Bosch Workshop Cabin Air Filters capture up to 99% of dirt and other environmental particles over 3 microns (up to 80% of particles under 3 microns), preventing them from entering the passenger cabin, which results in a noticeable improvement in air quality. Performance and durability are maximized by multi-layer media. 1st Layer - Larger particles are captured by the non-woven pre-filter layer. 2nd Layer - Elusive smaller particles are attracted and held by the electrostatically charged microfiber layer. 3rd Layer - Non-woven cover layer adds stability to protect the microfiber layer from damage and acts as an additional barrier to harmful elements. Bosch Workshop Cabin Air Filters feature high pleat counts that provide greater surface area for higher filtration capacity. Foam perimeter gasket (select part numbers where specified by OE) prevents harmful particles from bypassing the filter media. Rigid frame (select part numbers where specified by OE) adds stability for peak performance and long service life.