Integrity testing of HEPA filters mounted at the same outlet of the Air Handling Unit

They have request us as mandatory to perform aerosol leak testing on HEPA filters mounted in the same outlet of an Air Handling Unit destined to a grade D area. We are trying to elucidate how to implement this because the AHU is not prepared for such test, and up to now by injecting the aerosol in the plenum of the AHU just before the filters and introducing the photometer detector at the outlet air duct as we have figured out we have just obtained 0,04% of aerosol reduction at most, higher than the prescribed 0,01%, or is it that with such arrangement we can never pass below such limit???. If anyone has any experience or a practical reference on this we kindly ask to let us know

It sounds like your HEPA filters pull air from the plenum, is that correct? Are they fan powered HEPAs?

If so, I would recommend adding some sort of pre-filter, or duct work system to supply air to the HEPA filters to avoid the HEPA filters loading up (clogging). This will prolong the life of the HEPAs for a few reasons.

It sounds like your HEPA leak just failed, there could be a few causes for this
1 - The HEPAs do not meet requirements, and must be replaced, and retested
2 - The HEPAs have not been appropriately installed. There might be a leak around the outside of the HEPA where it seals with the can/housing.

Here is another possibility:
1 - The room ceiling is not sealed - you might have a drop ceiling, and Aerosol is leaking in from the plenum, through the ceiling, and into the room - this is also unnaceptable, as air is bypassing the HEPAs and going straight into the room from the plenum - obviating the purpose of the HEPA filters.

In any way, I think you need to find and fix the leak (HEPA, seal around the HEPA, or ceiling)