Normally, phlegm or mucus is watery. Mucus is expelled out of the lungs through the sweeping action of cilia, billions of microscopic “finger-like” projections on the lining cells of the air tubes. Cilia trap and eliminate pollutants and push mucus out of the lungs by moving it upward in one direction to the windpipe, the throat and mouth. The mucus is then swallowed and sterilized by the acid in the stomach, or coughed-out through the mouth.

When the bronchi is irritated by exposure to too much pollutants, viruses or bacteria, in the air, the bronchi swells and increases its secretion of mucus. Eventually, the mucus becomes thicker and stickier. Ciliary functions are impaired and the air passages become clogged by debris that cause even more irritation. Secondary bacterial infection sets in. An excessive amount of thick sticky mucus develops, which the person tries to expel. This causes the characteristic cough of bronchitis.