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 within Vault 22, in a locker on the oxygen recycling level. The room can only be accessed by the vault's adjoining cave system, due to debris blocking the normal entrance. A keycard is required for access.
Behind the scenesEdit
The HEPA 20 cartridge filter is based on the real world equivalent. A HEPA filter stands for "high-efficiency particulate air" filter and the number indicates how effective the filter is at controlling contaminants. The "20" indicates that the filter seen in-game would surpass the highest efficacy requirements of the real world, providing better filtration than those required in hospitals and surgical centers.