EXPOSURE CHAMBERS

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EXPOSURE CHAMBERS
Yves Alarie, Ph.D Professor Emeritus
University of Pittsburgh,USA
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A. BEST DESIGN There is no such thing as best
design chamber to answer all the needs of
inhalation toxicology studies. The best design
is a chamber that works according to the
following criteria Fairly uniform distribution
of contaminants, variation of 10-15 between
sampling ports is quite acceptable.
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Ammonia level should be verified with high animal
load particularly when reaction with pollutants
is possible, CI2, SO2, NO2, etc.If this airflow
presents a problem (high cost of pollutant,
availability of pollutant, cleaning, etc.) it can
be reduced, but probably not below 0.2 the
total volume of the chamber without creating
problems with temperature, ammonia, CO2,
humidity, etc.
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Animal load should be lower than 1 to 5 of
chamber volume. Calculate as follows take body
weight of each animal to be same as its volume
(i.e., 200 grams rat 0.2 Liter) and multiply by
number of animals. Thus to expose 100 rats of
200 grams (20 liters of rats) you need a minimum
chamber volume of 2,000 liters and preferably
there will be only one layer of animals. This
can be reduced to a 400 liter chamber (5) but
likely to need 2 layers. One layer is preferable
if working with highly reactive gases which will
react with fur of animals or when working with
large particles, but is not necessary with
aerosol around 1 µm in size and with non
reactive gases such as CO etc. or vapors such as
benzene, carbon tetrachloride etc.
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Do not use the chamber for mixing. The added
pollutant should be mixed with the incoming
diluting air prior to entering the chamber or
mixed at the top or entrance of the chamber. The
idea of placing baffles etc. in a chamber for
better mixing is nonsense. This simply provides
more surface area for interaction with gases or
aerosols. A baffle can be used to prevent large
particles to enter the chamber or at the entrance
of the chamber for proper mixing but never within
a chamber.
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• Do not use a fan in a chamber for mixing. A
  fan will increase turbulence which will cause
  aerosol coagulation, stratification and
  deposition on various surfaces.

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• With reactive gases, conditioning of the
  chamber should be done prior to loading animals
  in the chamber. Once this is done addition of
  animals may drop the concentration by at least
  50 and sometimes by as much as 95. This is not
  serious for a long-term chronic study. Indeed
  during the first week of such a study adjustment
  can be made to the delivery system to bring the
  concentration upward to the desired level. In
  cases of acute studies, if you want to know what
  the animals are likely to soak collect dead
  animals used for other experiments, keep them
  refrigerated and then place in your chamber.

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Permitted daily variation in exposure
concentration for chronic studiesReactive
gases 20 of desiredNon-reactive gases 15
of desiredAerosol lt 1 µm 20 of
desiredAerosol 1-5 µm 20 of desired
Big ProblemAerosol gt 5 µm Dont use
them in large chamber, nose or head only exposure
is more appropriate. However, relevance is in
question, rats do not inhale rocks!
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The only solution to this problem is to set up
elutriation systems to remove the large particles
or to reformulate the aerosol delivery system so
that the particle size is appropriate for an
inhalation study. Obviously this will not be
exactly the same as what consumers will be
exposed to. However, without doing this you may
end up with an LC50 of 1 ton/m3. While this may
be reassuring, it is not appropriate, since none
of the particles may have been inspirable or
respirable for the exposed animals.
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B. DESIGN FOR VERY HIGH LEVEL OF AEROSOL
EXPOSUREThe decision to use a chamber or head
only exposure is impossible to make without some
preliminary work with the chemical. This work
can be done using a small chamber and a few
animals. In general concentrations higher than
500 mg/m3 are difficult to work with unless the
particle size is small. Head-only or nose-only
exposure is indicated.
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There is an EPA recommended level of 5 gram/m3 of
aerosol as a level at which if no deaths occur
the chemical would be considered non toxic.
This is impossible to achieve for most solid or
liquid and keeping the particle size reasonably
small for animals to inhale and at the same time
using an inhalation chamber with the particle
size and exposure concentration remaining fairly
uniform from top to bottom.
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lt50 mg/m3 small particles O.K.
larger particles
O.K. gt50 mg/m3 small particles
O.K. larger
particles more difficult 500 mg/m3
small particles O.K. larger
particles large particles will deposit 5,000
mg/m3 small particles O.K. larger
particles too much deposition
1-3 µm MMD and below.
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C. CHEAP CHAMBER, GOOD RESULTS For acute,
sub-acute or chronic work using 10-20 rats or
20-50 mice an all-glass 100-liter aquarium is
just perfect. A cover can be made out of plywood
or plexiglass with the inside of the cover lined
with teflon or polyethylene film. Such a chamber
costs about 100 with all fittings.
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Operated at 100 liters/min, equilibrium
concentration will be reached in about 5 minutes
with uniform distribution of contaminants, gases
or aerosols.(6,7) It is not necessary to use
expensive stainless steel chambers with pyramidal
tops and bottoms and a glass door to observe the
animals.
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Murphys Law says you will break the glass
aquarium. So what? Just have a spare one on
hand. However, if you are exposing a large
number of animals for a chronic study it
obviously makes sense to have a stainless steel
chamber. However, you will break the glass door!
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D. EXPOSURE SYSTEMS FOR CALCULATION OF DOSE In
order to calculate the net dose received during
exposure to aerosols and reactive gases the
exposure concentration and duration of exposure
are needed. Two other variables are also needed
tidal volume (VT) and respiratory frequency (f or
BPM). These cannot be obtained with exposure
systems described above. They can be obtained
with head only exposure system with the body of
the animal held in an enclosure with a seal at
the neck, such a device is called a body
plethysmograph or a head-out body
plethysmograph.
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A pneumotachograph is a device for measuring
the rate at which air flows in and out of the
plethysmograph due to thoracic displacement with
each breath. The rate of airflow (ml/sec) when
integrated with time yields volume (ml) and a
record of tidal volume is obtained. Another way
of obtaining VT and f is to use a whole body
plethysmograph. With this device, the whole
animal is within an airtight enclosure and again
the pressure created with each breath is
measured. The pressure created with each breath
is more complex in its origin than with the body
plethysmograph. However, there is no need for
restraint of the animal.
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E. DO NOT EXPOSE ANIMALS UNLESS YOU ARE
SURE Prior to exposing animals you should have
good experience with an empty chamber, including
the housing cages and the contaminant delivery
system as well as the method for analysis of the
contaminant, and if you are dealing with an
aerosol, determinations of the particle size need
to be done. You should know how large a
difference there is between the nominal
concentration and actual concentration. If
there is a large discrepancy between the two you
should know why.
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F. RAPID GUIDE FOR EQUILIBRATION TIME AND ITS
USE As a gas or aerosol is introduced at a
uniform rate in an exposure chamber maintained at
a continuous airflow, the concentration within
the chamber increases until it is practically
constant. Assuming perfect mixing, from Silver,
(see reference above)
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F. RAPID GUIDE FOR EQUILIBRATION TIME AND ITS
USE As a gas or aerosol is introduced at a
uniform rate in an exposure chamber maintained at
a continuous airflow, the concentration within
the chamber increases until it is practically
constant. Assuming perfect mixing, from Silver,
(see reference above)
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Thus, the of the desired concentration w/b
obtained in time t is
The time required for equilibration of the
chamber to 99 can be calculated by setting
equation 2 equal to 99.
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or Transformed into logarithm form
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and t99 should be calculated for each situation
so that the time (min) to reach equilibration is
very short compared to the duration of exposure.
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G. AIR CHANGESAn air change is said to occur
when a volume of air equal to the volume of the
chamber has passed through the chamber (i.e., a
b). This is a convenient term used by
ventilation engineers, but is not, strictly
speaking, correct.When such occurs, from Eq (2),
only 63 of the air has been changed. From Eq
(5), the time for one air change must be
multiplied by a factor of 4.6 before 99 of the
expected change in air can be accomplished.
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Therefore, if a and b are equal, there is one
air change every 4.6 minutes or 12/hour, not
60/hour as one would think from the ventilation
engineers terminology given in the first
sentence above. The best thing to do is to
forget about air change and give a and b. Then
we know what you are doing.