Two types of prefilters are used when pollutants will be present in large enough quantities to rapidly clog HEPA or carbon filters.
1. 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.
2. Aluminum Prefilters will remain cool in moving air and their large surface area makes them ideal condensation media for airborne greases and mists, such as solder flux. These filters will also extend the life of more expensive carbon and HEPA filters by removing gross contaminants before they can reach sensitive filter surfaces.