The Pre Pleat Class 1 pleated air filter has been tested by Underwriters Laboratories, Incorporated and found to meet the stringent performance characteristics of a Class 1 air filter for flammability, as outlined in Standard 900. Class 1 air filter units are described as "Those that, when clean, do not contribute fuel when attacked by flame and emit only negligible amounts of smoke. The filter consists of a nominal 50% dust spot efficiency micro-fine fiberglass filtration media that has been bonded to a corrosion-resistant expanded metal backing, and then pleated into either standard or high capacity packs. Each pack is encased and sealed within a die-cut frame which is manufactured of special mineral-filled board to resist flammability. Pre Pleat Class 1 pleated air filters are offered in two inch depth in all of the most popular face sizes.