Paranasal sinuses are air cavities in the bones of the skull which connect to the nasal cavity.
These air cavities or channels are lined with mucous membranes. When the mucous membranes become swollen, the openings from the sinus into the nasal passages become obstructed. Mucous accumulates in the blocked-off sinus causing pressure, postnasal drip, headache and difficult breathing through the nose. However, there are many other causes of sinusitis like dental infections, infectious diseases, air pollution, temperature extremes and underwater swimming. Sinusitis can lead to serious complications if left untreated.