Laboratory fume hoods are the most important components used to protect laboratory workers from exposure to hazardous chemicals and agents used in the laboratory. Functionally, a standard fume hood is a fire-and chemical-resistant enclosure with one opening (face) in the front with a movable window (sash) to allow user access into the interior. Large volumes of air are drawn through the face and out the top to contain and remove contaminants from the laboratory.
The average velocity of the air drawn through the face of the hood is called the face velocity. It is generally calculated as the total volumetric exhaust flow rate for the hood, divided by the area of the open face, less an adjustment for hood air leakage. The face velocity of a hood greatly influences the ability of the hood to contain hazardous substances, that is, its containment efficiency. Face velocities that are too low or too high will reduce the containment efficiency of a fume hood. In most cases, the recommended face velocity is between 80 and 100 feet per minute (fpm). Face velocities between 100 and 120 fpm may be used for substances of very high toxicity or where outside influences adversely affect hood performance. However, energy costs to operate the fume hood are directly proportional to the face velocity. Face velocities approaching or exceeding 150 fpm should not be used, because they may cause turbulence around the periphery of the sash opening and actually reduce the capture efficiency of the fume hood.
Average face velocity is determined either by measuring individual points across the plane of the sash opening and calculating their average or by measuring the hood volume flow rate with a pitot tube in the exhaust duct and dividing this rate by the open face area. Containment may be verified by using one of the flow visualization techniques found in section 8.C.5 on fume hood testing. Once the acceptable average face velocity, minimum acceptable velocity, and maximum standard deviation of velocities have been determined for a hood, laboratory, facility, or site, they should be incorporated into the laboratory's Chemical Hygiene Plan.
Tracer gas containment testing of fume hoods has revealed that air currents impinging on the face of a hood at a velocity exceeding 30 to 50% of the hood face velocity will reduce the containment efficiency of the hood by causing turbulence and interfering with the laminar flow of the air entering the hood. Thirty to fifty percent of a hood face velocity of 100 fpm, for example, is 30 to 50 fpm, which represents a very low velocity that can be produced in many ways. The rate of 20 fpm is considered to be still air because that is the velocity at which most people first begin to sense air movement.
Most people walk at a velocity of approximately 250 fpm (about 3 miles per hour). Wakes or vortices form behind a person who is walking, and velocities in those vortices exceed 250 fpm. When a person walks in front of an open fume hood, the vortices can overcome the fume hood face velocity and pull contaminants out of the fume hood, into the vortex, and into the laboratory. Therefore, fume hoods should not be located on heavily traveled aisles, and those that are should be kept closed when not in use. Foot traffic near these hoods should be avoided, or special care should be taken.
Air is supplied continuously to laboratories to replace the air exhausted from the fume hoods and other exhaust sources and to provide ventilation and temperature/humidity control. This air usually enters the laboratory through devices called supply air diffusers located in the ceiling. Velocities that can exceed 800 fpm are frequently encountered at the face of these diffusers. If air currents from these diffusers reach the face of a fume hood before they decay to 30 to 50% of the hood face velocity, they can cause the same effect as air currents produced by a person walking in front of the hood. Normally, the effect is not quite as pronounced as the traffic effect, but it occurs constantly, whereas the traffic effect is transient. Relocating the diffuser, replacing it with another type, or rebalancing the diffuser air volumes in the laboratory can alleviate this problem.
Exterior windows with movable sashes are not recommended in laboratories. Wind blowing through the windows and high-velocity vortices caused when doors open can strip contaminants out of the fume hoods and interfere with laboratory static pressure controls.