The inside of the nose and air passages are coated with a lining of cells that produce mucus. This mucus helps to trap dust and pollen particles before they reach the lungs. The cells that produce mucus are surrounded by thousands of small hairs called cilia. These cilia pulse along at 10 to 20 beats every second. Cilia force a constant flow of mucus, with trapped particles of dust and pollen, to the back of the throat, where they are either swallowed or coughed out of the body.