Basic Documentation

Fume Hoods
Siemens Industry, Inc. 37
Topic Requirement(s) Commentary
Room Air
Cross Currents
U.S. OSHA, 29 CFR, Part 1910, Occupational Safety and Health Standards,
Subpart Z, Toxic and Hazardous Substances,1910.1450,C4(g):
General air flow should not be turbulent and should be relatively uniform throughout
the laboratory, with no high velocity or static areas; airflow into and within the hood
should not be excessively turbulent.
National Fire Protection Association, Standard NFPA 45, 2011
8.8.6 For auxiliary air fume hoods, auxiliary air shall be introduced exterior to the
hood face in such a manner that the airflow does not compromise the protection
provided by the hood and so that an imbalance of auxiliary air to exhaust air will not
pressurize the hood interior.
8.9.1 Chemical fume hoods shall be located in areas of minimum air turbulence.
A.8.9.1 A person walking past the hood can create sufficient turbulence to disrupt a
face velocity of 0.5 m/sec (100 ft/min). In addition, open windows or air
impingement from an air diffuser can completely negate or dramatically reduce the
face velocity and can also affect negative differential air pressure.
American National Standard for Laboratory Ventilation ANSI/AIHA Z9.5-2003
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.
For most laboratory chemical hoods, this requirement will mean 50 fpm (0.25 m/s)
or less terminal throw velocity at 6 ft. (1.8 m) above the floor.
6.3.5 Excessive cross draft velocities (>50% of the average face velocity) have
been demonstrated to significantly affect hood containment and should be identified
and alleviated. Ideally, cross draft velocities should be less than 30%.
Scientific Equipment & Furniture Association
SEFA 1–2006 Recommended Practices for Laboratory Fume Hoods
5.1 Location in Laboratory
Laboratory fume hoods should be so located within the laboratory to avoid
crosscurrents at the fume hood face due to heating, cooling, or ventilating inlets.
5.2 Safety Considerations
Laboratory fume hoods should be located away from high traffic lanes within the
laboratory because personnel walking past the sash opening may disrupt the flow
of air into the unit and cause turbulence, drawing hazardous fumes into the
laboratory.
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Virtually all standards point out that room air
currents at a fume hood’s open face area
(that is, cross drafts) can adversely affect
fume hood containment when the air currents
are above specific levels.
Excessive cross drafts typically result from:
• Improper room supply air diffusers
(conventional high velocity, mixing
diffusers rather than the perforated type).
• Having too few supply air diffusers
resulting in high supply air velocity from
each unit.
• Improper diffuser location (such as
directly above the fume hoods or on
room side walls).
• Room occupants using portable cooling
fans or having operable windows in the
open position.
• Too rapid movement of persons working
at or passing by fume hoods. Also,
having room doors open.
Both NFPA 45 and ANSI/Z9.5 recommend
that cross draft velocities should (ideally) not
exceed 30% of the average face velocity. For
a nominal 100 FPM face velocity the cross
drafts should therefore not exceed 30 FPM.
This is slightly less than 3 miles per hour
which is a person’s nominal walking speed.
For low velocity hoods, the allowable cross
draft is even lower.
Better fume hood containment can be
attained with good room air distribution than
with higher face velocities and poor air
distribution (higher cross current velocities).