The sinuses and nasal passages are air-filled and contain light mucus, which is produced by the body to trap inhaled dirt and foreign matter. The contaminated mucus is pushed out through tiny openings that serve as drainage passages to the back of the nose and throat. The mucus is then swallowed.
Anything that causes the mucous membranes lining the sinuses to swell may lead to infection by obstructing the body's method of draining foreign matter out of the body.
For example, one common cause of sinusitis is an upper respiratory tract infection such as the common cold. The sinusitis may be due to the cold itself, or may be a secondary bacterial infection that develops as a consequence of the stagnant mucus. Narrow drainage passages from the sinuses, a deviated septum and polyps or tumours can also obstruct drainage from the sinuses.
Nasal congestion due to other factors (e.g. allergy) also increases susceptibility to bacterial sinus infection.
Irritation of the sinuses also causes an increased production of mucus. For example, some people experience sinusitis as a consequence of exposure to cigarette smoke, dust mites or other inhaled irritants.
Other factors that may contribute to sinusitis include hay fever, food allergy, and tooth infection.