The average person who is moderately active during the daytime breathes about 20,000 liters of air every 24 hours. Inevitably, this air (which would weigh more than 20 kilograms) contains potentially harmful particles and gases. Particles, such as dust and soot, mold, fungi, bacteria, and viruses, deposit on airway and alveolar surfaces. Only small particles less than 3 to 5 microns in diameter penetrate to the deep lung. Fortunately, the respiratory system has defense mechanisms to clean and protect itself.

One such defense mechanism involves tiny muscular projections (cilia) on the cells that line the airways. The airways are covered by a liquid layer of mucus that is propelled by the cilia. These tiny muscles beat more than 1,000 times a minute, moving the mucus that lines the trachea about 0.5 to 1 centimeter per minute. Particles and pathogens trapped on this mucus layer are cleared to the mouth and swallowed.

Because of the requirements of gas exchange, alveoli are not protected by mucus and cilia—mucus is too thick and would slow movement of oxygen and carbon dioxide. Instead, the body has another defense system. Mobile cells on the alveolar surface called phagocytes seek out deposited particles, bind to them, ingest them, kill any that are living, and digest them. Phagocytes in alveoli of the lungs are called alveolar macrophages. When the lungs are exposed to serious threats, additional white blood cells in the circulation, especially neutrophils, can be recruited to help ingest and kill pathogens (foreign particles). For example, when the person inhales a great deal of dust or is fighting a respiratory infection, more macrophages are produced and neutrophils are recruited.

Last full review/revision August 2006 by Joseph D. Brain, ScD