Basic Documentation

Page 2 of 6 Siemens Industry, Inc.
Document No. 149- 976
To minimize these effects ANSI/AIHA Z9.5 states
that room air
velocities should be less than one-half
and preferably less than one-third of the face
velocity
1
. Standard NFPA 45 also states that room
air currents should ideally be less than 30% of the
fume hood face velocity.
2
Therefore, to use a face velocity setback of 60 fpm
and be in accord with the foregoing safety
standards, one must be sure that room air currents
at the fume hood will be less than one-half and
preferably less than 30% of the face velocity. This
means that for a 60 fpm face velocity setback, room
air currents must be less than 30 fpm and preferably
less than 18 fpm respectively.
3
These are very slow rates of air movement and are
comparable to the rate that smoke rises from a
smoldering candle in a calm room. With such low
room air velocity requirements, it is understandable
why many fume hood safety professionals express
admonition against reducing the face velocity to
such a low level.
The U.S. Occupational Safety and Health
Administration (OSHA) recommends that a fume
hood's face velocity range from 60 to 100 fpm, which
places 60 fpm at the extreme low end of their
recommendation. OSHA also recognizes the
problems with room air currents and cautions that
airflow throughout the entire laboratory room should
be “uniform and without higher velocity air currents.”
Standard AIHA’s Z9.5 specifically recommends that
the face velocity be at least 80 fpm.
4
The Sash Closure Alternative
An important safety consideration is expressed in
Standard NFPA 45 (Paragraph 6.8.3) which states:
"Laboratory hood sashes shall be kept closed
whenever possible. When a fume hood is
unattended, its sash shall remain fully closed."
1. American National Standard for Laboratory Ventilation
ANSI/AIHA Z9.5, 5.2.2: "Supply air distribution shall be
designed to keep air jet velocities less than half, preferably
less than one-third of the capture velocity or the face velocity
of the laboratory chemical hoods at their face opening."
2. National Fire Protection Association, Standard NFPA 45: A-
6-3.5: “Room air current velocities in the vicinity of fume
hoods should be as low as possible, ideally less than 30
percent of the face velocity of the fume hood.”
3. One-half of 60 fpm is 30 fpm. 30% of 60 fpm is 18 fpm.
4. American National Standard for Laboratory Ventilation
ANSI/AIHA Z9.5, 3.3.1: "Design face velocities for laboratory
chemical hoods in the range of 80
 100 FPM (0.41  0.51m/s)
will provide adequate face velocity for a majority of chemical
hoods." "This is the range recommended for a majority of
laboratory chemical hoods."
NFPA 45 also requires that laboratory occupants be
trained and reminded as a safety practice to close
fume hood sashes since the sash provides a
physical shield or barrier much like a face shield.
Leaving a fume hood sash open even when no one
is in front of a fume hood constitutes an unsafe
practice and exposes all occupants of a laboratory
room to needless hazards. Chemical processes or
experiments can generate large volumes of noxious
or hazardous fumes that may not be contained with
a 60 fpm face velocity. Gasses, flammable liquids, or
vapors in a fume hood may ignite and result in a fire.
In actual instances, explosive chemical reactions
have fractured glassware and propelled fragments
and chemicals throughout the room.
A guide document published by OSHA for its field
inspectors specifically states that the laboratory
facility chemical hygiene plan should include the
provision whereby “fume hoods shall be kept closed
at all times that do not require that the hood be
open”
5
.
Face Velocity Setback and User
Safety Training
Having an automatic provision to reduce face
velocity when a user moves away from the hood
may convey a message that conflicts with safety
practices. It infers that fume hood users are not
really expected to close fume hood sashes, and thus
weakens the effort to maintain and enforce
laboratory safe working practices. If a laboratory
accident or mishap is ultimately attributed to a
needlessly open fume hood sash, an automatic face
velocity setback provision might be construed as
evidence that safety practices were not being
enforced. This would substantially increase the
liability on the part of the laboratory facility and
probably also on the laboratory ventilation system
designer. As a matter of logic, if strict sash closure
practices are enforced, there is no benefit from an
automatic face velocity setback provision, since it
would not be needed.
Energy Savings
The claim is made that a typical fume hood user will
always leave the fume hood sash open even though
they only need access to the fume hood interior for
short periods—perhaps an hour or two a day. Thus,
an assumption is made that with a 60 fpm face
5. Reference: Occupational Safety and Health Administration,
Philadelphia Regional Instruction STD 1-23,1
February 11, 1991, Office of Technical Support