All laboratory hoods work on a very basic principle of containment. Negative pressure relative to the exterior is maintained within the interior of the hood to prevent any toxic vapours from escaping and air is drawn at a consistent rate into the hood opening. Experimental procedures are performed within the hood which is consistently and safely ventilated, usually by means of an extract blower and ductwork.
When exhausted to the external environment, chemical fumes are diluted many times over in the atmosphere and has a negligible effect to human health. When environment concerns are of importance, an extract treatment system, often referred to as a scrubber is installed to remove most of the vapours from the exhaust air stream. A suitable hood face velocity (the speed at which air is drawn into the opening of the hood) is of importance to the safe and effective operation of a fume hood. While excessive face velocities can often result in turbulence and reduce containment, insufficient velocities can also compromise hood performance.
In general, a hood inflow velocity of 0.5 m/s or 100fpm is recommended. Most of the hoods are commonly sized for a minimum inflow velocity (e.g. 0.5 m/s or 100 fpm) at full sash opening. However, when energy concerns are paramount, an economical way to decrease the amount of tempered air removed from the hood is to size the minimum face velocity of the hood at half-sash opening instead of full-sash opening.