Bosch P3909WS / F00E369128 Workshop Cabin Air Filter Performance and durability are maximized by mold-resistant multi-layer media 1st Layer - Larger contaminant 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 contaminants