The overhead scrubbers are located directly above the fume cupboard. They operate by having essentially 2 scrubbing beds separated by wind deflectors. The overhead units are used where roof height is adequate, fans located on the roof, or egress of scrubbers may be difficult. These scrubbers commonly find their purpose for perchloric acid fume cupboards as it negates the need of duct sprays from the exhaust plenum to scrubber inlet. The overhead scrubbers require careful design to maximise the scrubbing bed cross sectional area. As such, there is no one size fits all. The overhead scrubber has the lowest scrubbing capacity but best footprint where it is integrated with the fume cupboard.