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Lab Manager Fume Hood Webinar

It is a pleasure to be here today and I hope the attendees will find the next few minutes informative and that the information I share will help them have a safer experience when working with their fume hoods.   

Today we are going to focus on several areas that can make working with a fume hood more productive.

My experience is that people assume that because a fume hood is present in the lab , that the hood must be working properly and providing the intended safety and protection.  

If this hood was installed in a new lab you were about to occupy, would you question its performance? 

Wouldn’t you just assume that  because it is a new hood in a new lab that everything is ok?

In reality the fume hood is the most misunderstood and misused safety device in the lab.  

Many installed fume hoods are not providing the intended levels of safety. 

Today we will go through some of the critical components and discuss their impact on the hood’s overall ability to keep the user safe.

Today we will look at:

Fume Hood purpose and function
The Sash
Turbulence and Airflow
Face Velocity versus Containment
Making sure your hood is working
The need for training

By addressing each of these areas, you will gain an awareness and understanding to make working with a fume hood safer, more productive, and maybe even fun.

Let’s jump right in and start by looking at what a fume hood is and what it does. 

First we will discuss the Fume Hood’s  purpose and function

There are many things in labs that look like fume hoods, but are not.

There are ductless ventilated enclosures,
Biological Safety Cabinets or (BSCs), 
Canopy hoods, 
Educational hoods, 
Spot Extractors.  

But to be classified as a laboratory fume hood, there must be baffles,  an operable sash and it must be connected to the laboratory ventilation system.  

Laboratory or chemical Fume Hoods are not stand-alone devices like a Biological Safety Cabinet or ductless hood.   

In fact, the best fume hood in the world does nothing until it is connected to a properly designed, installed and maintained laboratory ventilation system.

The fume hood is just one component in a complex mechanical system.

Yet when looking at the safety warnings posted on the fume hood, it often says, “make sure the hood is working.”  

So What does “working” mean?  

To answer that question we first need to answer another question

That question is  what is it that a fume hood does?

The inside of the hood is called the fume chamber.  This is where your work is done.   
Inside the fume chamber there are four things the fume hood should do; 


The fumes, vapors, particles and mist produced within the fume chamber should be captured.  

Once captured, they should be contained within the fume chamber and not allowed to escape.  

While they are in the fume chamber, they should be mixed with air and diluted to a safe level.

Once properly diluted they are then exhausted outside and away from the building.

In the simplest of terms, when a fume hood is working properly, there is no loss of containment.  

Loss of containment exposes the user to various hazards. 

 That is why the fume hood also belongs to another class of products called Exposure Control Devices.

Much confusion comes from the fact that the fume hood is a hybrid device

Not only is it part of a complex mechanical ventilation system, it is a PPE designed to protect the user from various hazards. 

This hybrid classification is the source of a lot of misunderstanding.

The fume hood is much like a three-leg stool.  

To be stable and function properly the stool needs three even legs.  

The same is true with the fume hood.  Its legs are:

the hood itself, 
the laboratory ventilation system, 
and a properly trained user.  

Unless the fume hood’s performance is viewed in this holistic way, the fume hood will not function properly.   
All 3 “legs” must be balanced and functioning correctly to protect the user.

Hence, the big misconception  -- just because you have a fume hood in your lab doesn’t mean it is working or working safely.

Next let’s talk about the sash

From the user’s point of view, it is all about the sash.  

The sash is the interface between the fume hood and the user.  

It is the primary feature that provides you with protection.  

The sash is a protective barrier between you and the bad and dangerous stuff.  

Managing  the sash is largely common sense, the lower the sash position the better.  

While the sash provides protection, it is also a potential source of hazard. 

There can be contaminated air present at the edge of the sash.  

There can be a vortex of contaminated air just behind the sash, 

These vortices have the highest concentration of contaminants. 

You want to keep the edge of the sash or sashes as far away from your breathing zone as possible

You may have heard the term breathing zone before, but it is generally the area from your chest to the top of your head. 

You want the sash glass between you and the bad stuff.

And yes, there are different kinds of sashes, the most common is vertical rising which is a single sash glass that goes up and down like a window.  

There are combination or combo sashes which have sliding panels that move left and right that are inside a vertically rising frame.  

Regardless of sash type, the only reason to fully open the sash is for setup. 

A very common mistake is working with the hood in the full-open position . Lowering the sash  is better.

The sash movement should be smooth and easy. 

Many sashes can be operated with one hand from any position along the sash handle, and many newer hoods often have auto sashes that move with the assistance of a motor.  

Hood sashes are usually made from some type of safety glass, either tempered safety glass or laminated safety glass. 

This special safety glass is used to offer you more protection and minimize the likelihood of the sash itself  breaking and becoming part of an incident.  

But whatever type of sash your hood has, keeping it closed when not in use and opened as little as possible when you are using it is the right approach.  

The glass is there to protect you from hazards.

Fume Hoods are very expensive to operate.  

They require large amounts of air to function properly.  In most modern hoods, the amount of air the fume hood uses is determined by how much the sash is opened.  

The smaller the opening, the less air and energy the fume hood will use. So generally a closed sash uses much less air than an open sash.

One of the fume hood’s functions is to exhaust the polluted air outside and away from the building.  
As this air is exhausted, an equal amount of fresh air has to be supplied to the lab.  This supply air has to be conditioned.  

Heated or cooled and the humidity has to be controlled.  

The real cost of operating the hood is replacing the air that is being exhausted.  

When you exhaust less,  you need less replacement or supply air thus saving money.

While you might not have thought about it, fume hood fires are all too common.  

While there are many reasons for fires, let’s look at the role of the sash in a fire.  

In case of a fire, close the sash completely to minimize the amount of air feeding the fire. 

The sash also keeps the fire inside the hood and minimizes its spread.  

The proper actions when a fire does occur varies,  
you need to understand the SOP for your lab and respond accordingly.   

This is a somewhat complex topic that varies from lab to lab, because of the complexity, we have a training module that focuses just on fume hood fires.  

Fires are very dangerous and very expensive events. So preventing them should be the overriding objective.  

Small fires can quickly spread and become a major event.
In fact, a small fume hood fire resulted in the death of a researcher at UCLA, partially because she was not wearing a lab coat.

Given the material she was working with, she should have not only been wearing a lab coat, but a fire retardant lab coat.

Not only do fires harm people, they damage equipment, destroy work and even a small fire can take a building out of service for months, displacing the users and destroying research.   

But fires do happen and the sash is there to protect you.

Back to your breathing zone – given the complexity of laboratory ventilation systems and room conditions, it is nearly impossible for any  hood to contain 100% of the time.  
We can almost guarantee some loss of containment.  

In many of the demonstration videos, we use smoke or theatrical fog to help you visualize the airflow and show loss of containment, 
but in real life, the fugitive chemicals are likely to be invisible and maybe even odorless.  

What you can’t see can harm you.  

Whether or not you inhale those fugitive chemicals is greatly influenced by the position of the sash in relation to your breathing zone, 
keeping the sash closed as much as is reasonable helps protect you.  

Next, let’s talk about explosions, they come in all shapes and sizes and they can be caused in many ways.  
From a boiling beaker exploding to an explosion caused by flammable vapors. 

The most common byproduct of an explosion is shrapnel. 

In a fume hood, the most common shrapnel is broken glass

And with broken glass often comes chemical splashes.  

Again, that sash is there to be a barrier between the event and you.  

Like fires, explosions are all too common. 

They are harder to predict and to prevent; the difference is that the damage is usually concentrated on those using the hood or those nearby the explosion.
Remember, from the user’s point of view, the fume hood is a PPE.  

The fume hood is designed to protect the user.  

It is also important to understand that all the PPEs and safety devices that a fume hood user should be aware of.  

The standards, a  lab coat and safety glasses are a must.  

Things like gloves, face shields, goggles and maybe even a respirator may also be appropriate depending on what you are doing. 

You need to understand the risks and hazards of the materials you are working with.

But just in case there is an incident, you should also be aware of the fire extinguisher, the eyewash and the safety shower and how to use them properly.  

The sash is there to protect you and the degree of protection depends on how you use that sash.


Now let’s look at turbulence and airflow.  

While these are higher level concepts, it is important to understand the basics so you can maximize the hood’s performance. 

This is probably a good place for an analogy. 

Let’s compare a fume hood to a car.  

The car and the fume hood are both complex mechanical systems. 
As an operator,  you are responsible for how the car is used.  
But as a driver we would not expect you to fix mechanical issues, but we expect to know when the auto wasn’t performing correctly and be able to convey the symptoms to the mechanic.   

The same is true with the fume hood. You should understand enough about how it should be performing to report issues to those responsible for its mechanical performance.

First, let’s acknowledge that generally we can’t see air, if we could, much of what we are talking about would be merely common sense.  

It is important to know that air and water behave the same.  Both follow the physical laws of fluid dynamics. Air is a fluid. 

Think about a major water leak in your house, inches of water are everywhere in every room. You need to squeegee it all out. 
Think how difficult it is to move that water around and get it to go where you want it to go.  The same is true with air. 

An engineer once said to me, managing airflow was like herding frogs.

Mechanical ventilation and fume hood performance are anything but simple. 

But having a general understanding of what the air is doing allows you to do things that will make the hood work better 
and to spot things that may be impacting performance that need attention by facilities or maintenance. 

Just like water running downhill, air flows from high pressure areas to low pressure areas.  In a fume hood, the low-pressure area is in the plenum.  

The plenum is the space between the baffles and the back wall.  There is an opening at the top of the plenum where the ductwork is connected.  

There is a fan on the roof that creates the lowest pressure in the entire system.  You could say the fan is sucking the air through the hood.

Airflow is designed to flow from clean to dirty areas.  

The clean air from the corridor flows into the lab, 

the lab air flows into the hood through the sash opening, then into the plenum, 

then the ductwork to the exhaust fan where it is finally exhausted out of the building.  

Any pressure imbalance can cause the direction of that flow to change.  

Managing the room pressurization is a challenge to fume hood performance.

Pressure changes can reverse flow which can result in unsafe conditions in the lab.

In a properly designed and balanced lab, the lab is under slight negative pressure compared to the hallway.  

If you crack the door, you can usually feel the air rushing in.  The fume chamber has to be lower in pressure than the room to draw the air into the plenum.  

While all this looks good on paper, it is never that simple in reality.  

Maintaining proper balance is critical to the hood’s safe performance. 

Airflow is typically not something the user can control, but it is necessary to have a general understanding to verify that your fume hood is working. 

But there are some  components of airflow that the user can control.  

Let’s say that everything is balanced and working properly.  There is air entering the fume chamber through the sash and over-and- under the lower airfoil.  

Given the pressure difference, the air is trying to flow across the worksurface to the lower slot in the baffle, 

But if there are objects directly on the worksurface or in front of the baffles, you disrupt that airflow.
Think of the air as if it were water flowing down a river.  Smooth and calm.  

Now put some boulders in the river and a smooth flowing river turns into rapids.  

Objects in the hood do exactly the same thing to the airflow 
– these objects can create enough turbulence to cause the hood to lose containment, which could contaminate the lab air and create hazards for the users, the equipment and the work. 

As a user, there are things you can do to improve performance.  

Don’t block the baffle openings, keep object away from the side walls and from in front of the baffle openings

Elevate objects so they do not sit directly on the worktop. This allows the air to flow under them on its way to the baffles. 

work 6 inches behind the sash to prevent chemicals from being drawn out.

Move slowly and deliberately within the hood and when you remove objects.

All of these things  must be adhered to for the fume hood to have a chance at keeping you and your colleagues safe.



Now let’s look at turbulence.  Fume Hoods are turbulent devices.  If you could see the airflow and it were reddish orange, it would look very much like the flames of a fire.  

Pressure changes are continuous and that impacts the airflow.  Much like a campfire, no two seconds are exactly the same,  it is as if the fume hood were breathing. 

You have probably seen fume hood tests published by manufacturers.  

These tests are valid and useful, but you should understand how the tests were performed. Fume hoods are rigorously tested at their factories to assure their performance.

Fume Hood Manufacturers do the ASHRAE 110-AM ( as manufactured) test with an empty hood in a test room, under perfect room conditions.  

The reason for this test is just to seperate the hood's performance from influences of the room.  
This allows you to do an apples to apples comparison of various hoods. But it doesn’t tell you how that hood will perform in your lab.

In the lab, we never work with an empty hood and never are the room conditions perfect. 

Room conditions are changing minute to minute, even in the best labs, the rooms are far from perfect. 

In fact, in a lab building, the airflow in individual rooms resemble ocean currents with ebbs and tides, continuously moving and changing.
Your body also causes turbulence.  

When you stand in front of the hood your body acts much like an airplane wing.  There is a low- pressure area created in front of you.  
This low pressure area can suck chemicals out of the hood.  

That is why we work 6 inches into the hood, so that chemicals are less likely to be drawn out.  That critical 6 inches enables the hood to contain at a much better rate. 

Now, walk past a fume hood with an open sash.   That low pressure is now behind you, as you walk by it can suck chemicals out of the hood.   

Also, if you move your hands in a fast manner inside the hood or quickly remove objects from the hood, you can draw chemicals out.  All of these events are a loss of containment.  

These fugitive chemicals can then be inhaled by those at the hood, but they also pollute the air in the lab itself.

Another source of turbulence is cross drafts.  

The most common cross drafts are caused by high-velocity supply air registers being too close to the face of the hood 

Also when we have hoods too close to the door or too close to each other, there can be cross drafts. 

The magnitude and direction of the cross draft determine the  potential impact on the hood’s ability to contain.

Turbulence is a primary factor in the loss of containment. 

Reducing turbulence will help the fume hood have better containment. A component of turbulence is velocity. 

The faster we move in and around the hood the more turbulence we create.  The faster we open and close the sash, the more turbulence we create. 

When working with a fume hood, slow and deliberate movements are the best.

At the end of the day, there is only one way you can know how your fume hood is performing and that is to have it tested As Used in your lab. 


Face Velocity versus Containment

There is a major misconception that if the face velocity shown on the alarm reads 100 feet per minute the hood must be working.  

But there is NO direct correlation between face velocity and containment.  

Let me repeat that statement

There is NO direct correlation between face velocity and containment.  

Just because a fume hood has 100 fpm face velocity does not mean it is working safely.

In fact when doing ASHRAE 110 testing,  over half of the hoods that failed containment testing had the specified face velocity.

While face velocity is a component of containment, it is only one component.  

How fast the air is going into the hood, which is the face velocity merely determines how big the rapids or turbulence will be.  

Raw face velocity is more about turbulence. 

The direction of the airflow is determined by the differential pressure.  

The stronger the negative pressure at the back of the fume chamber, the more aggressively the air moves into the low-pressure parts of the hood - which is mainly the plenum.

Again the plenum is the space created between the baffles and the back wall of the hood.

This special hood was constructed with clear baffles so you can see inside the plenum.

By using colored smoke, you can see the path the smoke takes to the exhaust duct. 

Face velocity is a useful number and provides some indication of overall performance.

You should only work at a hood that has a velocity alarm and the face velocity is within the specified range. 
Here in the US, we will find hoods that were designed to operate between 60fpm and 100fpm. 

Occasionally you will find hoods with lower or higher design velocities.  

While 100 fpm has a logical basis, as the face velocity goes higher, particularly over 125fpm, excess turbulence is created, reducing the hood's ability to contain.  

So more isn’t necessarily better.  

And too low, Is more likely to have external movements cause loss of containment.

Again the way to know how a fume hood is performing at a specified velocity can be determined by ASHRAE 110  as-used (AU) testing.

It often helps to visualize the airflow within the hood.  Not everyone has access to a fog machine, so another way to visualize airflow  is to pour warm water on dry ice. 

 It is often said that if you can’t see a problem, you can’t fix it.   

Fume Hood Certified is starting to post videos on our YouTube channel that will help you visualize some of the issues we have been discussing.  

Today most labs have VAV or variable air volume controls.  

Remember, for your hood to function safely the room has to stay balanced.  
That means the room remains slightly negative to the hallway and the amount of supply air at any given time is roughly equal to the amount of air the fume hood is exhausting.

So we have both exhaust and supply volumes to manage.  

Let’s say the hood has an experiment running and you have the sash closed.  

The amount of air the hood is exhausting is low.  The room supply air terminal has closed down to keep the supply air coming in equal to the amount of air the hood is exhausting. 

Now you open the sash and the exhaust volume of the hood suddenly increases, 

But there is a lag time before the room controller recognizes this volume or pressure shift and readjusts the supply to get back to a stable condition.  

During this period of lag and instability, a fume hood is much more likely to have a loss of containment due to the differential pressure changes. 

Yet, it is highly possible the face velocity has remained within an acceptable range.  
Remember what I said, there is NO direct correlation between face velocity and containment.  

Before there were digital velocity alarms, hoods were often equipped with manometers.  These gauges read differential pressure.  

But because these readings were always fluctuating with the pressure changes, which was actually a better indication of containment, 

it was difficult for the User to determine when the hood was performing safely and when it wasn’t.  

The thinking was that a velocity monitor would be a better solution since it could be programmed to alarm when the face velocity was outside the specified range.

Yet over the years, I have seen hundreds of people working with hoods that were alarming and rather than addressing the problem they would simply mute the alarm.

While the velocity alarm is not perfect, it is better than nothing and should be used as indication of basic performance between regular hood tests and certifications.

Making sure your hood is working.

We have come somewhat full circle; how do you make sure your fume hood is working?  

The simple truth is that there is really no way to know for sure.  

But there are things we can and should do to improve the likelihood of it performing safely.  

The first thing you can do is understand what “is your hood working” means and be aware of signs that it isn’t.  

When you identify those red flags they should be reported so they can be addressed.  

Next, you should know who is responsible for your hood’s performance.  

If someone isn’t responsible, no one is responsible. 

Each organization can have a different organizational structure.  
Responsibility for safe performance can range from the user to mantiantence, to EHS, to a safety officer or a compliance officer. 
Again this hybrid nature of the fume hood, both an engineering control and a PPE tends to cloud responsibility.  

Do your part in ensuring you have had proper training on the use of the hood and make sure you are practicing best practices when you are working in the hood.

Like most safety equipment, there should be a regular checkup for the fume hood, at least annually and ideally twice a year.  
A trained inspector can spot many problems that can be addressed.  Often easy fixes.

Then there are self-audits the user and lab manager can do.

Internal EHS and Facilities people can perform various types of testing.  And when problems are suspected, an outside tester or troubleshooter can be brought in.  

You shouldn’t work at a fume hood that doesn’t have a current certification sticker on it.  

And remember that the hood is also a part of a large mechanical system that requires regular calibration and maintenance.  

How often do they check the fire sprinkler system in your building?  A laboratory ventilation system is much more complex and just as important.  
Shouldn’t the ventilation system be inspected as frequently?

With the fire sprinkler system, you generally only see the sprinkler heads in the lab.  You don’t see all those pipes, controls and pumps that make it work.  
Same is true with the laboratory ventilation system.  All you see is the hood itself.  
You don’t see all the things above the ceiling and on the roof that are necessary for the fume hood to work correctly.

So, having a safe and energy efficient hood is a holistic endeavor that requires many people working together.  

But at the end of the day, it is the user’s health and safety that is at risk.  As a frontline person make sure you are doing your part.  
By understanding the fume hoods function and knowing the signs that your fume hood may not be working,  you are in a much better position to get problems corrected. 

You are a critical part of the overall system.

The need for training

PPE as simple as a N95 Mask requires fitting and training, so why not a fume hood.  

The answer comes back to that  hybrid nature of a fume hood, it is more often classified as a ‘Engineering Control” rather than a PPE.  
As such, a hood’s performance is often considered outside the control of the user.  

Yet after testing thousands of hoods, we know that at least 25% of fume hood performance failures are caused by user work practices.  As a user, you are an important part of the system.
When giving workshops and speaking at conferences, 

I often say you may not have the budget to replace poorly performing hoods, 
you may not have the budget to fix poorly performing Laboratory Ventilation systems, 
but the one thing everyone can do is train the users.  

Training alone could fix a quarter of the problems and would  probably reduce the number injuries and deaths caused by preventable accidents.

Labs are inherently dangerous, with numerous hazards, but training can make them safer places to work.  

Fume Hood Certified is an education and training  company.  Our mission is to make labs safer one fume hood at a time. We are continually developing new courses that focus on the real-world needs of users.  If you have a special situation or would like to be a program tester for our courses, please contact us. 

Due to the complexity of laboratory ventilation systems, there are many things that impact performance that are outside the user’s control.  

We want you to be aware of what you can control and to at least understand what a properly functioning hood looks like, so if you feel your hood may not be functioning properly, you can report it to the proper people to have it checked.  Like the driver of the car that is making that funny click click click sound.

You might also compare this to a medical event.  You have a fever and feel horrible, you may have no clue what the root cause of the symptoms is, but these symptoms tell you something is wrong and that you should seek medical attention.  

Just because there is a fume hood in your lab, don’t automatically assume it is working properly. 
Situational awareness is key to your safety and wellbeing.

That covers the topics we outlined in the beginning and I want to thank everyone for spending this time with me.  

I would like to thank Lab Manager for putting this webinar together and to all of you for giving me the opportunity to share some of my fume hood experience with you.   

But we are not quite finished.  I would be happy to answer a few questions.  

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