Used air filters
Air filters are generally very fragile and can be easily damaged by careless handling, air pressure, vibrations or heat. Here we show a few filters after their useful life. Any dirt or dust that passes will enter the engine and sand the surfaces where it circulates.
When we take into consideration that the human eye can only see particles as small as bout 40 microns (a human hair is about 70 microns), it is easy to see how dust can get through. The most damaging particles are those between 2µ y 15µ, since they are big enough to damage the engine, but small enough to keep circulating in the oil.
Note the picture of the circular Toyota air filter that does not look very dirty. It is fabricated with a very restrictive paper and was already showing 25 water column inches of air restriction.