On an OEM/paper filter it has millions of microscopic holes. The dirt and dust which follow the air look for the path of least resistance. So when dust it trapped into one of the holes the air looks for a different, easier route. The dust does the same. This would mean that having dust in the holes doesn’t necessarily mean its cleaning the air better, for as soon as a hole is filled, the air/dust doesn’t head towards that hole anymore. Furthermore if you continue to use a filter past the point of being full, I would think it would start sucking dust in.
1986 190E 186K (For Sale)
(Maroon, Tan leather)
1996 C280 Sport 208K
(Brilliant Silver, Black Leather)
2002 CLK430 Sport 45K
(Brilliant Silver, Gray Leather , Birdseye Maple)