Having successfully completed over 10,000 HVAC service calls and with 20 years in the HVAC industry, I offer the follwing information. Air filters have nothing whatsoever to do with human beings, respiratory systems, or cleaning the air. The air filter has one purpose: to protect the equipment from the build up or accumulation of debris. Air filters that “clean” the air are much too restrictive and pushed the allowable external static pressure beyond the forced air units abilty to perform as designed. If however, you are inspecting an air handling unit that is designed for 5" of e.s.p. then yes, HEPA filters 12" thick can be used. But 99.9% of residential FAU blower assemblies are not capable of doing their job of moving air and on top of that pulling air through a restrictive filter.