REPLACMENT HEPA FILTER FOR DEFENDAIR 500 MODEL #F284 HEPA filter (F321). The HEPA filter has an efficiency rating of 99.97% against 0.3 micron oily aerosol particles. The HEPA filtration standard was developed in the 1940s to control radioactive dust particles in U.S. nuclear laboratories. HEPA 500 filters combine filtration ability with filter capacity. To maximize HEPA filter life, be sure to change the prefilters regularly.