Safety is a prime concern of all those who work in labs. Scientists use chemicals that dissolve other things, chemicals that burn, or chemicals that are poisonous to humans. Scientists also work with organisms that cause diseases, sometimes for which we have no cure. There are safety procedures for all these things, but to ensure no leak or accident presents a danger, labs are designed to keep people safe in multiple ways.
One major way labs keep people safe is to provide a lot of fresh air, and to constantly remove the air inside the lab, in case it has become contaminated by something. It is commonly accepted that air in standard bench labs should be replaced every 10 minutes. Another way to say this is 6 Air Changes per Hour, or 6 ACH.
As simple as that?
It is important to note that this 6 ACH figure is not universally accepted, and is not written in the building code. There are many standards which discuss lab ventilation, including:
- The ACGIH Industrial Ventilation Manual
- ANSI/AIHA Z9.5
- ASHRAE 62.1 and HVAC Applications Handbook
- NFPA 45
None of these standards gives a single, universal number. All of them argue that the correct ventilation is different for every situation. However, as designers and building owners, we often have to design the mechanical systems for a building before you know exactly what sort of lab will be inside it. We have to pick some number. 6 ACH is a good minimum for now. It’s possible to go lower, but you definitely don’t need to go higher in a normal bench lab. In addition, many institutions, including the NIH, specifically name 6 ACH.
ACH versus CFM
You’ll often hear discussion of CFM/SF (cubic feet per minute, per square foot). This gives each square foot of the floor of a building a certain amount of ventilation – often two cubic feet of fresh air every minute.
Let’s compare this number to 6 ACH.
- 6 ACH means the air is replaced every ten minutes. In an average lab with a nine foot ceiling, to achieve this goal, each square foot of floor area needs nine cubic feet of air every ten minutes.
- 2 CFM/SF means each square foot of floor area gets two cubic feet of air every minute, or twenty cubic feet of air every ten minutes.
2 CFM/SF is more than twice as much air as 6 ACH! That means your air handlers are twice as big, your shafts take up twice as much space in your building, and you’re spending twice as much to condition that air.
What about fume hoods and how do they 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).
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. 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 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.
For normal lab space, even labs with a lot of fume hoods, 2 CFM/SF is more than twice as much air as you need. For high-ventilation spaces like vivariums, which need an average of 13 ACH, 2 CFM/SF is still more than needed – but just by a little bit. If you are planning a whole building, and expect that perhaps 10% of the lab will be a vivarium, you can size your equipment based on the average: 0.9 x 6 ACH + 0.1 x 13 ACH = 6.7 ACH
Or, if it’s easier in CFM/SF, a 9-foot ceiling would make that just plain 1.0 CFM/SF. So, save money on capital costs and energy bills, check your ceiling heights, and cut that 2 CFM/SF in half.
Matthew Fickett, AIA, CPHC, LEED
Matthew, who leads SGA’s science and technology practice, is deeply involved with SGA teams in laboratory fit-out architecture and life sciences core and shell projects. As one of the leading laboratory architects in the industry, Matthew is in charge of orchestrating the rapid growth of SGA’s laboratory architecture expertise. His skillset extends beyond lab planning and includes leading project delivery in the true essence of SGA.