Figure 3-3. Compartment Model to Represent Particle Deposition and
Time-Dependent Particle Transport in the Respiratory Tract*
S
Seq
equ
uest
estered
ered iin
n T
Tiissu
ssue
e
S
Su
urf
rface T
ace Tran
ransp
spo
ort
rt
A
Ant
nter
eriior
or
N
Nas
asal
al
1
1
ET
ET
ET
E
Env
nviirronm
onment
ent
E
Extr
xtrath
atho
orrac
aciic
c
1
11
14
14
14
16
16
N
Nas
asoo--or
oroo-
0.
0.001
001
ET
ET
ET
100
100
2
22
G
GII T
Tra
ract
ct
phar
pharyynnx
x
15
15
LN
LN
Lar
Laryynx
ET
ET
ET
ET
11
11
11
nx
SEQ
SEQ
13
13
12
12
0.
0.03
03
10
10
0.
0.01
01
BB
BB
BB
SEQ
SEQ
SEQ
BB
BB
2
2
BB
BB
1
1
B
Brronc
onchi
hi
99
9
8
8
7
7
0.
0.03
03
2
2
0.
0.01
01
bb
bb
SEQ
SEQ
bb
bb
2
2
bb
bb
1
1
B
Brronc
onchi
hiol
oles
es
LN
LN
TH
TH
6
6
55
4
4
0.
0.0001
0001 0.
0.001
001 0.
0.02
02
0.
0.00002
00002
Al
Al
3
3
Al
Al
2
2
Al
Al
11
A
Allvveol
eolar
ar
10
10
3
3
22
11
In
Inte
terrsstitititiuum
m
T
Thor
hora
ac
ciic
c
93
PLUTONIUM
3. HEALTH EFFECTS
*Compartment numbers shown in lower right corners are used to define clearance pathways. The
clearance rates,
half-lives, and fractions by compartment, as well as the compartment abbreviations are presented in Table 3-6.
Source: ICRP 1994b
95
PLUTONIUM
3. HEALTH EFFECTS
Deposition of inhaled gases and vapors is modeled as a partitioning process that depends on the
physiological parameters noted above as well as the solubility and reactivity of a compound in the
respiratory tract (see Figure 3-4). The ICRP (1994b) model defines three categories
of solubility and
reactivity: SR-0, SR-1, and SR-2:
•
Type SR-0 compounds include insoluble and nonreactive gases (e.g., inert gases such as H
2
, He).
These compounds do not significantly interact with the respiratory tract tissues, and essentially all
compound inhaled is exhaled. Radiation doses from inhalation exposure of SR-0 compounds are
assumed to result from the irradiation of the respiratory tract from the air spaces.
•
Type SR-1 compounds include soluble or reactive gases and vapors which
are expected to be
taken up by the respiratory tract tissues and may deposit in any or all of the regions of the
respiratory tract, depending on the dynamics of the airways and properties of the surface mucous
and airway tissues, as well as the solubility and reactivity of the compound.
•
Type SR-2 compounds include soluble and reactive gases and vapors
which are completely
retained in the extrathoracic regions of the respiratory tract. SR-2 compounds include sulfur
dioxide (SO
2
) and hydrogen fluoride (HF).
Respiratory Tract Clearance.
This portion of the model identifies the
principal clearance pathways
within the respiratory tract. The model was developed to predict the retention of various radioactive
materials. The compartmental model represents particle deposition and time-dependent particle transport
in the respiratory tract (see Figure 3-3) with reference values presented in Table 3-7 (A,B). This table
provides clearance rates, expressed as a fraction per day and also as half-time (Part A),
and deposition
fractions (Part B) for each compartment for insoluble particles. ICRP (1994b) also developed modifying
factors
for some of the parameters, such as age, smoking, and disease status. Parameters of the clearance
model are based on human evidence for the
most part, although particle retention in airway walls is based
on experimental data from animal experiments.
The clearance of particles from the respiratory tract is a dynamic process. The rate of clearance generally
changes with time from each region and by each route. Following deposition of large numbers of
particles (acute exposure), transport rates change as particles are cleared from the various regions.
Physical and chemical properties of deposited material determine the rate of dissolution and, as particles
dissolve, absorption rates tend to change over time. By creating a model with compartments of different
clearance rates within each region (e.g., BB
1
, BB
2
, BB
seq
), the ICRP model overcomes problems
associated with time-dependent functions. Each compartment clears to other compartments by constant
rates for each pathway.