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Table 3-7. Reference Values of Parameters for the Compartment Model to
Represent Time-dependent Particle Transport from the
Human Respiratory Tract
Part A
Clearance rates for insoluble particles
Pathway
From
To
Rate (d
-1
)
Half-life
a
m
1,4
AI
1
bb
1
0.02
35 days
m
2,4
AI
2
bb
1
0.001
700 days
m
3,4
AI
3
bb
1
1x10
-4
7,000 days
m
3,10
AI
3
LN
TH
2x10
-5
No data
m
4,7
bb
1
BB
1
2
8 hours
m
5,7
bb
2
BB
1
0.03
23 days
m
6,10
bb
seq
LN
TH
0.01
70 days
m
7,11
BB
1
ET
2
10
100
minutes
m
8,11
BB
2
ET
2
0.03
23 days
m
9,10
BB
seq
LN
TH
0.01
70 days
m
11,15
ET
2
GI tract
100
10 minutes
m
12,13
ET
seq
LN
ET
0.001
700 days
m
14,16
ET
1
Environment
1
17 hours
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PLUTONIUM
3. HEALTH EFFECTS
Table 3-7. Reference Values of Parameters for the Compartment Model to
Represent Time-dependent Particle Transport from the Human
Respiratory Tract
Part B
Partition of deposit in each region between compartments
b
Fraction of deposit in region
Region or deposition site
Compartment
assigned to compartment
c
ET
2
ET
2
0.9995
ET
seq
0.0005
BB
BB
1
0.993-f
s
BB
2
f
s
BB
seq
0.007
bb
bb
1
0.993-f
s
bb
2
f
s
bb
seq
0.007
AI
AI
1
0.3
AI
2
0.6
AI
3
0.1
a
The half-lives are approximate since the reference values are specified for the particle transport rates and
are rounded in units of
days
-1
. A half-life is not given for the transport rate from Al
3
to LN
TH
, since this rate was chosen to direct the required amount of
material to the lymph nodes. The clearance half-life of compartment Al
3
is determined by the sum of the clearance rates.
b
See paragraph 181, Chapter 5 (ICRP 1994b) for default values used for relating
f
s
to
d
ae
.
c
It is assumed that
f
s
is size-dependent. For modeling purposes,
f
s
is taken to be:
f
s
=
0. 5
for d
ae
≤
2 . 5
ρ
/
χ µ
m and
f
=
0. 5
e
0 . 63 (
d
ae
ρ / χ −2. 5 )
s
for d
ae
>
2 . 5
ρ
/
χ µ
m
where
f
s
=
fraction
subject to slow clearance
d
ae
=
aerodynamic particle diameter/(μm)
ρ
=
particle density (g/cm
3
)
χ
=
particle shape factor
AI = alveolar-interstitial region; BB = bronchial region; bb = bronchiolar region; BB
seq
= compartment representing prolonged
retention in airway walls of small fraction of particles deposited in the bronchial region; bb
seq
= compartment representing prolonged
retention in airway walls of small fraction of particles deposited in the bronchiolar region; ET = extrathoracic region; ET
seq
=
compartment representing prolonged retention in airway tissue of small fraction of particles deposited
in the nasal passages; GI =
gastrointestinal; LN
ET
= lymphatics and lymph nodes that drain the extrathoracic region; LN
TH
= lymphatics and lymph nodes that
drain the thoracic region
Source: ICRP 1994b
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PLUTONIUM
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Particle transport from all regions is toward both the lymph nodes and the pharynx,
and a majority of
deposited particles end up being swallowed. In the front part of the nasal passages (ET
1
), nose blowing,
sneezing, and wiping remove most of the deposited particles. Particles remain here for about a day. For
particles with AMADs of a few micrometers or greater, the ET
1
compartment is probably the largest
deposition site. A majority of particles deposited at the back of the nasal passages and in the larynx (ET
2
)
are removed quickly by the fluids that cover the airways. In this region, particle clearance is completed
within 15 minutes.
Ciliary action removes deposited particles from both the bronchi and bronchioles. Though it is generally
thought that mucociliary action rapidly transports most particles deposited here toward the pharynx, a
fraction of these particles is cleared more slowly. Evidence for this is found in human studies. For
humans, retention of particles deposited in the lungs (BB and bb) is apparently biphasic. The “slow”
action of the cilia may remove as much as half of the bronchi- and bronchiole-deposited particles. In
human
bronchi and bronchiole regions, mucus moves more slowly when it is closer to the alveoli. For the
faster compartment, it has been estimated that it takes about 2 days for particles to travel from the bron-
chioles to the bronchi and 10 days from the bronchi to the pharynx. The second (slower) compartment is
assumed to have approximately equal fractions deposited between BB
2
and bb
2
, with both fractions
having clearance half-times estimated at 20 days. Particle size is a primary determinant of the fraction
deposited in this slow thoracic compartment. A small fraction of particles deposited in the BB and bb
regions is retained in the airway wall for even longer periods (BB
seq
and bb
seq
).
If particles reach and become deposited in the alveoli, they tend to stay imbedded
in the fluid on the
alveolar surface or move into the lymph nodes. Coughing is the one mechanism by which particles are
physically resuspended and removed from the AI region. For modeling purposes, the AI region is divided
into three subcompartments to represent different clearance rates, all of which are slow.
In the alveolar-interstitial region, human lung clearance has been measured. The ICRP model uses 2 half-
times to represent clearance: about 30% of the particles have a 30-day half-time, and the remaining 70%
are assigned a half-time of several hundred days. Over time, AI particle transport falls, and some
compounds have been found in lungs 10–50 years after exposure.
Absorption into Blood.
The ICRP model assumes that absorption into
blood occurs at equivalent
rates in all parts of the respiratory tract, except in the anterior nasal passages (ET
1
), where no absorption
occurs. It is essentially a 2-stage process, as shown in Figure 3-5. First, there is a dissociation