The sinuses are four pairs of hollow spaces in the bones of the face lined with mucous membranes and connected to the nose. The sinuses allow air to flow freely within the spaces in and out of the nose. Small hairs in the sinuses (cilia) regularly help move mucous out through the nose. When the small openings from the nose to the sinus cavities (sinus ostia) are blocked, or when too much mucous is produced, inflammation can occur within sinus cavities and cause acute sinusitis.

Approximately 31 million people in the U. S. suffer from acute sinusitis each year. Some individuals particularly susceptible to sinusitis have several bouts of sinusitis every year and many have chronic sinusitis.

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