ABOUT SINUSITIS: Four pairs of air-filled spaces in the head make up the sinus cavity. The sinuses moisten and warm the air we breathe, and filter out particles like dust, spores, and bacteria. Each sinus is lined with tissue or mucous membrane that produces a fluid to keep the sinuses moist and clean, and drains naturally through the nose. Colds or allergies may cause the membrane to become inflamed. The blockage can lead to bacterial growth and sinus infection. Symptoms include headaches, nasal congestion, post-nasal drip and facial pressure. In more severe cases, the inflammation can cause polyps to form, which must be surgically removed. Over 30 million people, or 14 percent of the U.S. population, suffer from chronic sinusitis; nearly a quarter of those develop polyps.