Prefilters are used when pollutants will be present in large enough quantities to rapidly clog HEPA or carbon filters. Dacron Filters are 4 ply with decreasing pore sizes for each ply. They are rated at 60% efficient for particles of 5 microns or larger. As such, they will remove most gross particulates, saving the HEPA for only the smallest particles. They are also used with carbon when the environment is dusty enough to reduce carbon filter effectiveness or as an after filter to contain possible carbon particle emission.