The average person who is moderately active during the daytime breathes about 20,000 liters (more than 5,000 gallons) of air every 24 hours. Inevitably, this air (which would weigh more than 20 kilograms [44 pounds]) contains potentially harmful particles and gases.
Particles, such as dust and soot, mold, fungi, bacteria, and viruses deposit on airway and alveolar surfaces. Fortunately, the respiratory system has defense mechanisms to clean and protect itself. Only extremely small particles, less than 3 to 5 microns (0.000118 to 0.000196 inches) in diameter, penetrate to the deep lung.
One of the respiratory system's defense mechanisms involves tiny, muscular, hair-like 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 upwards about 0.5 to 1 centimeter per minute (0.197 to 0.4 inch per minute). Particles and pathogens that are trapped on this mucus layer are coughed out or moved to the mouth and swallowed.
Corina Marinescu
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