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SECTION 4: Chemical Fume Hoods
One of the primary safety devices in laboratories where chemicals are
used is the laboratory fume hood. A well designed fume hood, when properly
installed and maintained, can provide a substantial degree of protection
for the experimenter, provided its proper use and limitations are understood.
The determination that a hood is necessary for a particular experiment
should be based on a hazard analysis of the experiment under consideration.
Such an analysis would include 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, and the number and sophistication of manipulations,
etc. More subjective factors such as the skill and expertise of the individual
performing the work should also be considered.
The EHS categorizes chemical fume hoods as described below, primarily
on the basis of face velocity.
Class A: Appropriate for most laboratory
operations requiring local exhaust ventilation to control the exposure
of lab personnel to hazardous materials; however, if absolute containment
is required, the use of a glove box should be considered. (Average face
velocity between 95 and 125 feet per minute)
Class B: May not be appropriate for some
laboratory operations. The hood user should evaluate its suitability
for current and anticipated uses on a case-by-case basis.
Class C: Not suitable for the manipulation
of hazardous materials.
Not all class B and C hoods must be brought up to the class A standard.
Many less hazardous operations can be performed in a hood not meeting
the class A criteria. It is the responsibility of the hood user to determine
whether corrective action is required, based on the materials used and
operations performed in the hood. EHS can provide assistance in this evaluation.
EHS is not responsible for initiating fume hood maintenance or ensuring
that it proceeds to completion. EHS functions as a technical advisor to
the hood user, the department and maintenance staff.
The Hood Inspection Sticker (top)
Each laboratory fume hood is surveyed by EHS annually. At the completion
of each survey, an inspection sticker is affixed to the hood. The hood
class is indicated as the background of the sticker (e.g., a large A,
B or C background). The following information may be found on the sticker:
Fume Hood Work Practices (top)
The protection afforded by a fume hood is only as good as the work practices
of the hood user. The following are general guidelines to be followed
when working in the hood:
1. Know the toxic properties of the chemicals with which you work.
Be able to identify signs and symptoms of overexposure.
2. 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 vapors from escaping the hood when air currents from people
walking past the hood, etc. interfere with airflow at the face of
the hood.
3. Keep the sash completely lowered anytime no “hands-on”
part of an experiment is in progress or whenever the hood is on and
unattended.
4. Never operate the hood unless there is some visual indication that
the hood is operating. A tissue or Kimwipe® taped to the sash
or inside the hood provides a good indicator of airflow.
5. Check the magnehelic gauge reading and compare it with the reading
documented on the hood inspection sticker. If the reading differs
significantly from that on the sticker, the hood may not be operating
sufficiently.
6. The hood is not a substitute for personal protective equipment.
Wear gloves, safety glasses, etc., as appropriate.
7. Visually inspect the baffles (openings at the top and rear of the
hood) to be sure the slots are open and unobstructed.
8. Do not block baffles. If large equipment is in the hood, put it
on blocks to raise it approximately two inches so that air may pass
beneath it.
9. Do not use the hood as a storage cabinet. Keep only the materials
necessary for the experiment inside the hood. If chemicals need to
be stored in the hood for a period of time, install shelves on the
sides of the hood, away from the baffles.
10. Keep the sash clean and clear.
11. Clean all chemical residues from the hood chamber after each use.
12. All electrical devices should be connected outside the hood to
avoid sparks which may ignite a flammable or explosive chemical.
13. DO NOT USE A HOOD FOR ANY FUNCTION FORWHICH 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 as to the uses for which they are designed.
Radioactive materials may only be used in hoods specially labeled
for radioactivity. If you have any questions about the capabilities
of a particular hood, contact Joan
Hutzly at 8-6251.
Other Laboratory Ventilation Equipment (top)
Be aware of ventilation equipment in a laboratory that are not considered
chemical fume hoods, such as laminar flow hoods, clean benches, biosafety
cabinets or elephant trunks.
A laminar flow hood has air that washes down from top to bottom and
are used to protect the working materials from contamination. These
do not provide protection for the worker and should not be used for
the manipulation of hazardous materials. Laminar flow hoods are not
tested by EHS.
Similarly, a biosafety cabinet is used to protect the material from
contamination, not the user. 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 chemicals.
Biosafety cabinets must be certified by an outside firm annually if
they are being used to protect the user. If they are only being used
for product protection, then the researcher must decide how often the
cabinet gets certified.
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