The principle of how foam air filters work is simple: "Open Cell" Polyurethane Foam is wetted with specially developed filter oil. The "sticky" filter oil is suspended in the path of the dirty air on the strands of the web-like cell structure of the foam. This makes it impossible for dirt to pass through the depth of the filter without sticking to the strands. As the outer strands become loaded with dirt particles, the wetted strands down stream start trapping dirt, allowing the entire thickness to be utilized. This prevents surface loading or air restriction for 80% of the service life of the air filter element. When the filter is sufficiently dirty, it can be easily washed, re-oiled, and re-used. http://www.unifilter.com/whyfoam.html