The addition of the DOP filter makes the DefendAir HEPA 500 a high efficiency three-stage air filtration unit, ready for any sensitive job requiring first-pass HEPA filtration. With the standard pre-filter, DOP Stage 2 pre-filter and the HEPA primary filter installed, the HEPA 500 passes laboratory DOP testing conducted per ANSI standards, achieving an efficiency rating of 99.97% against oily aerosol particles down to 0.3 microns.