The air we breath contains lots of little particles of matter such as pollen, and dust. Particles of matter that are smaller than 10 microns in diameter are small enough to get into the lungs and cause damage.
Particles larger than 10 microns in diameter are filtered out of the air by the nasal cavity and the ciliated cells lining the bronchi. These large particles are either removed through coughing or are carried up the bronchi by the cilia and swallowed. Particles less that 10 microns in diameter can get all the way down into the alveoli in the lungs. They can cause tissue damage as they hit the lung tissue. These tiny particles can also damage the lung by causing irritation which can lead to scarring of the lung tissue.