2 CFM/SF Is Wrong | Part 2: What About Fume Hoods?

By: Matthew Fickett AIA, LEED, CPHC | Principal

Part 1 of this series showed that 2 CFM/SF doesn’t measure lab ventilation the right way, and also usually provides more than twice as much air as we need.  In this part, we’ll discuss one main factor in lab ventilation: fume hoods.

What for?

Scientists use all kinds of chemicals and organisms which can be harmful to people in their work, and the first priority of lab design is keeping people safe from all these hazards. We talk about this safety in levels of containment:

• Primary containment is the bottle, dish, or other container which holds the potentially dangerous material.
• Secondary containment is an environment in which the scientist can do work with the material, but still protect the rest of the lab from it.

Fume hoods are an example of secondary containment.

How do fume hoods work?

A fume hood pulls air from the rest of the lab in through the open front, then sends it up an exhaust duct.  This lets a scientist reach in through the open front of the hood to do work, but ensures that there’s no chance they will breathe any contaminated air.  To put it another way: the main job of a fume hood is to ensure that once any air has gone in the front, it never comes back out the same way.

Older fume hoods do this in a sort of brute-force way: just move enough air, and you can be sure none is going the other direction!  Modern fume hoods are designed to carefully manage the airflow, so that a much smaller amount of air movement still provides the same safety.

Different fume hoods use different amounts of air to create a safe environment, but a very conservative upper limit is 700 CFM (Cubic Feet per Minute) for a 6’ wide fume hood.  It could easily be less than a third of that!

Where does it come from?

A common mistake is to imagine that the air a fume hood needs is extra, in addition to the 6 ACH (Air Changes per Hour) that the lab requires.  Actually, you can use the same air for both.  This isn’t a compromise in safety; it’s correct design.  Let’s break it down.

Remember that the standard, 6 ACH, is air changes per hour.  That is, new air arrives, and the old air leaves.  So, for every bit of air in the lab, it must get removed (somehow) and replaced (somehow).  In a lab by itself, there’s a duct delivering air through a supply diffuser in the ceiling, and a different duct removing that same air through an exhaust grille in the ceiling.

Recall that fume hood is taking air from the lab, and sending it up a duct to the lab exhaust system.  Therefore, if you add a fume hood to the lab, all you have done is add another way for air to leave the lab.  You can think of a fume hood as just a very complicated-looking exhaust grille.  There’s no change to the total amount of air being moved through the lab; it’s just that some of it leaves through a normal exhaust grille, and some of it leaves through the fume hood.

How many hoods can you have?

Consider our worst-case example, a 700 CFM hood.  Remember that in Part 1, we discussed how a 6 ACH target means that in a typical 9’-0” tall lab space, you are getting 0.9 CFM/SF.  700 CFM divided by 0.9 CFM/SF tells us that for every 778 square feet of lab, we are already getting enough clean air to supply the fume hood. 

To put it another way, you can have one fume hood per 778 square feet of lab space, without needing any extra air at all.  If you used a higher-efficiency fume hood, one which only needs 250 CFM, you could have one fume hood per every 278 square feet of lab space.

What else?

In part 3, we will look at vivariums.