For HVAC and HEPA type air filters, UL 900 establishes smoke and flammability limits for clean air filters according to two classifications:
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UL Class 1 – air filters which, when clean, do not contribute fuel when attacked by flame and emit only negligible amounts of smoke. A Class 1 air filter unit shall not produce flame or sparks when subjected to the flame exposure. Spot-flame tests described in the testing standard, and during the flame-exposure test, shall not cause the development of an area of more than 2 1/2 square inches as measured below the smoke-density time curve.
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UL Class 2 – air filters which, when clean, burn moderately when attacked by flame, or emit moderate amounts of smoke, or both. A Class 2 air filter unit shall not produce flame or extensive sparks, which are sustained beyond the discharge end of the test duct, described in this Standard, when subjected to the flame exposure test, and shall not cause the development of an area of more than 9 square inches as measured below the smoke-density time curve.
Titled Standard for High-Efficiency Particulate Air Filter Units, UL 586 defines procedures for confirming the performance of HEPA filters. It also defines minimum construction components, airflow and resistance parameters, testing equipment and a minimum criterion flame test.
Camfil Farr has published an overview of UL 900 and UL 586. Click here to view that document.