|If you live in a city, chances are you inhale 17,000 pints of air that contain about 20 billion foreign particles, including dirt, dust and chemicals.
Most larger particles get trapped in your stiff nostril hairs, according to The Incredible Machine. Smaller particles can make it deep into your throat and bronchial tubes, where they are caught in the lining of sticky mucus and are forced up by an escalator of upward-sweeping hairs called cilia.
Particularly offensive particles are coughed or sneezed out. Most, however, are raised just high enough to be swallowed and ultimately removed with other waste through the digestive tract.
So, now you know why you should cover your nose and mouth when you sneeze or cough.