The paranasal sinuses are a collection of air cells that open into the nasal cavity. There are generally four pairs of sinuses including the frontal, ethmoid, maxillary, and sphenoid sinuses. Sinusitis occurs when there is inflammation and/or infection of the paranasal sinuses.Problems with the lining of the sinuses include environmental allergies, inability to adequately fight infection, production of abnormally thick mucus, and the tendency to form polyps.Drew V. Collins, MD is here to discuss sinusitis and how Greenville Health System is here to help you.