Environmental Health and
Safety (EHS) informs laboratory personnel in proper hood
procedures, ensures that chemical hoods are surveyed annually
with a calibrated anemometer, reports hood malfunctions to
laboratory personnel and Engineering & Maintenance immediately,
and performs follow-up airflow surveys promptly after hood
repairs are completed.
Laboratory Personnel
ensure that work in chemical hoods is appropriate for the type
of hood available and the quality of ventilation present, use
hoods according to recommended guidelines for safe use, and
coordinate modifications, maintenance, repair, and new equipment
needs with EHS and/or Engineering & Maintenance.
Engineering & Maintenance
performs preventive maintenance on fans, ductwork, and hoods,
makes repairs or maintenance as identified by annual surveys or
by users, and notifies EHS and coordinates with laboratory
personnel when hoods must be turned off for repair, maintenance,
or other operations. Engineering & Maintenance will notify EHS
to certify a hood that has been repaired.
Project Managers ensure
all chemical hoods have been commissioned and are working
properly at the completion of a renovation project.
The New York City Fire
Department requires all chemical hoods to operate at an average
minimum face velocity of 100 feet per minute (fpm), with no
single point lower than 75 fpm.
A chemical hood should be used
when:
- handling chemicals with
significant inhalation hazards such as toxic gases, toxic
chemical vapors, volatile radioactive material, and
respirable toxic powders
- carrying out experimental
procedures with strong exothermic reactions
- handling chemicals with
significant vapor pressure
- chemical vapors generated
could cause a fire hazard
- working with compounds
that have an offensive odor
Rating Classifications
Chemical hood surveys are conducted at a sash height of 18”.
- PASS - Chemical
hoods with an average face velocity between 100 and 150 fpm
with no single point below 75 fpm are passing. The optimum
range for face velocity is 100 to 130 fpm.
- RESTRICTED USE ONLY
– Chemical hoods with an average face velocity between 151
and 180 fpm are acceptable for restricted use only.
Laboratories should not use acutely toxic, highly hazardous,
or carcinogenic chemicals in hoods deemed for restricted use
only.
- FAIL – Chemical
hoods with an average face velocity less than 100 fpm or
greater than 180 fpm or have any single test point less than
75 fpm are failing.
User Guidelines (Standard
Operating Procedures)
Position the hood sash to a maximum of 18 inches high to
ensure proper airflow velocities at the work opening. Adjust the
sash to shield from splashes or flying objects. In addition to
aiding in proper airflow, the sash acts as a physical barrier in
the event of an unplanned incident in the hood
Keep hood storage to an
absolute minimum. Excess materials in the hood disrupt airflow
and can act as a barrier or cause airflow to bounce back across
the face of the hood. Do not take up hood space and block
ventilation by storing unused equipment or chemicals in hoods.
If large items must be kept in the hood, contact EHS for
evaluation, certification and a smoke test.
Keep all work at least six
inches inside the hood. The capture ability of a hood may not be
100% at the front of the hood.
Avoid cross drafts. Someone
walking rapidly past the work opening can create a cross draft
that may disturb the direction of airflow and cause turbulence.
Keep the hood clean. Remove old
experimental glassware and clutter. Wipe up spilled chemicals or
residues. Ensure the glass sash remains clear for visibility.
Do not heat perchloric acid in
standard chemical hoods. Perchloric vapors may create explosive
perchlorates in the ductwork. Contact EHS if you are performing
perchloric acid digestions.
Confirm that the chemical hood
is operational. Check the air flow gauge if so equipped. Check a
telltale (a piece of paper attached to the bottom of the sash).
The telltale should be noticeably pulled toward the back of the
hood.
Separate and elevate each
instrument. Use blocks or racks to elevate equipment one to two
inches off the hood deck surface so that air can easily flow
around all apparatus with no disruption.
Avoid opening and closing the
hood sash rapidly and swift arm movements in front or inside the
hood. These actions may cause turbulence and reduce the
effectiveness of hood containment.
Use extreme caution with
ignition sources inside a hood. Ignition sources such as
electrical connections and equipment, hot plates, controllers,
and open flame can ignite flammable vapors or explosive
particles from materials being used in the hood.
Never put your head inside a
hood while operations are in progress. The plane of the sash is
the imaginary boundary that should not be crossed except to set
up or dismantle equipment.
Report airflow problems and
problems with the physical structure of the chemical hood to
Engineering & Maintenance as soon as possible. If a hood
fails while working with highly hazardous materials, immediately
close the sash. Leave the immediate area and contact EHS for
further assistance.
Lower the sash to a 2” opening
when the hood is not being used.
Chemical Hood Surveys and
Maintenance
EHS surveys all chemical hoods on an annual basis to verify that
the air velocity at the opening is within an acceptable range.
The Chemical Hood Test Record label is affixed to the external
surface of the hood and will indicate the inspection date, the
average face velocity measurement at 18” sash height, and the
rating classification (e.g., Pass, Restrict, or Fail). If the
hood is found to be in the restricted or failing range, EHS will
conduct a hazard assessment of the operations conducted in the
hood and inform laboratory staff if the hood is acceptable for
use until it is repaired and re-certified. If the hood is found
not to be acceptable for use, EHS will post a sign on the hood
informing laboratory staff not to use the hood.
EHS will automatically submit a
work order to Engineering & Maintenance to repair all
non-passing hoods and post a sign on the chemical hood to notify
laboratory personnel. Questions about restrictions on the use of
hoods during repair should be directed to EHS.
Engineering & Maintenance
conducts repairs to chemical hoods and flow monitors.
Engineering & Maintenance will notify EHS when a chemical hood
and/or monitor have been repaired. EHS will re-survey the hood
to verify the air flow is at passing levels.
Shutdown Notification
Notifications will be posted when chemical hood fans will be
shut down. For minor shut downs affecting only a few hoods,
notifications will be placed directly on each hood affected. For
large shutdowns affecting large areas (e.g. an entire building),
notifications will be posted in elevators and other public areas
as with other utility shut downs.
