2CFM/SF is Wrong | Part 1: Lab Ventilation Basics
November 12, 2021
By: Matthew Fickett AIA, LEED, CPHC | Principal
What’s the purpose of lab ventilation?
Science is inherently dangerous. Scientists use chemicals that dissolve other things (solvents and acids), chemicals that burn (sometimes all by themselves), 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?
Of course not! 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
- IMC
- NFPA 45
- OSHA
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. If you’re curious for more, the NIH Design Requirement Manual is a good place to start.)
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.
Table 1: Comparison of Lab Ventilation Rates for a 9’-0” ceiling height
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?
But wait, I hear you saying. What about fume hoods? Don’t they require tons of extra air? I don’t want to design a building which prevents scientists from using all the fume hoods they need.
You don’t have to! We’ll discuss this in Part 2 of this series.