HEPA 20 cartridge filter
They can be found in Vault 22 in a locker on the oxygen recycling level. It can only be accessed by the cave in Vault 22 because the door leading to it is blocked by debris. They are used for repairing the air filtration system in Brotherhood of Steel's hidden bunker for the Still in the Dark quest. Since it is a quest item, it cannot be dropped without the use of console commands.
To locate the HEPA filters you first need to locate the key card in the fourth level to be able to open the door. It's in the first room on the left beyond the door labeled "Quarters". Then go to the third floor. Open the metal door to the cave and turn right at the fork. Follow the cave to find another wooden door saying "Oxygen Recycling". The HEPA filters are located in a locker in the upper right corner of the room. This is the only locker which is not empty.
Behind the scenesEdit
A High Efficiency Particulate Air filter is a real standard for air filters. The "20" suffix refers to its MERV factor: a filter's ability to remove particulates from the air on a scale of 1 to 20.