source such as an infection of the adjacent teeth . </P> <P> These conditions may be treated with drugs such as decongestants, which cause vasoconstriction in the sinuses; reducing inflammation; by traditional techniques of nasal irrigation; or by corticosteroid . </P> <P> Malignancies of the paranasal sinuses comprise approximately 0.2% of all malignancies . About 80% of these malignancies arise in the maxillary sinus . Men are much more often affected than women . They most often occur in the age group between 40 and 70 years . Carcinomas are more frequent than sarcomas . Metastases are rare . Tumours of the sphenoid and frontal sinuses are extremely rare . </P>

Where are the paranasal sinuses found and what are their functions