Case Studies in the Human Respiratory System - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 262

Fig. 8.20 Local DE of all
wood dust within regions of
the nasal cavity. The nasal
regions are defined in Fig. 8.1

20%

Pine Dust
Heavy Oak Dust
Light Oak Dust

15%

10%

5%

0%

1 2 3 4 5 6 7 8 9 10

Nasal Region

There are two major clearance mechanisms of inhaled wood particles in the nasal
cavity: the mucociliary mechanism and by physical means. Particles deposited on
the ciliated epithelium of the nose are cleared by the mucociliary action, i.e. cilia
beating that moves the mucus secretion to remove particles towards the pharynx. The
anterior one-third of the nose is not covered by ciliated epithelium but by cutaneous
epithelium like the skin. The clearance mechanism of deposited wood particles in
these regions is assumed to be by physical means, such as sneezing, wiping and
blowing (Swift and Kesavanathan 1996), which lead to a slower clearance than in
the ciliated epithelium region. In a study of wood dust deposition in the nose of
furniture workers (Hadfield 1972), wood particles were found to accumulate in two
major regions: (i) in a small oval-shaped area on the anterior part of the nasal septum
near the floor of the nose, and (ii) on the anterior part of the middle turbinate. In
the current CFD study, it is noticeable that these regions also exhibit high particle
deposition efficiency. Inhaled wood dust depositing on these regions is retained, and
any toxic substance which it may contain remains in contact with this part of the
epithelium for a longer time than on other regions that are covered by the ciliated
epithelium (Fry and Black 1973) and therefore damages the exposed layer of soft
tissues.
Under the assumption of spherical particles, and with the different wood types
exhibiting different densities, a comparison in terms of their aerodynamic properties
(inertia and settling properties) can be made through the equivalent aerodynamic
diameter d ae defined as

d p ρ p
1000

d ae = (8.25)

This means that, very dense particle with a small diameter can have the same
aerodynamics as one with a large diameter, less dense particle if their d ae are the
same.
A parameter used for normalising impaction-dominant deposition studies is the
inertial parameter, IP, given by

= Qd ae

IP (8.26)