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.
Chemical fume hoods are inspected annually by the Office of Environmental Health and Safety. Labels are affixed to the fume hood that indicate the sash height for adequate containment. All fume hoods are equipped with a magnehelic gauge or other continuous monitoring device to monitor the flow of air. Proper readings of the monitoring device can also be found on the inspection label.
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
3. Keep the sash completely lowered anytime no “hands-on”
part of an experiment is in progress or whenever the hood is on and
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
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
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 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 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 (see picture below) 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.