Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 important determinant in the particle overload phenomenon. The contribution of extractable
2 organics has been shown to be inconsequential in this response (this subject is discussed in more
3 detail in Chapter 1 0). It is, however, important to note that dose determination for inhaled
4 compounds remains problematic among toxicologists (Dahl et al., 1991).
5 The dosimetric aspects considered will include deposition in the conducting airways and
6 alveolar regions, normal particle clearance mechanisms and rates in both regions, clearance rates
7 during lung overload, elution of organics from the particles, particle transport to extraalveolar
8 sites, and the interrelationships of these factors in determining the target organ dose.
9
10 4.2. REGIONAL DEPOSITION OF INHALED PARTICLES
11 The regional deposition of particulate matter in the respiratory tract is dependent on the
12 interaction of a number of factors, including respiratory tract anatomy (airway dimensions and
13 branching configurations), ventilatory characteristics (breathing mode and rate, ventilatory
14 volumes and capacities), physical processes (diffusion, sedimentation, impaction, and
15 interception), and the physicochemical characteristics (particle size, shape, and density) of the
16 inhaled particles. Regional deposition of particulate material is usually expressed as deposition
17 fraction of the total particles or mass inhaled and may be represented by the ratio of the particles
18 or mass deposited in a specific region to the number or mass ofparticles inspired. The factors
19 affecting deposition in these various regions and their importance in understanding the fate of
' .
20 inhaled diesel exhaust particulate matter are discussed in the following sections. It is beyond the
21 scope of this document to present a comprehensive account of the complexities of respiratory
22 mechanics, physiology, and toxicology. Where appropriate, the reader is referred to publications
23 that provide a more in-depth treatment of these topics (Weibel, 1963; Brain and Mensah, 1983;
24 Raabe et al., 1988; StOber et al., 1993).
25
26 4.2.1. Physical Processes, Physiological/Anatomical Considerations, and Particle
27 Characteristics
28 Oeposition of particles may occur through several processes or combinations thereof,
29 including diffusion, sedimentation (gravitational settling), interception, electrostatic
30 precipitation, and impaction. It is important to appreciate that these processes are not necessarily
31 independent but may, in some instances, interact with one another. such that total deposition in
32 the respiratory tract resulting from these processes may be less than the calculated probabilities
33 for deposition by the individual processes (Raabe, 1982). Depending on the particle size and
34 mass, varying degrees f deposition may occur in the nasopharyngeal, tracheobronchial, and
35 alveolar regions of the respiratory tract.
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