The 5000 D Tar Trapping Pre-filter traps larger particles, easing the burden on HEPA and carbon filters which helps extend filter life. The pre-filter is a cotton/polyester belnd with a sticky substance on one side to trap the tar. They comes in a package of 10 because the AllerAir AirMedic Pro 5 HDS (formerly AllerAir 5000DS) needs a new pre-filter every 2-3 months for maximum filtration.