These requirements cover tests to determine combustibility and the amount of smoke generated for air filter units of both washable and throwaway types used for removal of dust and other airborne particles from air circulated mechanically in equipment and systems installed in accordance with the Standards for Installation of Air Conditioning and Ventilating Systems@ NFPA 90A (Other Than Residence Type)@ and for Installation of Warm Air Heating and Air Conditioning Systems@ NFPA 90B (Residence Type). Since the combustibility and smoke generation of an air filter unit@ after a period of service@ depends upon the nature and quantity of the material collected by the filter@ the test requirements of this standard@ for classification purposes@ apply only to air filter units in a clean condition. Consequently@ when filters are susceptible to the accumulation of combustible deposits@ it is intended that maintenance and inspection practices should be followed as proposed in Appendix B of NFPA 90A.