[danielheath]

Your body constantly sheds skin flakes (dust).

Tiny mites find those flakes, cover them in enzymes that break down skin, then wait for them to break down enough to eat.

When a puff of air lifts the dust into your lungs, those digestive enzymes attack your lungs.

Your body produces extra mucus in response, to trap dust on the way in, and inflames the tissue surrounding any damage to prevent infection taking hold. Neither response is pleasant.

[EricE]

And if you think that's bad, just do an air sample after running a vacuum. Micron filters or not. It's one of the many reasons I love my central vacuum system - it exhausts outside my home :)