Improper service of air filters lets dirt in. Not cleaning excess dirt out of housing before removing element can let dirt piled on element fall into inlet.
Continous blowing with air opens pores in filter media that you can't see with your eye and let's fine dirt through.
Dirt can get into the opposite side of filter by sliding into housing that has not been cleaned and be pulled in the engine.
We had a problem with a very large concrete floor resurfacer that had a 4cyl--2300cc industrial engine that started burning oil and lost power. Called the place that sold it. and guy came out, did some checking, pulled the head and the cylinders were trashed, the intake had concrete dust in it. Looked at the air intake hoses and piping and it was OK. Called Donaldson, who made the housing, explained how operators were servicing air filters, the answer was improper and "overservicing" of the air filter. Use restriction gauge, only replace filter when gauge said to, leave it alone no matter how dirty it looked and never blow out filters. Never had any more problems.
Had I never been involved, would have never believed it.

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