When you breathe, air travels down your nasal cavity, into the trachea, and into your bronchial tubes. These tubes split up into many other tubes that each lead to sacs called alveoli.
The body has been designed to defend your lungs from elements in the air that might damage them. Hairs in the nasal cavity filter out the dust and other debris in the air that may come into your lungs. Next, special tissues kill off the bacteria threatening to enter your body through the nose. The bronchial tubes are lined with mucus that catch the smallest dust particles coming towards the lungs. Finally, any particles that get through this security system are terminated by white blood cells.