PRINCIPLES OF TOXICOLOGY - Biology East Borneo

44 ABSORPTION, DISTRIBUTION, AND ELIMINATION OF TOXIC AGENTSSolid and Liquid Particulates The lung can be thought of as consisting of three basic regions: thenasopharyngeal region, the tracheobronchiolar region, and the distal or alveolar region. Particles thatare roughly 5 µm or greater in diameter are generally deposited in the nasopharyngeal region. If theyare deposited very close to the surface, they can be sneezed out, blown out, or wiped away. If they aredeposited slightly farther back, they may be picked up by the mucus-blanketed cilia lining the lung inthis region (the mucociliary “escalator”) and moved back up into the nasopharyngeal region, wherethey may be swallowed and absorbed in the GI tract in accordance with their solubility and absorptioncharacteristics. Particles that fall into the size range of 2–5 µm generally reach the tracheobronchialregion before they impact the lung surface. Most of these particles are also cleared by the mucociliaryescalator back up to the nasopharyngeal region, where they are either eliminated directly or swallowedand absorbed or excreted in the GI tract. Particles smaller than 1 µm in diameter may reach the alveolarregions of the lung. Absorption in the lung, if it takes place at all, will most likely take place in thealveolar region, although there may be some absorption in the tracheobronchiolar region, particularlyif the material is soluble in the mucus.Size is probably the most important single characteristic determining the efficiency of particulateabsorption in the lung. Size determines the region of the lung in which the aerosol is likely to bedeposited. Even within the range of very small particles that reach the alveolar region and may beabsorbed there, size is inversely proportional to the magnitude of particle deposition.Figure 2.5 shows the dependence of lead deposition in the human lung on the size of the leadparticles in an artificially generated lead sesquioxide aerosol that was inhaled by the subjects. Size isexpressed as diffusion mean equivalent diameter (DMED), a measure of mean particle diameter. Theamount deposited was calculated as the difference between the amount of lead that entered the lungand the amount that the subject exhaled. Thus, the lung regions in which the particles were actuallydeposited were not identified. However, the DMEDs for all three aerosols were less than 1 µm. For astandard subject, with a breathing cycle of about 4 s, the lung deposition of lead varied from about 24percent for particles with a DMED of 0.09 µm to 68 percent for very small particles with a DMED ofFigure 2.5 Deposition in the human lung of lead particles of various sizes: • DMED, 0.02 µm; " DMED, 0.04µm; H DMED, 0.09 µm. Data from Chamberlain et al., 1978, Table 5.2.