490I
INDOOR AIR POLLUTION
PART 1Laboratory work and chemical testing involves procedures
that could contaminate the air inside occupied spaces. The
nature of the contaminants varies widely: high humidity
from steam baths, odors from hydrogen sulfi de analyses,
corrosion capabilities of alkalies and acids, solubility of
acetone, explosive properties of perchloric acid, health haz-
ards of bacteriological aerosols, and poisonous properties of
nickel carbonyl. Ideally, best procedure is not to emit; but
the next best is to remove or exhaust directly and as close
to the point of origin for safety of laboratory personnel and
protection of property.
To achieve this end, the accepted methods used for
containment and removal of contaminants is by restrict-
ing the contaminant procedures to within an enclosure or
hood. Simultaneously the air is drawn across the hood face
to capture and remove the contaminants before escaping
into the room.
In the design of a fume exhaust system utilizing hoods the
following factors must be analyzed and evaluated: Capture
velocities, Fume hood design, Seven basic hood designs,
Makeup air source, Air distribution, Exhaust system, Exhaust
duct materials, Exhaust air treatment, Special systems.CAPTURE VELOCITIESAir fl ow rates required for hood exhaust systems are based on
a number of factors, the most important of which is capture
velocity. For most applications these will range from 50 to
200 fpm. The lower fi gure is used to control contaminants
released at low speed into relatively quiet room air (15 to
25 fpm). The higher fi gure is used to control contaminants
released at high rates. Under special conditions hood face
velocities as low as 25 fpm have been used with industrial
type hoods.
Conclusions regarding optimum face velocity selec-
tion are rather mixed. In conceptual design of a lab facilitythis is given much thought and argument, especially when
air conditioning is to be included. For every 1000 cfm of air
exhausted through hoods, 3 to 4 tons of refrigeration are
required to be added to system capacity for makeup air.
At $1,000 per ton of refrigeration the cost of exhausting
1000 cfm could range from $3,000 to $4,000. This cer-
tainly adds to hood burden and capital outlay.
Some design emphatically forbid hood face veloci-
ties less than 100 fpm. Attempts have been made to
relate face velocity to hood service by compromising
fume hood usage with the added responsibility of super-
vision by laboratory personnel to insure that fume hood
usage is restricted to the type contaminant for which face
velocities were selected. To this end, Brief (1963) offers
a method of hood classifi cation as a step toward economy
of design and operation. He classifi ed type “S” hoods for
highly toxic contaminants (threshold limit values less than
0.1 ppm) as having face velocities from 150 to 130 fpm.
Type “A” hoods for moderately toxic contaminants (TLV’s
of less than 100 ppm) can be sized for face velocities of
100 to 80 fpm. Hoods for non-toxic contaminants, Type
“B” (TLV’s above 100 ppm), are sized for a face velocity
from 60 to 50 fpm.
It should be emphasized that TLV’s should be used with
care and not as sole criteria since they represent airborne
concentrations that most workers may be exposed to repeat-
edly during a normal work day of 8 hours duration for a
working lifetime.
Fume hood effi ciency depends on the amount of air
exhausted and hood design. To assure fl exibility of operation
and maximum safety to lab personnel, a fume hood should
be designed for exhaust air rates ample for complete removal
of all contaminants. This may be a logical step when only
one hoods or two are involved in a single facility. However,
with more than two, generous exhaust through all hoods can
impose a heavy initial and operating cost penalty on the air
conditioning system. From actual experience with labora-
tory design, it is diffi cult to select a one-hood design that
will satisfy all situations.C009_001_r03.indd 490C009_001_r03.indd 490 11/18/2005 10:29:56 AM11/18/2005 10:29:56 AM