Sinus infection may be initiated caused by anything that interferes with airflow into the sinuses and the drainage of mucus out of the sinuses. The sinus openings (ostea) may be blocked by swelling of the tissue lining and adjacent nasal passage tissue, for example with common colds, allergies, and tissue irritants such as OTC nasal sprays, cocaine, and cigarette smoke. Sinuses can also become blocked by tumors or growths that are near the sinus openings.
The drainage of mucous from the sinuses can also be impaired by thickening of the mucous secretions, by decrease in hydration (water content) of the mucous brought on by disease (cystic fibrosis), drying medications (antihistamines), and lack of sufficient humidity in the air. The epithelial cells have small hairlike fibers, called cilia, which move back and forth to help the mucus move out of the sinuses. These small cilia may be damaged by many irritants, especially smoke. This can prevent them from assisting the mucus in draining from the sinuses.
Stagnated mucus provides an environment for bacteria, viruses and in some circumstances (for example, AIDS or immunodepressed persons) fungus to grow within the sinus cavities. In addition, the microbes themselves can initiate and exacerbate sinus blockage. The most commonly infected sinuses are the maxillary and ethmoid sinuses.
http://www.medicinenet.com/sinusitis/page2.htm
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