Electra Silver 9400 Electrostatic Filters provides over 94% arresting in a premium quality extruded aluminum, silver anodized frame with a durable and attractive finish. At the heart of these electrostatic furnace filters are three layers of high arresting material sandwiched between 2 layers of expanded metal. A special impregnated black polypropylene has an EPA registered anti-microbial material to inhibit mold and bacteria growth while the center non-woven layer retains debris in a maze of fibers designed to maximize flow even when soiled. Our electrostatic filters rated by independent testing laboratory at over 94% peak arresting, these furnace filters have 10 times the arresting of a disposable fiberglass filters.