The SIP PERFORMANCE double-layer air filter inserts have two layers. The first layer has medium sized micropores to filter coarse dirt particles. The second layer has very fine micropores to filter microscopic size dust particles - these dust particles cause the most damage. The filters are made of self-extinguishing polyurethane foam and consist of two layers joined by a special hot molding process, which does not change the filter properties. The outer coarse layer consists of red filter material: open polyurethane foam with a permeability of 45 ppi (pores per inch). The fine-pored second layer consists of black filter material: open polyurethane foam with a permeability of 60 ppi (pores per inch).