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Why the salt aerosol must consist of salt particles of a certain size?
The deposition of inhaled particles in the lung is primarily governed by the
mechanisms of inertial impaction, gravitational sedimentation, Brownian diffusion,
and, to a lesser extent, the mechanisms of electrostatic precipitation and interception.
Deposition is minimal for particles in the size range 0.4–0.7 μm that have minimal
intrinsic mobility. Deposition of larger particles results from significant gravitational
and inertial transport, whereas deposition of smaller particles is mainly due to
diffusive transport. Data show that total lung deposition is a poor predictor of clinical
outcome, and that regional deposition needs to be assessed to predict therapeutic
effectiveness. The spatial distribution of deposited particles and, as a consequence,
drug efficiency is strongly affected by particle size. Large particles (> 6 μm) tend to
mainly deposit in the upper airway, limiting the amount of drugs that can be delivered
to the lung. Small particles ( < 2 μm) deposit mainly in the alveolar region and are
probably the most apt to act systemically, whereas the particle in the size range 2-6
μm are be best suited to treat the central and small airways.
In the figure 1 the size distributions of mass (red bars) and number (blue bars) of
salt particles produced by generator IIRIS-136 are presented. It is seen that the most
numerous are particles with diameter 0.3 μm, whereas the particles with diameter 3
μm have the largest mass portion of generated salt aerosol. It should be noted that
when sodium chloride are introduced into the environment, where the relative
humidity exceed 75%, the solid salt particles will turn into droplets of salt solution
with size more than twice of solid size. Thus dry NaCl particles with diameters less
than 0.5 μm grow to their maximum size in the first bronchi. Although it takes longer
path in airways for more massive particle to grow they deposit predominately in the
upper and central airways. The smaller more numerous particles assure more uniform
coverage of alveolar and small airways surfaces by salt solution compared with larger
more massive particle, but require longer exposition time to get equivalent mass of
salt deposited. The deposition salt particle is enhanced due to electrical charges they
© Copyright by IIRIS
Number density [cm-3]
Mass concentration [mg·m-3]
Diameter [m]
Figure 1. Number and mass distribution of salt particles generated by Iiris-136. Unlike to the total
number density or total mass concentration of generated particle the form of their distribution by size
is rather independent of the working regime of generator.
carry. The surfaces of fragments of salt crystals acquire charges of opposite sign
during the grinding in generator. Measurements have shown that there are particle
with both sign charges in salt aerosol that reaches patients in cocoon, but positively
charged particles are dominant and salt aerosol has, therefore, positive space charge.
C. Darquenne, Particle deposition in the lung,
Encyclopedia of Respiratory Medicine, 2006, Pages 300–304
The deposition of inhaled particles in the lung is primarily governed by the
mechanisms of inertial impaction, gravitational sedimentation, Brownian diffusion,
and, to a lesser extent, the mechanisms of electrostatic precipitation and interception.
Deposition is minimal for particles in the size range 0.4–0.7 μm that have minimal
intrinsic mobility. Deposition of larger particles results from significant gravitational
and inertial transport, whereas deposition of smaller particles is mainly due to
diffusive transport. Apart from particle size, several factors affect particle deposition,
including particle characteristics, physiological and environmental factors, and lung
anatomy.
© Copyright by IIRIS
Figure 1. Mechanisms of deposition of inhaled particles in the respiratory tract. The mechanisms of
deposition are not restricted to the zone of the lung as shown in the figure. For example, deposition by
sedimentation can also occur in the acinus (see text for details).
Figure 2. Typical curve showing the effect of particle size on total deposition in the human lung. Total
deposition is expressed as a percentage of the total number of inhaled particles
© Copyright by IIRIS
Alan T. Cocks, Rohantha P. Fernando, The growth of sulphate aerosols in the human
airways. Journal of Aerosol Science, Volume 13, Issue 1, 1982, Pages 9-19.
Abstract
Fundamental heat and mass transfer considerations have been used to estimate growth
rates in the human airways for inhaled sulphuric acid, ammonium bisulphate and
ammonium sulphate droplets with initial radii of 0.1 and 1 μm. Maxwell's equations
for bulk diffusion and heat transfer, modified for the effects of molecular (Knudsen)
flow, have been utilized and solutions to the growth expressions obtained numerically
using interpolation of tabulated physical data. The calculations predict that
equilibrium radii in the lungs will be between two and three times the initial radii and
that droplets of initial radii of 0.1 μm will reach their equilibrium size in - 0.1 sec
whereas 1 μm particles will take ≥ 10 sec. Similar growth curves are predicted for all
three solutes and hence neutralization of inhaled H2SO4 aerosols by NH3 released in
the respiratory system would be expected to have a negligible effect on the rate of
growth of the droplets.
G.A. Ferron, B. Haider, W.G. Kreyling. A method for the approximation of the
relative humidity in the upper human airways. Bulletin of Mathematical Biology,
Volume 47, Issue 4, 1985, Pages 565-589.
Abstract
In order to determine the growth of inhaled aerosol particles in the human respiratory
tract the relative humidity in a lung model has been calculated using a numerical
method. The computations take into account different types of airflows, enhanced
transport mechanisms and an optimized wall temperature profile in the upper airways.
These parameters are varied to fit experimental temperature data. Under certain
conditions the corresponding relative humidity shows a maximum near the first
bifurcation, which exceeds the final humidity in the alveoli. This high humidity forces
dry NaCl particles with diameters less than 0.5 μm to grow to their maximum size in
the first bronchi. Thereafter the droplets loose water and reach their final size in the
terminal bronchioles.
Darquenne, C. Aerosol deposition in health and disease. Journal of Aerosol
Medicine and Pulmonary Drug Delivery.Volume 25, Issue 3, 1 June 2012, Pages
140-147.
Abstract
The success of inhalation therapy is not only dependent upon the pharmacology of the
drugs being inhaled but also upon the site and extent of deposition in the respiratory
tract. This article reviews the main mechanisms affecting the transport and deposition
of inhaled aerosol in the human lung. Aerosol deposition in both the healthy and
diseased lung is described mainly based on the results of human studies using
nonimaging techniques. This is followed by a discussion of the effect of flow regime
on aerosol deposition. Finally, the link between therapeutic effects of inhaled drugs
and their deposition pattern is briefly addressed. Data show that total lung deposition
is a poor predictor of clinical outcome, and that regional deposition needs to be
assessed to predict therapeutic effectiveness. Indeed, spatial distribution of deposited
particles and, as a consequence, drug efficiency is strongly affected by particle size.
Large particles (> 6 μm) tend to mainly deposit in the upper airway, limiting the
© Copyright by IIRIS
amount of drugs that can be delivered to the lung. Small particles ( < 2 μm) deposit
mainly in the alveolar region and are probably the most apt to act systemically,
whereas the particle in the size range 2-6 μm are be best suited to treat the central and
small airways.
Number density [cm-3]
Mass concentration[mg·m-3]
Diameter [m]
© Copyright by IIRIS