Sinuses are air-filled spaces in the skull behind the face. They are kept moist and clean by a lining of mucosa. Things such as pollen, smoke, and chemical fumes can bother the mucosa. Constant exposure to irritants can cause ongoing swelling of this lining. It can also harm tiny hair-like cilia that cover the mucosa. Cilia help carry mucus toward the opening of the sinus. Damage to cilia keeps mucus from draining from the sinuses.