Chemistry Reference
In-Depth Information
mucosa in the nose into the olfactory nerve in the brain,
as shown in rats (e. g for cobalt) (Persson
et al.
, 2003).
Particle size is the primary determinant both for
how many particles are deposited in the respiratory
system and for the region of the respiratory systems in
which they are deposited (Schulz
et al.
, 2000). The nose
is an effi cient fi lter not only for water-soluble gases
but also for particles. It is a much more effi cient fi lter
than the mouth for particles of approximately 1
greater than 100-
m aerodynamic diameter have a low
probability of entering the human respiratory sys-
tem, and particles less than 50-
µ
m frequently enter.
Under most circumstances, most inhaled particle mass
is exhaled in the same breath, and the remainder is
deposited in the respiratory system.
Particle size is the most important determinant of the
location of deposition of inhaled particles (Schulz
et al.
,
2000). The upper airways—head airways and tracheo-
bronchial region—serve as the site of deposition of much
of the inhaled particle mass and serve to protect the more
vulnerable alveolar region from exposure to inhaled
particles. Particles greater than 10-
µ
m
or larger (Heyder and Rudolf, 1977; Task Group on
Lung Dynamics, 1966). During heavy physical work,
breathing through the mouth is necessary (Proctor,
1977). Even during calm breathing, inhalation through
the mouth occurs to some extent, and this affects the
amount of particles deposited in the lungs (Camner,
1981; Camner and Bakke, 1980).
The main mechanisms for deposition of particles in
the lung are impaction, sedimentation, and diffusion
(Raabe, 1982). The effect of impaction increases with
particle size and air velocity, and it is most important
in the nose, throat, and the larger bronchi (i.e., where
the velocity of the air is highest). The effect of sedimen-
tation increases with particle size and decreases with
air velocity. It is most important in the smaller airways
and the alveoli. The effect of diffusion is of importance
only for particles smaller than a few tenths of a microm-
eter, and the effect increases with decreasing particle
size. Electrostatic effects may also be important for the
deposition of particles, at least under some conditions.
Melandri
et al.
(1977) have shown that unipolar electro-
static charges on particles increase the total deposition
because of electrostatic attraction between the particle
charge and the image charge on the airway wall.
Because the nose very effectively humidifi es the air,
the relative humidity is already 98-99% in the subglot-
tic space (Ingelstedt, 1956). Particles that are hygro-
scopic can increase in size by increasing their water
content and thus be deposited in the lungs quite differ-
ently from what could be expected from measurements
at normal relative humidity. For example, particles of
sodium chloride have been shown to increase seven
times in diameter when they are inhaled (Dautrebande
and Walkenhorst, 1964).
Several theoretical models for the deposition of
particles in the lung have been proposed. The fi rst
was published in 1935 by Findeisen. The International
Commission on Radiological Protection (ICRP) has
used Findeisen's model with some smaller modifi -
cations (Task Group on Lung Dynamics, 1966). The
model agrees fairly well with several experimental
results (Raabe, 1982; Task Group on Lung Dynam-
ics, 1966; Task Group on Metal Accumulation, 1973).
A more recent model is also useful (National Institute
for Public Health and the Environment, 2002). Particles
µ
m aerodynamic
diameter are mainly deposited in the head airways, also
known as the extrathoracic region. Particles less than
10-
µ
m diameter may be deposited in the tracheobron-
chial airways or the alveolar region. Particles deposited
in the larger bronchi are more likely to be cleared to the
pharynx by the mucociliary ladder and swallowed into
the gastrointestinal tract, whereas particles deposited in
small terminal and alveoli are less effi ciently cleared.
Some examples of numerical values of deposition in
tracheobronchial and peripheral lung structures (alve-
olar) for particles with varying mass median aerody-
namic diameter are given in Table 1. Theoretical models
predict the deposition in different parts of the respira-
tory tract as a function of particle size. In real situations
there are always large variations in the particle size.
However, theoretical calculations with the ICRP model
show that estimates obtained using the aerodynamic
mass median diameter give a rather good approxima-
tion of the deposition of an aerosol even when large
size variations occur.
More sophisticated theories of particle deposi-
tion have been developed (e.g., the model by Yu and
Taulbee, 1977). This model agrees very well with theo-
retical estimates of the total deposition by Heyder
et al.
(1975). The particle-size range of 0.3-3
µ
m was used
during several breathing patterns. Compared with the
ICRP model, the model by Yu and Taulbee gives lower
values for deposition.
One diffi culty with the theoretical models is that
even if they give a good average estimate of deposi-
tion, they do not take into account the large individual
differences that are known to exist. The part of the total
deposition in the lung that is deposited in alveoli has,
for example, been shown to vary at least by a factor of
two, even among relatively young healthy male non-
smokers (Albert
et al.
, 1967; Camner and Philipson,
1978; Lippmann
et al.
, 1971). The penetration of par-
ticles (6
µ
m) to the alveoli has been found to correlate
with lung function parameters as forced expiratory
volume in 1 second (FEV
1
) and airway resistance
( Svartengren
et al.
, 1984; 1986).
µ
