Health Assessment Document for Diesel Emissions - NSCEP | US ...
Health Assessment Document for Diesel Emissions - NSCEP | US ...
Health Assessment Document for Diesel Emissions - NSCEP | US ...
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1 important determinant in the particle overload phenomenon. The contribution of extractable<br />
2 organics has been shown to be inconsequential in this response (this subject is discussed in more<br />
3 detail in Chapter 1 0). It is, however, important to note that dose determination <strong>for</strong> inhaled<br />
4 compounds remains problematic among toxicologists (Dahl et al., 1991).<br />
5 The dosimetric aspects considered will include deposition in the conducting airways and<br />
6 alveolar regions, normal particle clearance mechanisms and rates in both regions, clearance rates<br />
7 during lung overload, elution of organics from the particles, particle transport to extraalveolar<br />
8 sites, and the interrelationships of these factors in determining the target organ dose.<br />
9<br />
10 4.2. REGIONAL DEPOSITION OF INHALED PARTICLES<br />
11 The regional deposition of particulate matter in the respiratory tract is dependent on the<br />
12 interaction of a number of factors, including respiratory tract anatomy (airway dimensions and<br />
13 branching configurations), ventilatory characteristics (breathing mode and rate, ventilatory<br />
14 volumes and capacities), physical processes (diffusion, sedimentation, impaction, and<br />
15 interception), and the physicochemical characteristics (particle size, shape, and density) of the<br />
16 inhaled particles. Regional deposition of particulate material is usually expressed as deposition<br />
17 fraction of the total particles or mass inhaled and may be represented by the ratio of the particles<br />
18 or mass deposited in a specific region to the number or mass ofparticles inspired. The factors<br />
19 affecting deposition in these various regions and their importance in understanding the fate of<br />
' .<br />
20 inhaled diesel exhaust particulate matter are discussed in the following sections. It is beyond the<br />
21 scope of this document to present a comprehensive account of the complexities of respiratory<br />
22 mechanics, physiology, and toxicology. Where appropriate, the reader is referred to publications<br />
23 that provide a more in-depth treatment of these topics (Weibel, 1963; Brain and Mensah, 1983;<br />
24 Raabe et al., 1988; StOber et al., 1993).<br />
25<br />
26 4.2.1. Physical Processes, Physiological/Anatomical Considerations, and Particle<br />
27 Characteristics<br />
28 Oeposition of particles may occur through several processes or combinations thereof,<br />
29 including diffusion, sedimentation (gravitational settling), interception, electrostatic<br />
30 precipitation, and impaction. It is important to appreciate that these processes are not necessarily<br />
31 independent but may, in some instances, interact with one another. such that total deposition in<br />
32 the respiratory tract resulting from these processes may be less than the calculated probabilities<br />
33 <strong>for</strong> deposition by the individual processes (Raabe, 1982). Depending on the particle size and<br />
34 mass, varying degrees f deposition may occur in the nasopharyngeal, tracheobronchial, and<br />
35 alveolar regions of the respiratory tract.<br />
2/1/98 4-2 DRAFT--DO NOT CITE OR QUOTE