3Ì 5925 Ð2 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 12 times the TLV value with half masks and up to 16 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.