Recently a thoughtful reader had a question for the Safety Guys.

The email exchange read like this:
TR: Your article points out that hood function can be compromised by “misuse.” You specifically cite a condition that we constantly face here in my laboratory, that is the blockage of the back bottom slot by reagent bottles and overloading of the hood. The face velocity (at appropriate sash height) meets the required flow in spite of these conditions. That being the case, is it necessary to remove these items? We operate under the belief that as long as the face velocity meets specifications that we can use the hood without rearranging or removing the contents. Is there an OSHA standard that addresses this situation?

Our authoritative reply went something like:

SG: THAT DEPENDS.

Now, let us elaborate on that answer. Exhaust hoods come in many different colors. OK, seriously, straight talk from here on, we promise. Exhaust hoods are available in many different designs and that is what we want to discuss here—basic design principles and proper operation of one specific type of hood. There are chemical fume hoods for working with acid fumes and chemical vapors. For working with biological materials there are clean air benches and biosafety cabinets that serve entirely different purposes. One problem with all hoods, but definitely more so with chemical hoods than others in our experience, is that as a cabinet space in a lab they tend to accumulate stuff and are frequently used for storage. Since the reader’s question dealt with chemical fume hood operation we will focus our answer and this column on their design features and proper operation.

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. Their design attempts to create a capture zone in front of the hood, draw contaminates away from the worker and into the hood where they are exhausted.