One focus of this issue of Forensic Magazine is laboratory equipment. And one of the most important fixtures in any laboratory, including a well-outfitted forensic lab, is the chemical fume hood. Fume hoods are available in many different designs, such as constant air volume, variable air volume, ductless bench top hoods, and auxiliary or supply air hoods. Regardless of the specific type selected, there are a few design parameters and basic operational principles that should be used across the board. Chemical fume hoods are for working with gases, acid fumes, and chemical vapors.
Chemical fume hoods are designed to capture and exhaust contaminates resulting from working with chemicals. They are sometimes referred to as wet benches as the chemicals used (solvents, corrosives, etc.) are usually liquid. Fume hood design endeavors to create a capture zone in front of the hood to draw contaminates away from the worker and into the hood where they are exhausted. This column provides guidance on design features and proper operation.
Laboratory Ventilation Design Considerations
Before we get to the specifics of fume hoods there are a few basic ventilation rules that should be incorporated into all labs where potentially toxic or hazardous chemicals are used. First is to provide adequate general ventilation. This means a non-recirculating supply with exhaust to the outside.1,3 This general supply is not to be relied on for protection from exposures but ensures a continual supply of fresh outside air to prevent the buildup of contaminate concentrations. One recommendation for general ventilation is to provide at least six air changes per hour for your chemical laboratory.1 This means that six times the total volume of air in the lab is supplied every hour.
Next, laboratories where chemicals are used must be under negative pressure in relation to the adjacent non-laboratory rooms, offices, and corridors.2 In the event of a spill or release the contaminates will be kept in the lab and exhausted rather than spread into surrounding areas. You generally want the air to flow from low hazard areas to high hazard areas. Proper pressurization is maintained by balancing the ventilation system. A good rule of thumb is to provide make up air equal to 90% of exhaust volume.1
One final recommendation is to make sure cabinets and equipment do not block or interfere with supply or exhaust vents. Too often we inspect labs to find things stored on top of cabinets or in front of vents, completely disrupting the air flow.
Fume Hood Design Principles
Basic principles of aerodynamics are used to promote a smooth flow of air into the hood. This is termed laminar flow, and the sides and the sill (bottom edge) of the hood are shaped like a foil to allow the airstream into the hood with a minimum of turbulence. In addition, the sill is also raised slightly off the bottom or floor of the hood to create flow across this surface.
Since undisturbed entry is key for hood performance, placement of the hood in the laboratory should receive attention. Do not locate fume hoods near doors, busy walkways, or room air supply and exhaust vents. Try to keep room air currents to less than 20% of the hood face velocity by separating the hood from supply and exhaust locations to the extent practical.1 Air turbulence, especially near the hood entry can affect its ability to contain and exhaust contaminates.
The next most important design parameter for good hood performance is face velocity, or the speed that air flow enters the hood. Air velocity needs to be just right. If it is too slow contaminates will not be contained or exhausted out. Too fast and turbulence and eddies can lead to slipstreaming and dumping contaminates into the laboratory. Face velocity is a function of the total exhausted volume and the area of the opening. The basic equation from physics is that velocity is equal to the volume divided by the area. So if the area increases, for example by opening the hood sash, the velocity drops. Conversely, if the area is reduced by closing the sash the face velocity increases for constant volume hoods. In addition, since the velocity is directly related to the exhaust volume, if the volume is reduced the velocity does likewise. Keep in mind however that different hood designs use this principle differently so bypass hoods, auxiliary air hoods, and VAV (variable air volume) hoods operate slightly differently. Full descriptions of these new hood types are beyond the scope of this column but may appear in future articles.