Chemistry Reference
In-Depth Information
by mechanical forces on solid materials to break them
down into particles.
The size distribution of larger or intermediate size
particles in aerosols is often log-normal.
The mass median diameter is the diameter of
a particle with the median mass, that is, half of the
mass lies on either side of the median diameter. The
mass median diameter is a measure of the mass distri-
bution of the aerosol. The mass median aerodynamic
diameter (MMAD) is important in determining pul-
monary deposition of particles with penetration to the
small airways and alveoli of particles less than 10-
because they can penetrate into and deposit in the
alveolar or gas-exchange region. Thoracic particles are
those that can penetrate the upper airways to the tho-
racic airways and beyond, whereas inhalable particles
are all particles that can be deposited anywhere within
the respiratory tract.
All particles, even very small particles, are effi ciently
fi ltered by the upper airways when breathed through
the nose, but when the nasal passages are bypassed
and particles are breathed through the mouth, there
is a particularly high deposition for ultrafi ne parti-
cles, less than 100-nm diameter. In healthy subjects
breathing through a mouthpiece at rest, the percent-
age of inhaled ultrafi ne carbon particles deposited
(number deposition fraction) varied from 55% for par-
ticles 65 nm MMAD to 80% for the smallest particles
8 nm MMAD (Daigle
et al.
, 2003). This contrasts with
a deposition fraction for fi ne particles under the same
circumstances of well under 50% (Beckett
et al.
, 2005).
The deposition fraction of particles further increases
with exercise and is greater in subjects with mild
asthma than in normal subjects (Chalupa
et al.
, 2004).
The biological fate and also the toxicological effects
of deposited particles depend on the site of deposition
in the airways and on particle characteristics, including
size, solubility in airway lining fl uid, and other physi-
cal characteristics (Oberdörster and Driscoll, 1997).
For a more detailed discussion of aerosol proper-
ties, generation, and measurement, see Mercer (1973),
Stockham and Fochtman (1977), and USEPA (2004).
µ
m
aerodynamic diameter. Particles greater than 10
m aer-
odynamic diameter are effi ciently fi ltered by the nasal
passages, whereas particles smaller than this may pen-
etrate to the alveoli, which are more sensitive to injury
and may serve as a route for translocation to other parts
of the body.
An alternate convention when describing the size of
particles in ambient air pollution is to divide particles
into categories of total suspended particulate (TSP)
comprising all particles suspended in air; particulate
matter less than 10-
µ
m mean aerodynamic diameter
(PM
10
, which are likely to penetrate beyond the upper
airway fi ltering mechanisms) and particulate mat-
ter less than 2.5-
µ
m (PM
2.5
), which are more likely to
be deposited in the terminal bronchioles and alveoli
(as shown in Table 1) and to have systemic effects.
In occupational health, particles are sometimes
categorized into size categories according to the por-
tions of the respiratory system they can reach when
inhaled. Using this classifi cation, “respirable particles”
with a mass median aerodynamic diameter less than
approximately 5-
µ
3.1.1 Absorption of Gases and Vapors
The ability of vapors and gases to penetrate the air-
ways depends on their solubility in water. The higher
the solubility in water, the higher up in the tracheobron-
chial tract the gas will be absorbed. For example, sulfur
dioxide is highly soluble in water, and more than 99%
of concentrations found in typical air pollution epi-
sodes is absorbed in the nose (Speizer and Frank, 1966).
Other less important factors determining the effi ciency
of uptake of vapors and gases include the rate of air-
fl ow (increased absorption with increased airfl ow) and
the concentration of the gas (Fiserova-Bergerova, 1983).
However, vapors and gases of metallic compounds are
usually insoluble in water and will reach the alveoli,
where they can penetrate the air-blood barrier.
µ
m are considered more hazardous,
TABLE 1 Calculation of Total Absorption into the
Body as a Function of Two Different Rates of Alveolar
Absorption and Different Particle Sizes for a Specifi c
Deposition and Clearance Model
Tracheobronchial - Total
Particle
Alveolar
absorption Size (%)
(MMAD)
deposition
nasopharyngeal into body
(
µ
m)
(%)
when alveolar absorption is:
100%
50%
0.1
50
9
50.4
26.7
0.5
30
16
30.8
16.6
3.1.2 Deposition of Particles
2.0
20
43
22.2
12.6
5.0
10
68
13.4
8.6
Most of the absorption of less soluble particles occurs
in the lower part of the respiratory tract. However,
it has been shown that some transfer of metals in the
form of nanoparticles (ultrafi ne particles) and/or solu-
tions may occur by a direct transfer from the olfactory
10.0
5
83
9.2
6.8
From Task Group on Metal Accumulation, 1973.
MMAD, Mass median aerodynamic diameter.
Gastrointestinal absorption is assumed to be 5%.
