SECTION 6B: Controlling Chemical
Fume Hoods and Laboratory Ventilation (top)
One of the primary safety devices in a laboratory is a chemical fume hood.
A well-designed hood, when properly installed and maintained, can offer
a substantial degree of protection to the user, provided that it is used
appropriately and its limitations are understood.
This section covers a number of topics aimed at helping laboratory workers
understand the limitations and proper work practices for using fume hoods
and other local ventilation devices safely.
There are basically two types of fume hoods at Princeton, they are:
Constant volume – where the exhaust flowrate or quantity of air pulled through the hood is constant. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the hood increases proportionally. Thus, higher face velocities can be obtained by lowering the sash.
And variable air volume (VAV) - where the exhaust flowrate or quantity of air pulled through the hood varies as the sash is adjust in order to maintain a set face velocity. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the hood stays the same while less total air volume is exhausted.
How a Fume Hood Works (top)
fume hood is a ventilated enclosure in which gases, vapors and fumes are
contained. An exhaust fan situated on the top of the laboratory building
pulls air and airborne contaminants in the hood through ductwork connected
to the hood and exhausts them to the atmosphere.
The typical fume hood found in Princeton University laboratories is equipped
with a movable front sash and an interior
baffle. Depending on its design, the
sash may move vertically, horizontally or a combination of the two and
provides some protection to the hood user by acting as a barrier between
the worker and the experiment.
The slots and baffles direct the
air being exhausted. In many hoods, they may be adjusted to allow the
most even flow. It is important that the baffles are not closed or blocked
since this blocks the exhaust path.
The airfoil or beveled frame around
the hood face allows more even airflow into the hood by avoiding sharp
curves that can create turbulence.
In most hood installations, the exhaust flowrate or quantity of air pulled
through the hood is constant. Therefore, when the sash is lowered and
the cross-sectional area of the hood opening decreases, the velocity of
airflow (face velocity) through the hood increases proportionally. Thus,
higher face velocities can be obtained by lowering
Fume Hoods (top)
A fume hood is used to control exposure of the hood user and lab occupants
to hazardous or odorous chemicals and prevent their release into the laboratory.
A secondary purpose is to limit the effects of a spill
by partially enclosing the work area and drawing air into the enclosure
by means of an exhaust fan. This inward flow of air creates a dynamic barrier
that minimizes the movement of material out of the hood and into the lab.
In a well-designed, properly functioning fume hood, only about 0.0001%
to 0.001% of the material released into the air within the hood actually
escapes from the hood and enters the laboratory.
When is a Fume Hood Necessary?
The determination that a fume hood is necessary for a particular experiment
should be based on a hazard analysis of the planned work. Such an analysis
- A review of the physical characteristics, quantity and toxicity
of the materials to be used;
- The experimental procedure;
- The volatility of the materials present during the experiment;
- The probability of their release;
- The number and sophistication of manipulations; and
- The skill and expertise of the individual performing the work.
Good Work Practices (top)
The level of protection provided by a fume hood is affected by the manner
in which the fume hood is used. No fume hood, however well designed, can
provide adequate containment unless good laboratory practices are used,
- Adequate planning and
preparation are key.The
hood user should know the Standard
Operating Configuration (SOC) of the hood and should design
experiments so that the SOC can be maintained whenever hazardous
be released. The SOC refers to the position of the sash. A schematic
drawing of the SOC is displayed on the front of each chemical fume
- Before using the hood, the user should check
survey sticker to determine where the sash should be positioned for optimum containment for that particular unit.
- The hood user should also check
gauge or other hood performance indicator and
compare its reading to the reading indicated on the hood survey
the reading differs significantly (15% or more for a magnehelic
gauge) from that on the sticker, the hood may not be operating
Items contaminated with odorous or hazardous materials should be removed
from the hood only after decontamination or if placed in a closed outer
container to avoid releasing contaminants into the laboratory air.
When using cylinders containing highly toxic or extremely odorous gases,
obtain only the minimal practical quantity. Consider using a flow-restricting
orifice to limit the rate of release in the event of equipment failure.
In some circumstances, exhaust system control devices or emission monitoring
in the exhaust stack may be appropriate.
To optimize the performance of the fume hood, follow
the practices listed below:
- Mark a line
with tape 6 inches behind the sash and keep all chemicals and equipment
behind that line during experiments. This will help to keep materials
from escaping the hood when disturbances like air currents from people
walking past the hood, etc., interfere with airflow at the face of the
Bad placement of materials.
Good placement of materials.
Best placement of materials.
- Provide catch basins
for containers that could break or spill, to minimize the spread of
- Keep the sash completely
lowered any time an experiment is in progress
and the hood is unattended. Note: Lowering the sash
not only provides additional personal protection, but it also results
in significant energy conservation.
- Never use a hood to control exposure to hazardous substances without
first verifying that it
is operating properly.
- Visually inspect the
baffles (openings at the top and rear of the hood)
to be sure that the slots are open and unobstructed. For optimum
performance, adjust the baffles when working with high temperature equipment
and/or heavy gases or vapors. See figure below for suggested
Normal baffle position - all open.
Slot position for high temperature equipment, such as hot
plates. Lower slot is minimized since heated vapors tend to
Slot position for heavy gases and vapors. Upper slot
- Do not block slots. If
large equipment must be placed in the hood, put it on blocks
to raise it approximately 2 inches above the surface so that air
may pass beneath
it. See figure below.
Poor placement of large equipment
Good placement of large equipment
- Place large or bulky
equipment near the rear of the fume hood.
Large items near the face of the hood may cause excessive air turbulence
and variations in face velocity.
- Do not use the hood
as a storage device. Keep
only the materials necessary for the experiment inside of the hood.
If chemicals must be stored in the hood for a period of time, install
shelves on the sides of the hood, away from the baffles. See Use
of Hood as a Storage Device for more information.
- Keep the hood sash clean
- Check area around the
hood for sources of cross drafts, such as open windows,
supply air grilles, fans and doors. Cross drafts may cause turbulence
that can allow leaks from the hood into the lab.
- Extend only hands and
arms into the hood and avoid leaning against it. If
the hood user stands up against the face of the hood, air currents
by turbulent airflow may transport contaminants into the experimenter's
- Clean all chemical residues from
the hood chamber after each use.
- All electrical devices
should be connected outside the hood to avoid
electrical arcing that can ignite a flammable or reactive chemical.
- DO NOT USE A HOOD FOR
ANY FUNCTION FOR WHICH IT WAS NOT INTENDED. Certain
chemicals or reactions require specially constructed hoods. Examples
are perchloric acid or high pressure
reactions. Most special use hoods are labeled with the uses for
they are designed. See Common Misuses of Fume Hoods
for more information.
Common Misuses and Limitations (top)
Used appropriately, a fume hood can be a very effective device for containment
hazardous materials, as well as providing some protection from splashes
and minor explosions. Even so, the average fume hood does have several limitations.
- Particulates: A fume hood
is not designed to contain high velocity releases of particulate contaminants
unless the sash is fully closed.
- Pressurized systems: Gases
or vapors escaping from pressurized systems may move at sufficient
to escape from the fume hood.
- Explosions: The hood is
not capable of containing explosions, even when the sash is fully
If an explosion hazard exists, the user should provide anchored
barriers, shields or enclosures of sufficient strength to deflect
or contain it.
Such barriers can significantly affect the airflow in the hood.
A conventional fume hood must not be used for perchloric
acid. Perchloric acid vapors can settle on ductwork, resulting
in the deposition of perchlorate crystals. Perchlorates can accumulate
on surfaces and have been known to detonate on contact, causing
injury to researchers and maintenance personnel. Specialized
perchloric acid hoods, made of stainless steel and equipped with
a washdown system
must be used for such work.
- Air Foil Sills: Many fume
hoods are equipped with flat or rounded sills or air foils which direct
the flow of air smoothly across the work surface. Sills should not be
removed or modified by the hood user. Objects should never be placed
on these sills. Materials released from containers placed on the sills
may not be adequately captured. In addition, an object on the sill may
prevent the quick and complete closure of the sash in an emergency.
- Spill Containment Lips: Most
modern fume hoods have recessed work surfaces or spill containment
to help contain minor liquid spills. In many cases, these lips
are several inches wide. Containers of liquids should not be placed
on the hood
- Horizontal Sliding Sashes: The
hood user should never remove sliding sashes. Horizontal
sash hoods are designed and balanced with no more than
half the face open at any time. Removal of sashes may reduce
below acceptable levels.
- Tubing for Exhaust: Tubing
is frequently used to channel exhaust to the hood from equipment
some distance away. This is not an effective control method.
- Connections to the Exhaust System:
Occasionally, a researcher may need local exhaust ventilation
other than that provided by an existing fume hood. A new device
may not be
connected to an existing fume hood without the explicit approval
of the department's facilities manager or Special Facilities
Adding devices to even the simplest exhaust system without adequate
evaluation and adjustment will usually result in decreased performance
of the existing hood and/or inadequate performance of the additional
- Microorganisms: Work involving
harmful microorganisms should be done in a biosafety cabinet, rather
than a chemical fume hood. See the Biosafety
Manual for more information.
- Highly Hazardous Substances:
A well designed fume hood will contain 0.999 - 0.9999% of the contaminants
released within it when used properly. When working with highly dangerous
substances needing more containment than a fume hood offers, consider
using a glove box.
- Pollution Control:
An unfiltered fume hood is not a pollution control device. All contaminants
that are removed by the ventilating system are released directly
the atmosphere. Apparatus used in hoods should be fitted with condensers,
traps or scrubbers to contain and collect waste solvents or toxic
- Waste Disposal: A fume hood should
not be used for waste disposal. It is a violation of environmental regulations
to intentionally send waste up the hood stack. As described above, the
hood is not a pollution control device.
Fume Hood as a Storage Device
Fume hoods are designed specifically to provide ventilation for the protection
of lab occupants during chemical manipulations. The airflow they provide
is greatly in excess of that needed for storage of closed containers of
even the most toxic of volatile materials. Storing materials in this way
is, therefore, a misuse of an expensive piece of equipment.
In general, the storage of chemicals in fume hoods is strongly discouraged.
Materials for more information about proper storage of flammable,
toxic, or odorous chemicals.
The realities of available space and equipment in some laboratories may
make it difficult or impossible to completely prohibit the use of hood
workspaces for storage. In such a case, the following general policy is
Hoods Actively in Use for
Storage of materials should be minimized or eliminated altogether. Materials
stored in the hood can adversely affect the containment provided. In addition,
the hood is frequently the focus of the most hazardous activities conducted
in the laboratory. The presence of stored flammable
or volatile, highly
toxic materials can only exacerbate the problems resulting from an explosion
or fire in the hood. Even if they are not directly involved in such an event,
attempts to control or extinguish a fire may result in the spilling of stored
Hoods Not in Active Use
Materials requiring ventilated storage (e.g., volatile and highly toxic,
or odorous substances) may be stored in a hood if they are properly segregated
and the hood is posted to prohibit its use for experimental work.
All fume hoods at Princeton University are equipped with some type of continuous
airflow monitoring device, either in the form of a magnehelic gauge, a color
coded flow indicator or a face velocity monitor. Some are equipped with
Each hood also has a survey sticker with important information to help
determine whether the particular hood is functioning properly and is appropriate
for the work to be performed.
Static Pressure Gauge (Magnehelic)
Most fume hoods on campus are equipped with static pressure gauges that
measure the difference in static pressure across an orifice in the duct,
or between the laboratory and the fume hood exhaust duct. Most of the devices
are aneroid pressure gauges, such as magnehelics, that are mounted on the
front of the hood above the sash.
The gauge is a flow rate indicator with a scale that reads in units of
pressure, rather than velocity. Changes in the magnehelic reading are
not linearly proportional to changes in face velocity; therefore it should
only be used as an index of hood performance.
The magnehelic gauge reading at the time of the most recent hood survey
is shown on each fume hood evaluation sticker. A difference of 15% or
more in the magnehelic reading from that shown on the sticker is an indication
that the flow rate in the duct, and thus the face velocity, may have changed
significantly since the last survey. If the user notices such a change,
or has any other reason to suspect that the hood is not operating properly,
contact EHS at 258-5294 for a re-survey of the hood.
Color Coded Flow Indicators
Some hoods are equipped with FlowSafe devices, rather than magnehelic gauges.
This device constantly measures the face velocity of the hood and, using
a needle that either points to green (for good) or red, indicates whether
or not the hood is functioning properly.
Face Velocity Monitors
Some of the newer hoods have constant
face velocity measuring devices. An LED readout of the face velocity is
found on the device on the top corner of the hood opening. The readout indicates
the actual face velocity of the hood, and should be a negative number, to
reflect that the direction of flow is negative, into the hood, rather than
positive, out of the hood.
Many hoods in areas of Frick, Moffett and E-Quad are equipped with sash
position alarms. These hoods are designed to operate with the hood sash
lowered to approximately 20 inches above the base of the hood, in order
to conserve energy by exhausting air at a lower flow rate than would otherwise
When the sash is raised above 20 inches, a buzzer will sound and a red
light will begin flashing, alerting the hood user and other laboratory
occupants that the hood face velocity is now likely to be below 100 feet
per minute. In the event that the sash must be raised above 20 inches,
such as when large equipment must be installed or removed, the buzzer
can be turned off manually, but the light will continue flashing until
the sash is lowered below the 20 inch mark.
All chemical manipulations performed in an alarm equipped hood should
be done with the sash opening at 20 inches or less.
Every chemical fume hood on campus should have a survey sticker affixed
to the front of the hood in a conspicuous location. The sticker contains
basic information about hood performance as of the most recent survey and
should be consulted each time the hood is used.
The EHS Hood Number
is a unique identifier for the particular hood. Refer to this number when
discussing problems with a particular hood.
The Inspection Sticker is aligned on the hood so the arrow is in the proper location for the maximum safe sash position.
The Flow Monitor Reading
is the reading of the magnehelic gauge or other continuous monitoring
device at the time of the survey. Where the hood has two possible exhaust
rates, as is the case for some hoods in Frick, the reading corresponding
to each rate may be indicated as, for example, 0.31/0.42.
The Inspected on
date is the date of the last hood survey. Hoods that have not been surveyed
within the past year should not be used until tested by EHS.
line gives the name of the EHS technician who surveyed the hood.
If hood performance is judged to be unsuitable for use with hazardous
chemicals, a sticker with this information is placed on the hood instead
of the survey sticker.
Do not use a hood that has no survey sticker. If a survey is needed,
call EHS at 258-5294.
Evaluation and Maintenance
EHS surveys each fume hood annually. The face velocity of the fume hood
is measured with the sash in the Standard
Operating Configuration (SOC). The inspection sticker is positioned on the hood so the arrow is in the proper location for the maximum safe sash position.
of the continuous monitoring device is recorded on the hood sticker.
After each performance survey, a written report of the results is furnished
to the individual responsible for the hood (e.g., the Principal Investigator
or laboratory manager), the Chemical
Hygiene Officer for the department, and the Special Facilities staff
for the laboratory building.
When Problems are Noted
There are several factors that can affect the performance of the hood, resulting
in low face velocity or turbulent airflow. These include mechanical problems
or exhaust slots blocked by large objects or excessive storage.
If a problem is found during the hood survey, a written notice will be
provided on-site to the laboratory or taped to the sash of the fume hood.
If the problem requires the need for work practice changes (e.g., blocked
exhaust slots or excessive storage), the laboratory worker should make
the recommended changes and call EHS at 258-5294 to have the hood resurveyed.
If maintenance is necessary, the laboratory worker may send a copy of
the written notice to the building Special Facilities staff to request
maintenance. EHS does not initiate maintenance or ensure that it is completed.
Special Facilities will contact EHS when the work is complete to have
the hood resurveyed.
Providing maintenance for fume hoods is a function of the Facilities Department,
and is performed by Special Facilities personnel and the MacMillan shops.
Since the hood user is the person most aware of how a hood is being used
on a day to day basis, it is the responsibility of the hood user to determine
that maintenance is necessary and to request that it be performed.
If a hood user believes that the hood is not performing adequately, the
following steps should be taken:
- An inadequate face velocity may result from obstructions to the
airflow in the hood. These may be caused by large quantities of equipment
the hood or by paper or other material drawn into the exhaust
slots. The user should first check for such obstructions and remove
- The user may obtain initial maintenance through Special Facilities.
If Special Facilities is unable to correct the problem, they will
assistance from the MacMillan maintenance shops.
- The hood sash should be lowered until repairs are complete. Place
a sign on the hood reminding users not to use the hood.
- If maintenance efforts are not sufficient to correct the deficiency,
engineering changes may be necessary. When notified of such a situation,
the user or a department representative should request an evaluation
of the problem by the Facilities Engineering Department.
Exhaust Systems (top)
Many laboratories use equipment and apparatus that can generate airborne
contaminants, but cannot be used within a fume hood. Examples include gas
chromatographs, ovens, and vacuum pumps.
Other types of local exhaust ventilation systems may be required to control
contaminants generated by these operations. Such systems must not be installed
without explicit approval of the building facility manager, Facilities
Engineering and/or maintenance personnel. See Common
Misuses of a Fume Hood for more information.
An elephant trunk is a flexible duct or hose connected to an exhaust system.
It can only capture contaminants that are very close to the inlet of the
hose, typically less than a distance equal to one half of the diameter of
Elephant trunks can be effective for capturing discharges from gas chromatographs,
pipe nipples or the end of tubing. However, the effectiveness of the elephant
trunk should be carefully evaluated before they are used to control releases
of hazardous substances.
A canopy hood in a laboratory is constructed in a similar fashion to the
overhead canopy hoods seen in kitchens. In order for the canopy hood to
be able to capture contaminants, the hood requires a relatively large volume
of air movement, making them somewhat costly to operate. The canopy hood
works best when the thermal or buoyant forces exist to move the contaminant
up to the hood capture zone.
One of the biggest problems with canopy hoods is that, in most cases,
they are designed such that the contaminated air passes through the individual's
breathing zone. The airflow is easily disrupted by cross currents of air.
For the most part, canopy hoods should only be used for exhaust of non-hazardous
There are many types of slot hoods, each suited for different types of operations.
In general, a slot hood requires less airflow than a canopy hood and is
much more effective than an elephant trunk or canopy hood, when installed
Slot hoods are best used for operations that require more working room
than a fume hood and where a limited number of low toxicity chemicals
are used. The placement of the opening(s) and the velocity of airflow
are based on the application, particularly dependent upon the vapor density
of the chemical(s).
Examples of good uses for slot hoods are darkrooms and acid dipping operations.
Downdraft hoods or necropsy tables are specially designed work areas with
ventilation slots on the sides of the work area. This type of system is
useful for animal perfusions and other uses of chemicals with vapor densities
heavier than air.
or odorous gases should be used and stored in gas cabinets.In the event
of aleak or rupture, a gas cabinet will prevent the gas from contaminating
Gas cabinets should be connected to laboratory exhaust ventilation using
hard duct, ratherthan elephant tubing, since such tubing is more likely
to develop leaks. Coaxial tubing should be used for delivering gas from
the cylinder to the apparatus. Coaxial tubing consists of an internal
tube containing the toxic gas, inside another tube. In between the two
sets of tubing is nitrogen, which is maintained at a pressure higher than
the delivery pressure of the toxic gas. This ensures that, in the event
of a leak in the inner tubing, the gas will not leak into the room.
There are two general types of glove boxes, one operating under negative
pressure, the other operating under positive pressure. Glove boxes consist
of a small chamber with sealed openings fitted with arm-length gloves. The
materials are placed inside the chamber and manipulated using the gloves.
A glove box operating under negative pressure is used for highly toxic
gases, when a fume hood might not offer adequate protection. A rule of
thumb is that a fume hood will offer protection for up to 10,000 times
the immediately hazardous concentration of a chemical. The airflow through
the box is relatively low, and the exhaust usually must be filtered or
scrubbed before release into the exhaust system.
A glove box operating under positive pressure may be used for experiments
that require protection from moisture or oxygen. If this type of glove
box is to be used with hazardous chemicals, the glove box must be tested
for leaks before each use. A pressure gauge should be installed to be
able to check the integrity of the system.
A conventional fume hood should not be used for work with viable biological
agents. A biosafety cabinet is specially designed and constructed to
offer protection to both the worker and the biological materials.
Similarly, a biosafety cabinet should generally not be used for work
with hazardous chemicals. Most biosafety cabinets exhaust the contaminated
air through high efficiency particulate air (HEPA) filters back into the
laboratory. This type of filter will not contain most hazardous materials,
particularly gases, fumes or vapors. Even when connected to the building
exhaust system, a ducted biosafety cabinet may not achieve a face velocity
of 95 - 125 feet per minute, making it inappropriate for use with hazardous
Use of a "ductless fume hood" is strongly discouraged. These devices
work by using a fan to draw air into a chamber, through one or more
and back into the laboratory. EHS and several professional safety and engineering organizations do
not recommend the use of ductless fume hoods for several reasons. First,
it is difficult to determine whether the filters are functioning adequately
or need to be changed; thus, the potential for recirculating toxic materials
into the laboratory is significant. In the event of a chemical spill,
the hood is usually not able to contain the spilled material or the potentially
high concentrations of chemical vapors.
Second, the face velocity of the hood is normally below 80 feet per minute.
The hood is normally designed such that the air does not flow smoothly
and evenly through the hood. Both of these characteristics make it likely
for disruption of airflow or turbulence, causing unfiltered air to leak
into the laboratory.
Clean benches are similar to appearance as a fume hood however do not exhaust air from the laboratory. A clean bench is a device that draws air from the lab through a HEPA filter and vents the filtered air downwards onto a work surface to keep the materials within free from particulate contamination. These devices are not to be used with hazardous materials as they provide no personal protection. Do not store materials on top of this hood as this will block the filter, overload the motor, and provide poor product protection.
6C: Personal Protective Equipment
6A: Controlling Chemical Exposures - General