The SR 221 Pre-filter should always be used for particle, chemical cartridge and combined filters such as Sundstrom's Wildland First Responder Kit-(SR-100). The pre-filter protects the main filter against premature clogging by larger particles, for example dirt and dust. The SR 5153 Pre-filter holder secures and protects the pre-filter against handling damage. Also used for face-fit tests together with test disc SR 322. Comes in packs of 10.