Health Assessment Document for Diesel Emissions - NSCEP | US ...
Health Assessment Document for Diesel Emissions - NSCEP | US ...
Health Assessment Document for Diesel Emissions - NSCEP | US ...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
1 volume basis (Table 4-1 ). This is due to larger alveolar diameters in humans and concomitantly<br />
2 lower surface area per unit of lung volume.<br />
3 The alternative, perhaps more accurate physiologically, is to consider deposition rate<br />
4 relative to exposure concentration; the deposition rate will initiate particle redistribution<br />
5 · processes (e.g., Clearance mechanisms, phagocytosis) that transfer the particles to various<br />
6 subcompartments, including the alveolar macrophage pool, pulmonary interstitium, and lymph<br />
7 nodes. Over time, there<strong>for</strong>e, only small amounts of the original particle intake would be<br />
8 associated with the alveolar surface.<br />
9<br />
10 4.3. RESPIRATORY TRACT CLEARANCE RATES<br />
11 4.3.1. Tracheobronchial Clearance<br />
12 The dynamic relationship between deposition and clearance is responsible <strong>for</strong><br />
13 determining lung burden at any point in time. Clearance of highly insoluble particles from the<br />
14 tracheobronchial region is mediated primarily by mucociliary transport and is a more rapid<br />
15 process than those operating in alveolar regions. Mucociliary transport (often referred to as the<br />
16 . mucociliary escalator) is accomplished by the rhythmic beating of cilia that line the respiratory<br />
· 17 tract from the trachea through the terminal bronchioles. This movement propels the mucous<br />
18 layer containing deposited particles (or particles within alveolar macrophages [AMs]) toward the<br />
19 larynx. Clearance rate by this system is determined primarily by the flow velocity of the mucus,<br />
20 which is greater in the proximal airways and decreases distally. These rates also exhibit<br />
21 inte.rspecies and individual variability. Considerable species-dependent variability in<br />
22 tracheobronchial clearance has been reported, with dogs generally having faster clearance rates<br />
23 than guinea.pigs, rats, or rabbits (Felicetti et al., 1981). The half-time (t 112 ) values <strong>for</strong><br />
24 tracheobronchial clearance of relatively insoluble particles are usually on the order ofhours;<br />
25 those <strong>for</strong> alveolar clearance may be hundreds of days in humans and dogs. The clearance of<br />
26 particulate matter from the tracheobronchial region is generally recognized as being biphasic or<br />
27 multiphasic (Raabe, 1982). Some studies have shown that particles are cleared from large,<br />
28 intermediate; and small airways with t 112 of0.5, 2.5, and 5 h, respectively. However, reports have<br />
29 indicated that clearance from conducting airways .is biphasic and that the long-term component<br />
30 <strong>for</strong> humans may take much longer <strong>for</strong> a significant fraction of particles deposited in this region<br />
31 and may not be complete within 24 h, as generally believed (Stahlhofen et al., 1990).<br />
32 Although most of the particulate matter cleared from the tracheobronchial region will<br />
33 ultimately be swallowed, the contribution of this fraction relative to carcinogenic potential is<br />
34 unclear. With the exception of conditions of impaired bronchial clearance, the desorption t 112 <strong>for</strong><br />
35 particle-associated organics is generally longer than the tracheobronchial clearance times,<br />
2/1/98 4-6 DRAFT --DO NOT CITE OR QUOTE