Fume hood testing not only measures performance, but also demonstrates how user behavior influences the hood’s ability to keep them safe.
A chemical fume hood is an exposure control device and its purpose is to contain hazards and protect the user from exposure. A fume hood is also part of the overall laboratory mechanical system and there are a variety of potential causes of failure that can occur, both within and outside the user’s control. A cause of failure that is often overlooked is the user's own work practices.
While this article focuses on chemical fume hoods, many of the key takeaways below apply to ventilated enclosures as well.
The only way to know for sure if a hood is losing containment is to run a test. When doing containment testing, at least 25 percent of failures are caused by user work practices. If a fume hood is performing poorly, the lab manager may not be able to replace the hood or revamp the laboratory ventilation system, but they can educate the user to help protect themselves by ensuring good practices and expanding their knowledge of how fume hoods perform best.
The biggest challenge with fume hood performance is that users usually cannot see the hazards—most are invisible and odorless. Therefore, there is no easy way to know if your fume hood is working properly and protecting you. Many think that face velocity, which is defined as the speed of the air entering the sash opening, is an indication of safety. The ANSI/AIHA Z9.5 Laboratory Ventilation states: “Face velocity had been used historically as the primary indicator of laboratory hood performance for several decades. However, studies involving large populations of laboratory fume hoods tested using a containment-based test like the ANSI/ASHRAE Standard 110, ‘Method of Testing the Performance of Laboratory Fume Hoods,’ reveal that face velocity alone is an inadequate indicator of hood performance.” In fact, most hoods that fail containment testing have acceptable face velocity readings.
So, what causes loss of containment in a fume hood? The two biggest causes are turbulence and pressure shifts (room pressure versus fume chamber pressure).
Air is a liquid; it follows the laws of fluid dynamics. It flows from high pressure to low pressure. Picture a smooth flowing river, the surface is only moved by wind. Add lots of boulders, and now there are rapids. The smooth flowing river has transformed into a raging obstacle course. People do the same in and around fume hoods. Their actions create turbulence in the airflow that increases the chance of loss of containment.
By definition, a fume hood is an enclosure that has a movable sash, an upper airfoil, lower airfoil, and baffles. An enclosure without these features is named a ventilated enclosure.
To the user, the most important feature of a fume hood is the sash. The sash, or sash panels, are the pieces of transparent material—usually glass—that are located at the front of the fume chamber and are movable. The sash position has a huge impact on the airflow within the fume chamber, but the sash is also a barrier between the fume chamber and your breathing zone. It offers protection from other hazards, such as fire and explosion. Using the sash properly is one of the most important things you can do, not only to protect yourself, but to also save energy.
The next component to focus on is the baffles. These are normally located in the rear of the hood, and along with the back wall, create the exhaust plenum. The exhaust plenum has the lowest pressure within the hood, so the air naturally wants to flow there. There are usually slots or holes in the baffles to allow the air to flow into the exhaust plenum. There are many baffle designs, and some perform better than others. The baffles are the most critical component in fume hood performance, so users need to pay close attention to them.
The lower airfoil is also a critical component. The sash normally closes onto this airfoil. It is the nose of the work surface. There are many designs, but typically, they are constructed of metal and are designed to create a stream of air that sweeps across the work surface rearward toward the baffles.
The purpose of most fume hood tests, like the ASHRAE 110 and EN 14175, are to test the validity of the fume hood design as manufactured (AM) and to test the interface of the fume hood with the laboratory ventilation system as installed (AI). Both tests are useful tools to ensure that the system is working to specification. What they do not focus on is the user, with the exception of the ASHRAE 110 - AU (as used) test. The much more common AM and AI tests are conducted using empty hoods; without people and with very limited movement.
Even when the hood tests well, your actions at the hood can negatively impact its ability to properly contain. As the user, you are a critical element of the ventilation system.
Let’s examine some specific examples of fume hood use.
As a best practice, the lower the sash handle is, the safer the user will be. Hoods should never be used at full open—this is for setup only. Working with an 18-inch opening on a vertical sash is far safer than working at full open.
When you stand in front of the sash opening, your body acts like an airplane wing; the air being drawn into the hood flows over your shoulders and around your sides, creating a low-pressure zone directly in front of you. This low-pressure area in front of you will attempt to pull air from inside the hood outward, creating a loss of containment and possible exposure. This is why you always work at least six inches behind the sash to keep chemicals out of the area of reverse flow.
Next, extend your hands and arms into the hood and move them about. What are they doing to the airflow? Picture yourself in a canoe, your arms are the paddles, and just as the paddles can displace large amounts of water, your arms can displace large amounts of air creating turbulence and disrupting the airflow. This is a recipe for loss of containment. When working in the hood, move your hands and arms slowly and deliberately.
Do not place objects directly on the work surface. The airstream flowing over and under the lower airfoil is attempting to sweep chemical fumes back toward the baffles. Placing objects directly on the work surface is like putting boulders in that smooth river. It creates rapids, greatly increasing the chances for loss of containment. Everything in the hood should be elevated off the surface one to two inches. Blocks, shelving, or elevators can be used to lift the objects.
Baffles have been designed to organize the airflow through the fume chamber into the exhaust plenum. Never tape notes and such to the baffles or use the hood for storage. Blocking any of the slots or penetrations in the baffles will disrupt the airflow and create turbulence, which increases the loss of containment.
It is not just what happens in the hood that matters, but what happens around the hood as well. For instance, just walking past the hood with a raised sash is almost a guarantee there will be loss of containment. Why does loss of containment matter? The fugitive chemicals escaping the hood contaminate the room air, which means everyone in the lab is breathing these potentially hazardous chemicals.
There is a long list of dos and don’ts beyond what has been outlined here. Remember, users are a critical element of the overall fume hood system. How lab staff work in and around the hood can create hazards that may impact the safe performance of the hood, and everyone in the lab.
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