Basic Documentation

Laboratory Ventilation Codes and Standards
Siemens Industry, Inc. 24
Topic Requirement(s) Commentary
Face Velocity
Setback
California OSHA – Division of Occupational Health & Safety (DOSH) Subchapter 7.
General Industry Safety Orders, Group 16. Control of Hazardous Substances,
Article 107. Dusts, Fumes, Mists, Vapors and Gases
5154.1. Ventilation Requirements for Laboratory-Type Hood Operations:
(2) When a laboratory-type hood is in use to contain airborne hazardous substances
and no employee is in the immediate area of the hood opening, the ventilation rate
may be reduced from the minimum average face velocity of at least 100 feet per
minute to a minimum average face velocity of 60 feet per minute if the following
conditions are met:
(A) The reduction in face velocity is controlled by an automatic system which does not
require manual intervention. The automatic system shall increase the airflow to the
flow required by (c)(1) when the hood is accessed
(B) The laboratory-type hood has been tested at the reduced flow rate according to
the tracer gas method specified in Section 7, Tracer Gas Test Procedure, of
ANSI/ASHRAE 110-1995, Method of Testing Performance of Laboratory Fume
Ho
ods, which is hereby incorporated by reference, and has a hood performance rating
of 4.0 AU 0.1 or less. The test may be performed with or without the mannequin
described in the ANSI/ASHRAE 110-1995 tracer gas method.
The tracer gas test need only be performed once per hood. However, if employers
have chosen to perform the tracer gas test on subsequent occasions, it is the most
recent record of test results and test configuration that shall be maintained pursuant to
subsection (c)(2)(C).
(C) The record of the most recent tracer gas test results and the "as used" test
configuration shall be maintained as long as the automatic system is operable and
thereafter for five years.
(d) Operation. Mechanical ventilation shall remain in operation at all times when hoods
are in use and for a sufficient time thereafter to clear hoods of airborne hazardous
substances. When mechanical ventilation is not in operation, hazardous substances in
the hood shall be covered or capped off.
A face velocity setback (automatically
reducing the exhaust flow of an unattended
fume hood (when the sash is left open) is
sometimes promoted as a means to reduce
fume hood air consumption and thus reduce
energy usage. Be certain that local code
requirements do not prevent reducing the
face velocity under these conditions.
• This attempt to save energy only pays off if
lab workers neglect to close the sashes.
Selecting such a system constitutes
sanctioning unsafe practices.
• A fume hood must provide containment
whether or not a user is present. If the face
velocity is reduced, the fume hood becomes
more susceptible to room cross currents
and other factors that adversely affect a
fume hood’s ability to maintain safe
containment. (See Fume Hoods: Room
Cross Currents.)
• The face velocity setback provision may be
perceived by fume hood users that they can
leave sashes open and that this unsafe
practice is okay. This undermines the
chemical hygiene plan.
• A sash left open presents is an
unnecessary hazard to all persons in the
room since it does not provide a physical
barrier from potentially dangerous
conditions that might develop in the
unattended fume hood. For example, high
volumes of toxic fumes can be suddenly
given off from unexpected chemical
reactions or an explosion within the hood
can propel objects such as glass fragments
out from the hood interior.
Safer alternatives to face velocity setback
include sounds that alert room occupants of a
sash left open (chirps, beeps or tones) and
automatic sash closure devices.