In search of increased operating efficiency, face velocity of fume hoods has been increasingly reduced. Even though “High-Efficiency fume hoods” test well as per ASHRAE or EN standards, but when placed in an inadequately designed lab ventilation system, these hoods cannot function as designed, resulting in a loss of containment. By reducing the face velocity, the fume hood’s ability to dilute contaminants is put into question. Since the volume of incoming air is reduced, in the likely event of a loss of containment, the fugitive contaminants leaked out will be at a relatively higher concentration.
Hoods at a lower face velocity are also more susceptible to cross drafts. When fume hoods operate at 60fpm, cross drafts should be limited to less than 20fpm. However, the lower boundary for perceptible air speed for human beings is around 30fpm. Hence, it wouldn’t even be possible, without a complex and expensive detection system to tell if cross drafts were affecting fume hood containment at lower face velocities.