Primary HEPA filter cartridge with poly-prefilter for model 981 designed to handle ultra fine sub-micron particulate. Prefilter - 2 ply, multi-density. Polyester media. Efficiency: 30-35 percent per ASHRAE 52 test method. H E P A Filter - 99.97 percent efficiency. using thermally generated. Dioctylphthalate (DOP), with particles 0.3 micron or larger size as the challenge.