3Ì 5911 Ð1 R pre filter. Pre filters are used along with 3Ì 6000 series gas filters to protect the wearer against fine dust particles and liquid aerosol up to 4 times the TLV value with half masks and up to 4 times the TLV value with full face respirators. 501 retainer is used to clamp pre filters on to gas filters. Pack of 2 pre filters.