The airways in the lungs are lined with a thin, watery mucus blanket. The mucus sits upon tiny, hair-like strands called “cilia.” The cilia and mucus blanket work together to keep the lungs clean and free from infection.
The lungs are very clean—even though they are directly connected to the outside air, which is full of germs and dust. The reason everyone’s lungs are not full of dust and dirt is because of the amazing job the lungs do to keep themselves clean. It happens like this: The mucus blanket catches germs and dust that make it past the nose and into the lungs. The cilia then sweep the mucus blanket, like a conveyor belt, up to the back of the throat where it is swallowed. Healthy lungs usually sweep up about a cup of mucus every day. There are also germ-fighting cells that ride this conveyor belt and attack any invading germs or other foreign particles.
So, this combination of germ-attacking cells that ride the conveyor belt, as well as any germs and dust that get swept to the back of the throat and swallowed, is the reason why people don’t constantly have pneumonia and lungs full of dust.