For HVAC and HEPA type air filters, UL 900 establishes smoke and flammability limits for clean air filters according to two classifications: fire
- UL Class 1 – air filters which, when clean, do not contribute fuel when attacked by flames and emit only negligible amounts of smoke. A Class 1 air filter unit must not produce flames or sparks when exposed to flames. Spot-flame tests described in the testing standard, and during the flame-exposure test, must not cause the development of an area of more than 2 1/2 square inches as measured below the smoke density time curve.
- UL Class 2 – air filters which, when clean, burn moderately when attacked by flames, or emit moderate amounts of smoke, or both. A Class 2 air filter unit must not produce flames or extensive sparks that are sustained beyond the discharge end of the test duct, as described in this standard, when subjected to the flame exposure test, and must 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 verifying the performance of HEPA filters. It also defines minimum construction components, air flow 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.