Air Filter, Class 1 : An air filter that, when clean, does not contribute fuel when attacked by flame and emits only negligible amounts of smoke when tested in accordance with UL 900, Standard for Safety for Air Filter Units
NFPA 97 Standard Glossary of Terms Relating to Chimneys, Vents, and Heat-Producing Appliances, 2003 ed. http://travelwidpinx.info/nfpa-97-standard-glossar y-of-terms-relating-to-chimneys-vents-and-heat-pro ducing-appliances-2003-edition.html, (Std: Glossary of Terms Relating to Chimneys, Vent, and Heat-Producing Appl - Amer) {241 NFC}