3Ì 5935 Ð3 R pre filter. Pre filters are used along with 3Ì 6000 series gas filters to protect the wearer from fine dust particles and liquid aerosol up to 50 times the TLV value with half masks and up to 200 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.