Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 269

Fig. 8.28 Average deposition
efficiency for asbestos and the
MMVF at different fibre
lengths

industry, especially since asbestos has been found as the culprit for asbestosis and
mesothelioma.
The deposition of the fibres was performed through the Lagrangian tracking
method. In this tracking scheme, the fibre was considered as a point location in
space with an equivalent aerodynamic diameter. The fibre is considered deposited
onto a wall when the distance between the wall and the point in space is less than half
the equivalent aerodynamic diameter. Thus its elongated shape is not considered, and
deposition by interception is not achieved. Deposition by interception can occur if the
particle travels close enough to a surface of the airway passages where an edge of the
particle touches the surface. Usually the fibre length determines whether the particle
will be intercepted. Typical fibres with a diameter of 1 m

have been found to deposit in the bronchial tree (Sussman et al. 1991). Although the
nasal airway is more treacherous in comparison to the tracheobronchial airway, the
nose is much larger in size and the effects of deposition of long fibre lengths may
be marginal. The work by Su and Cheng (2005) also discusses this issue, where de-
position in the nasal airway was mainly due to impaction and that short fibres could
have the same deposition fraction as long fibres in a specific nasal airway region or
subregion, as long as the inertial parameters for both fibres are in the same range.
However for asbestos ( d ae ≈

μ m and a length of 200 μ

m) at low flow rates such as 7.5 L/min, the inertial
parameter is low and deposition by interception may be more significant for very
long fibre lengths.
The deposition of the MMVFs and asbestos fibres over a range of the inertial
parameter is used to compare against previous studies of total deposition efficien-
cies of spherical particles in the nasal cavity (Fig. 8.29 ). The deposition efficiency
increases as the inertial parameter increases. The range in which the deposition
efficiency reaches 90 % or more is from 20,000 (Cheng et al. 2001) to 50,000
(Su and Cheng 2005). The simulation results used the TC-model show the combined
deposition efficiency for the left and right cavity. For IP < 10,000 the deposition
efficiency is higher for the simulation than for Su and Cheng (2005). The higher

1 μ