They are used for repairing the air filtration system in the 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.
Six filters can be found in Vault 22 in a locker on the oxygen recycling level which can only be accessed by the cave in Vault 22 because the door leading to it is blocked by debris.
To locate the the filters, you first need to find the keycard in the Common Areas level to be able to open the door. It is in the first room on the left beyond the door labeled "Quarters", which is on the lower level.
Then return to the elevator and go to the third floor - food production. Follow the hallway in front of you and turn right. The first door on the left opens into the cave access room. Open the metal door to the cave and turn right at the fork. Follow the cave to find a wooden door labelled "Oxygen Recycling". The HEPA filters are located in a locker in the upper right corner of the room.
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.