UNI air filters. The principal 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 the dirt to pass through 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 surface life of the air filter element. When the filter is sufficiently dirty it can be easily washed, re-oiled and re-used. Applications: 1987-92 Honda TRX 250X 1993-08 Honda TRX 300EX 2009-12 Honda TRX300X