If they allow unrestricted airflow, they'll be guilty of allowing too much dirt to pass. You can see the problem. Not at all. They do a fairly good job for most airborne debris, but they have their limitations. Remember, they don't call me Tiny for nothing.
Most air filters have a paper media, though there are also cotton gauze and foam filters.
I've heard there's an oiled foam filter that works sort of like flypaper. Fortunately, my encounter was with a more porous paper filter and, being small as I am, I slipped on through. When I last measured myself I was somewhere around fifteen microns. About a sixth the width of a human hair. Gasoline that is squirted into the combustion chamber mixes with the air and airborne particles, including sand, salt and other minerals, which is then compressed by the piston and ignited.