These are disposable dust filter pads designed to be used on top of the 6000 series gas and vapour cartridges. They are held in place using a filter retainer (sold separately). The use of the filter pad gives you additional particulate protection as well as helping extend the life of the filter cartridge. The filters are supplied in packs of 10. 5925 is an FFP2 filter with an assigned protection factor of 10 (APF 10). 5935 is an FFP3 filter with an assigned protection factor of 20 (APF 20). Conforms to EN 143:2000 *Please note the information is for guidance purposes only. A proper risk assessment by qualified personnel should be carried out before selecting an appropriate filter cartridge.