A High Efficiency Particulate Air (HEPA) filter is an extended-media, dry type filter with a rigid casing enclosing the full depth of the pleats. In order to be called a HEPA, it has to meet or exceed specific criteria. The efficiency of your HEPA filter should be provided at purchase. The filter medium is made up of densely packed fibres through which air flows. This medium is folded into a series of pleats to increase the surface area and channel the air in a uniform direction. The pleats are held together with separators that can be made of various materials, commonly aluminium or glue. This is then packed into a frame to keep it rigid. The frame is sealed to prevent air leaks, so only air that has passed through the fibrous medium gets through the filter.