Natural defense against large particles

Nose hairs traps large particles
Mucus in nose, mouth and throat helps remove
large particles

Smaller Particles

Smaller particles travel deeper into respiratory
system. Some gets exhaled.
Accumulates into alveolar sacs over the period of
exposure causing fouling of exchange surface.
Breathing becomes increasingly difficult.

Ultrafine particles (<0.1μm)

Some may actually pass with air through the alveoli
and are incorporated into the bloodstream.
Once believed that they are very small and lack
sufficient impact strength to damage the alveolar sacs
and are simply exhaled.