Sweeps filtered air from the BACK of the hood to the FRONT. 1. use an ELECTRICAL BLOWER to draw contaminated room air through a prefilter. 2. The prefilter, which is similar to a furnace filter, only REMOVES GROSS CONTAMINANTS and needs to be cleaned or replaced on a regular basis. 3. The high efficiency particulate air or (HEPA) filter removes 99.9% OF PARTICLES THAT ARE 0.3 MICRON or larger.