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The lungs are protected by a series of defense mechanisms in different regions of the respiratory tract.

When a person breathes in, particles suspended in the air enter the nose, but not all of them reach the lungs. The nose is an efficient filter. Most large particles are stopped in it, until they are removed mechanically by blowing the nose or sneezing.

Some of the smaller particles succeed in passing through the nose to reach the windpipe and the dividing air tubes that lead to the lungs 

These tubes are called bronchi and bronchioles. All of these airways are lined by cells. The mucus they produce catches most of the dust particles. Tiny hairs called cilia, covering the walls of the air tubes, move the mucus upward and out into the throat, where it is either coughed up and spat out, or swallowed.