Your sinuses are air-filled cavities in your face and head, lying behind your eyebrows and your cheekbones and between your eyes. Normally, the sinuses are lined with a mucous membrane that functions to produce and circulate normal mucus into your nose, which helps filter dust and particles as well as moisten the air that you breathe. When these cavities become inflamed or swollen, you have sinusitis. This usually happens because a virus, allergy, or other inflammation creates swollen mucous membranes that block the natural drainage pathway of the sinuses. The most common cause of acute sinusitis is the common cold.