A type of filter used to remove all particles that may be present in the flow of gas into or out of a process. Absolute filters are used for ensuring the sterile flow of air or oxygen to biological reactors as well as for clean rooms and sterile cabinets used for analytical work. Unlike an air filter, the pore sizes are smaller than the expected particle size. With a typical uniform pore size of 0.2 μm, the pressure drop is greater than that of air filters made from fibrous materials.