A fume hood is designed to capture chemical contaminants at the source, contain them within the hood chamber, dilute them with a large volume of incoming air and safely exhaust them. Most Exposure Control Devices (ECD), especially Fume Hoods, are not standalone devices and are a part of a complex Lab Ventilation System (LVS).
Loss of Containment is an unplanned and uncontrolled release of the containments from ECDs and Hoods. One of the primary reason being the lack of support of the system to the functioning of ECDs and hoods, causing the contaminants to become fugitive and leak out into the lab.
Loss of Containment = Breathing in Chemicals From the Hood
Face Velocity is commonly used as a parameter to determine if the fume hoods are functioning as expected.
Even a prescribed Face Velocity does not guarantee a high containment confidence, as fume hood containment is dependent on several other factor. However, face velocity is an important parameter for Dilution within the fume hood.
By diluting the chemical fumes with a large volume of incoming air, their concentrations are lowered so in the event of a Loss of Containment, the risk of breathing in concentrated chemicals is reduced.
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.
To compensate for frequent losses of containment, labs are designed for a high number of Air Changes per Hour (ACH). A higher ACH is considered to be safer but this may not be true. It has been proven that an ACH rate above 12 does not have a significant impact on lab Indoor Air Quality. Moreover, it also causes turbulence in the lab, affecting the fume hood performance. Fume Hoods are one of the most significant sources of exhaust in the lab.
Modulating the hood face velocity directly results in a change in the ACH Rate. The lower the face velocity, the lower will be the ACH rate in the lab. Since ACH is the biggest driver of energy costs in a facility, there is high incentive to keep the face velocity as low as acceptable. However, If the ACH is too low, it is a major health and safety risk and if it is too high, then there is a high energy cost involved, for no additional safety.
Fume Hoods are supposed to protect against exposure to chemicals. But for several reasons including improperly designed Lab Ventilation Systems, they lose containment several times in the day and expose users to these toxic chemicals. Even if hoods show the prescribed face velocity of 100 fpm (0.5m/s), this is no indicator of hood safety and containment.
