Laboratory
Hoods and Ventilation Enclosures
Problems and Pitfalls
Douglas B. Walters, Ph.D., CSP, CCHO; President, KCP, Inc.; 6807 Breezewood Rd;
Raleigh, NC 27607; (919) 851-1465; waltersdb@earthlink.net
Ventilation is
the first line of defense used to protect workers in chemistry laboratories.
This discussion provides insight on the design and operation of laboratory
hoods, and specialized ventilated enclosures.
Emphasis is on the do's and don't for proper control and containment of
the potentially hazardous materials present in chemistry laboratories. Included
is a discussion of basic hood and ventilation design, where hoods should be
located, and how they should be operated to optimize containment. Routine testing, monitoring and preventive
maintenance is discussed. Specialized
vented enclosures such as, balance enclosures, microscope enclosures, and
enclosures for high-throughput, automated equipment and other equipment is also
discussed. The complicated engineering
control known as a laboratory chemical hood is discussed in clear,
easy-to-understand language so that everyone from the beginner to those familiar
with the dynamics of airflow will be enriched from the discussion.
The
key to safe handling of chemicals is the use of properly designed, installed
and functioning engineering controls, i.e., laboratory ventilation, laboratory
chemical hoods, and vented enclosures.
The two most important references for anyone seriously involved with
laboratory ventilation and hoods are the most recent versions of the first two
references cited at the end of this paper (1,2). The references also list several easy-to-read, hard copy
references on laboratory hoods and ventilation (3-8). In addition,
there are several workbooks available by Jeff Burton (9). Finally, several Internet sites are also
listed in the references (10-24).
Because of the constantly changing nature of the internet and the
websites, it is always advisable to check for new sites by using a search
engine like google (25) and searching under fume hoods, laboratory chemical
hoods, etc. Note much of the Internet
information is not a primary or secondary reference source and particular
attention should be given to validity of the information.
Laboratory operations that involve or give off potentially hazardous chemicals (e.g., toxic, flammable, explosive, odiferous) should be performed in a properly functioning hood or vented enclosure. By definition a hood is any opening that exhausts air. A laboratory chemical hood is different from a biological safety cabinet, laminar flow hood, glove box, or vented enclosure. This Manual only addresses laboratory chemical hoods and vented enclosures.
The
first step is to be sure the general laboratory ventilation system is working
correctly. Each laboratory is different
and has unique needs. The architects
and engineers that design and build such facilities should be experienced in
laboratory ventilation. Testing and
evaluation should be conducted by professionals/organizations who are familiar
with the safety and the engineering aspects of the system and who understand
the requirements of the laboratory users.
All hoods and vented enclosures should be certified, as described below,
when they are installed, and commissioned before they are used.
There
are a few basic rules to remember about ventilation design. Incoming, (i.e., supply, or makeup) air must
be sufficient to replace and compensate for the air exhausted, otherwise hood
containment will be compromised and contaminants will escape. Exhaust fans should be outside the building
to ensure that hood ductwork is under negative pressure. This is done so that if a leak develops in
the ductwork the exhaust will not escape back into the building. Exhaust fans should be outside and as far as
possible and downwind from all air intakes.
Hood exhaust air should not be recirculated. Also limit the horizontal runs of ductwork and make it as short
as possible. Hoods should not be
piggybacked into other hood ducts without increasing the fan capacity.
Hoods
should not be located in a major traffic flow aisle, near open windows,
doorways, next to desks, near air conditioner ducts or other sources of
cross-drafts. Locate hoods as far to
the rear as possible in a laboratory to assure emergency escape routes. Where possible locate hoods at the dead end
of aisles to minimize traffic flow in front of the hood.
Careful
selection, installation, maintenance, and evaluation of laboratory chemical
hoods and vented enclosures are imperative.
Laboratory chemical hoods are incorrectly called fume hoods, because
they control other aerosols besides just fumes (e.g., gases, vapors, mists,
fogs, particulates). Always consult a
qualified, and trained safety professional with specific experience whenever
hoods are selected, installed and used.
Hazardous
chemicals should be weighed and handled only where there is adequate
ventilation, such as in a hood or vented enclosure. Many chemicals present hazards of various types. Hazardous chemicals should be weighed or
used in a hood or vented enclosure and not on the open bench where dispersion
into the laboratory air is possible.
Appropriate protective equipment, specific to the particular hazard,
should always be used. Always consult
your Chemical Hygiene Officer or Safety Office and read the Material Safety
Data Sheet (MSDS) before weighing and handling any hazardous chemical.
Workers
should never be exposed to hazardous chemicals. Adequate ventilation is defined as ventilation that is sufficient
to prevent worker exposure. Chemical
concentrations should always be below the OSHA Permissible Exposure Limit
(PEL), NIOSH Recommended Exposure Limit (REL), ACGIH Threshold Limit Value
(TLV), or AIHA Worker Environmental Exposure Level (WEEL) for those chemicals
for which these values have been established.
Exposure should always be kept to a minimum. Remember smell has little to do with dose. Just because you can’t smell it doesn’t mean
you’re not being exposed. The vapors of
many chemicals can be at a hazardous concentration level without any noticeable
odor. The same applies to dusts, mists and fumes. The body can sometimes breakdown chemicals by metabolism into
hazardous metabolites. Similarly,
by-products of chemical reactions can be extremely hazardous. Planning for the handling and control of
these toxic metabolites and by-products should be part of the experimental
procedure. Hence, always enclose as
much of the work being performed as possible in a hood or vented enclosure.
All
engineering controls are not created equal.
Many hoods have specific functions.
For example, canopy hoods (like those over a kitchen stove) should only
be used when the effluents from the operation are forced (e.g., by heating)
upward toward the hood. Otherwise the
exhaust is drawn across worker and his/her breathing zone no matter how careful
the work practices. Slot hoods at the
back of laboratory benches are of very limited usefulness. The effective capture range of a 1 to 2 inch
slot hood is only 8 to 12 inches regardless of the length of the slot. Even doubling the airflow will not
substantially increase the capture range.
The same is true of a simple elephant trunk type opening. As a rule of thumb, the effective capture
range is about one duct diameter, i.e., a 6” duct effectively captures
effluents only within 6” from the opening, beyond that the effluents are
dispersed into the room.
Laboratory
chemical hoods are deceptively simple in design. Just because the motor can be heard or a tissue moves at the hood
face does not mean the device is containing properly. Hood exhaust volume (cfm), and face velocity (fpm), are just two
parameters that can be measured. A
specific value for either does not mean the enclosure is working properly. The key is whether the hood contains the
hazardous material, i.e., chemical containment. Containment should be as close to the source as possible,
however, working in closed systems like a glove box is usually not feasible,
desired, or practical. The next best
thing is to control the hazard along the path, before the hazard reaches the
receiver, i.e., the worker. This means
partially enclosing the source using a laboratory chemical hood, or vented
enclosure where there is a front opening for access. Reliance should not be on personal protective equipment like
respirators. If respirators are needed
this means the enclosure is not containing the hazard. In addition, respirators provide a false
sense of security, and OSHA requires a respirator fit program, which requires a
lot of time, resources, and stringent documentation.
There
are several reasons face velocity and exhaust rate alone should not be used to
determine performance. These parameters
are influenced dramatically by the room air supply, cross-drafts caused by
operator movements, laboratory traffic flow, location of room doors, the nature
of the equipment and processes being enclosed, and the location and type of
room supply and exhaust air diffusers.
In addition, use of higher exhaust rates than necessary is
expensive. A single 6-foot laboratory
chemical hood operating 24/7 at 100 fpm costs about $5,000/year to
operate. Remember active hoods should
always be left running, even after hours.
Otherwise the materials used in the hood will diffuse back into the work
area. Airflows that are too high can
also cause turbulence at the face and compromise containment.
Vented
enclosures are intended as alternatives and complements to traditional
laboratory chemical hoods. Vented
enclosures are less expensive to buy, install and maintain than traditional
hoods. Because they use less air they
are cheaper to operate. Traditional
laboratory chemical hoods are expensive to operate, but a 3 ft. vented enclosures
can be operated 24/7 for only about $500/yr.
Hence, vented enclosures often enable a lot more bench space and
operations to be ventilated. Vented
enclosures can be custom designed for specific operations (e.g., balance
enclosures, microscopes, high-throughput robotic and automatic equipment
enclosures, histopathology operations).
When properly designed they have demonstrated excellent containment
(14). In addition, they usually can be
installed or removed in a matter of hours; an important advantage in today’s
rapidly changing corporate world. In
such cases alarmed and monitored HEPA filters remove the contaminated effluents
so the exhaust can be recirculated.
If
the material used in the hood or vented enclosure is non-volatile and capable
of being removed by a HEPA (high efficiency particulate air) filter is may be
acceptable to recirculate the exhaust air back into the laboratory. There are some important restrictions
however. For example, vented enclosures
used for particulates should be alarmed at the face so that when the face
velocity drops below a set point an alarm will sound indicating that the HEPA
filter is becoming loaded.
Alternatively, the pressure drop across the HEPA filter can be measured,
continually monitored and alarmed. An
increase in pressure means the filter is becoming loaded and should be replaced.
If
trace amounts of known, specific, volatile chemical are used it also may be
possible to use bag–out charcoal filters to remove the effluents and
recirculate the exhaust. If this is
done the charcoal filter should be equipped with chemical-specific detectors
and alarms to indicate when breakthrough occurs. Charcoal filters should not be used for anything more than trace
amounts of volatile chemicals or where several different chemicals are used. Input from a safety professional is crucial.
It’s
advisable to require proof of containment by the hood manufacturer before
purchase. Hood containment should also be tested again after installation and
before actual use. This provides a
baseline for the hood performance that is valuable in future hood
evaluations. The American Society of
Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) has a specific
test for containment (ASHRAE 110-1992, “Fume Hoods, Methods of Testing Performance
of Laboratories,” 25,12,13). ASHRAE
110-1992 is specific for laboratory chemical hoods, but reputable manufacturers
of vented closures will always provide similar proof of containment for their
products (15).
There
are several types of laboratory chemical hoods, e.g., conventional, by-pass,
auxiliary air, vertical sash, horizontal sash, combination horizontal/vertical
sash (i.e., HOPEC), radioisotope, perchloric acid, floor mounted (improperly
known as walk-in), bench level, constant volume, and variable air volume hoods.
For a full discussion of each type and
purpose of these hoods consult one of the references cited above in the
beginning of this Section. It should be
noted that auxiliary air hoods are no longer recommended. These hoods were for were originally designed
for energy savings, but this design is not practical since the auxiliary air
had to be conditioned before it was used; thus negating any energy savings.
A
hazard is the danger that can result when an unwanted event occurs. Hazards are evaluated according to their
potential to cause harm, based on an analysis of their properties and their
conditions of use. Risk is a measure of
the probability and the consequences of the hazards resulting from an activity
or condition. Risk assessment is an
evaluation of the probability, extent, and impact that an undesirable event
could have on the health and welfare of humans and the environment.
Chemicals
often present biological, chemical, environmental, or physical hazards. A hazardous chemical or chemical mixture is
one that can fit into one of three categories.
1. It has undesirable biological effects, either acutely or chronically,
with reasonable regard to size and duration of dose, type of exposure and the
physical state of the material needed to produce such effects. 2. Its toxicity information is not
available, but is highly suspect because of its structural or functional
similarity to known toxic agents. 3. It
is combustible, flammable, explosive, caustic, corrosive or highly reactive
(e.g. an oxidizers). OSHA, NFPA, EPA,
DOT, CPSC, and other organizations have specific definitions for specific
hazardous chemicals, classes and substances.
Hazardous chemicals should always be weighed or used in a hood or vented
enclosure and not on the open bench where dispersion into the laboratory air is
possible. Appropriate protective
equipment, specific to the particular hazard, should always be used. Always consult your Chemical Hygiene Officer
or Safety Office and read the Material Safety Data Sheet (MSDS) before weighing
and handling any hazardous chemical.
Once
a hood is installed the responsibility shifts to the users. A periodic and routine maintenance and
inspection schedule should be established.
This includes inspection of motors, fans, pulleys, belts and routine
lubrication. Always have an emergency
plan for hood or electrical failure.
The safety department or Chemical Hygiene Officer should establish a
routine hood monitoring and evaluation program. Safety personnel should use separate velometers or anemometers
that are calibrated each year and visually inspect and evaluate every hoods at
least annually. Each hood should be
marked with a date of inspection, face velocity and the proper operating sash
height.
All
hood users should receive proper training before they ever use a hood. Except for gloved hands and protected arms
no part of the workers body should ever enter a hood, especially the head. This is why the term walk-in hood is no
longer used. Always wear proper
personal protective clothing including appropriate eye protection. Always lower the sash whenever the hands-on
operation is completed. Keep the sash
fully closed when the hood is unattended.
Never shut off a hood unless the hood is clean and decontaminated. Hoods should not be overcrowded and should
not be used to store unwanted or unused chemicals, materials or equipment. Avoid clutter in hoods. Keep light fixtures clean, ground electrical
outlets and keep them free from corrosion and in proper working order. Inspect baffles to ensure they have not
slipped shut. Malfunctioning sashes and
broken glass should be repaired before the hood is used. All work in a hood should be carried out at
least 15 cm (6 in) from the sash edge of the hood. A yellow line 15 cm from the edge can serve to indicate the limit
of usage. Adequate airflow and the
absence of excessive turbulence are necessary for safe operation. Ensure that equipment does not block the hood
baffles. It may be necessary to place
large equipment on 2” blocks to permit adequate and uniform airflow.
When
feasible apparatus used in hoods should be fitted with condensers, traps or
scrubbers to contain or collect hazardous effluents. The hood should never be used as a means for disposing of
chemicals (e.g., by evaporation). The
effective operation of a hood depends on many factors including face velocity,
the overall ventilation pattern of the room, the adequacy of room air supply,
and the methods of working at the hood.
Before each use, be sure that the hood exhaust system is working
properly. Routinely inspect the hood
immediately before it is used.
Always
keep your head outside of the hood face.
Specialized vented enclosures and workstations are now commercially
available for many operations like weighing, microscopy, pipetting, and use of
automated equipment. Flexible ductwork
or "elephant trunk" hoods should be used over the exit ports of
equipment like gas chromatographs and atomic absorption spectrometers to remove
toxic effluent from the laboratory environment. Remember that in the event of an accident or fire every item in
the hood may be involved, including those stored in the hood.
The
Americans With Disabilities Act resulted in new commercially available hood
designs, and new advances in laboratory design for access by disabled
persons. Hood standards like those of
ANSI, ASHRAE, NFPA (27), and SEFA (28) are continually being revised. Laboratory chemical hoods, and vented
enclosures and workstations are continuing to evolve and represent an exciting
and dynamic aspect of chemical health and safety.
References
1. ANSI/AIHA, Laboratory Ventilation Standard Z 9.5, American National Standards
Institute/American Industrial Hygiene Association, 2700 Prosperity Ave., Suite
250, Fairfax, VA 22031, most recent version.
2. ACGIH, Industrial
Ventilation Manual, American Conference of Governmental Industrial
Hygienists, 1330 Kemper Meadow Dr., Suite 600, Cincinnati, OH 45350-1634, most
recent edition.
3. DiBerardinis, L.J., Baum, J.S., Furst, M.,
Gatwood, G.T., Seth, A.K., Guidelines for
Laboratory Design, 3rd
ed., John Wiley & Sons, New York, 2001
4.
DiNardi, S.R., ed. The Occupational
Environment-Its Evaluation and Control, American Industrial Hygiene
Association, 2700 Prosperity Ave., Suite 250, Fairfax, VA, 1997.
5.
Alaimo, R.J., ed. Handbook of Chemical
Health and Safety, American Chemical Society, Oxford University Press,
Washington, DC, 2001.
6.
Stricoff, R.S., Walters, D.B., Handbook
of Laboratory Health and Safety, John Wiley & Sons, New York, 1995.
7. National Academy of Sciences Report, Prudent Practices in the Laboratory:
Handling and Disposal of Chemicals, National Research Council National
Academy Press, Washington, DC, 1995.
8.
Saunders, G.T., Laboratory Fume Hoods A
User’s Manual, John Wiley & Sons, New York, 1993.
9.
Jeff Burton, Industrial Ventilation
Guidebook, and other related ventilation workbooks, study guides and short
courses, available from IVE, Inc. 2974 S. Oakwood Dr., Bountiful, UT 84010, and
the American Industrial Hygiene Association, 2700 Prosperity Ave., Suite 250,
Fairfax, VA 22031.
10.
http://www.usg.edu/ehs/guidelines/fume_design.phtml
11.http://www.epa.gov/labs21century/conf/index.htm
12.
http://www.safelab.com/RESOURCES.HTM#technical_papers
13.
http://www.labhoodpro.com/
14.
http://www.flowsciences.com/publications/hutton_vbse_study.html
15.
http://www.flowsciences.com
16.
http://www.ehs.csus.edu/articlesfumehood.htm
17.
http://web.mit.edu/iho/www/fumehood/intro.html
18.
http://www.cehs.siu.edu/Chemical/hood/Default.htm
19.
http://www.uwm.edu/Dept/EHSRM/LAB/fume.html
20.
http://www.pp.okstate.edu/ehs/links/fumehood.htm
21.
http://www.ehs.berkeley.edu/pubs/factsheets/09fumehd.html
22.
http://web.uvic.ca/ohs/fumehoods.html
23.
http://www.fpm.wisc.edu/safety/gsp/fumehood.htm
24.
http://www.princeton.edu/~ehs/labmanual/sec6-2.html
25.
ANSI/ASHRAE Standard 110-1995 – Guideline
Method of Testing Performance of Laboratory Fume Hoods, American National
Standards Institute / American Society of Heating, Refrigeration, and Air
Conditioning Engineers, 1791
Tullie Circle, NE, Atlanta, GA 31329.
26. Gottschall, W.G., Walters,
D.B., Laboratory Health and Safety
Dictionary, John Wiley
& Sons, New York, 2001.
27. NFPA 45, Fire
Protection for Laboratories Using Fume Hoods, National Fire Protection
Association, 1 Batterymarch Park, Quincy MA 02269, most recent version.
28. SEFA 1.2-1996, Standards for Laboratory Fume Hoods, Scientific Equipment and Furniture Association, 1028 Duchess Dr., McLean VA 22102