How do the lungs clear out dust that settles in them and sticks to inner surface of their walls?
The mucociliary elevator
The mucociliary escalator is actually one of the human body's greatest barriers against infections. It works because of two different cells that line the airways.
Goblet cells : these line the airways, and produce mucus (snot), which is used to trap dust particles/bacteria etc.
Ciliated epithelial cells : which have tiny projections called cilia extending from them like lots of little fingers. The cilia are constantly in motion.
Now try to imagine them working together; thousands of tiny little fingers, all gently moving mucus upwards towards the throat.
When it gets to the throat, we usually clear it by gently coughing. From here, it is usually swallowed. (I know, I know. It's gross. but we all do it).
The last thing anybody needs is large amounts of bacteria creeping into their lungs, which would lead to pneumonia .