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 11.

FIGURES

Figure 1. Schematic of mechanisms that affect fiber deposition and retention in the lungs. The deposition depends on all the indicated parameters in a complex fashion. However, larger diameter particles are affected more by gravitational settling, impaction, and interception, resulting in greater deposition further up in the respiratory tract. The saddle points, or carinae, in the branching respiratory tree are often a focal point for deposition of larger diameter fibers. Smaller diameter particles are affected more by diffusion and can collect in the smaller airways and gas exchange region (alveoli). Particle removal from the lungs is primarily effected by the cilia coating the non-gas exchange regions of the lungs; the cilia push mucus produced in the lungs and any particles trapped in the mucus out of the lung and into the gastrointestinal tract in a matter of hours or days. Some fibers are sufficiently soluble in lung fluid that they can disappear in a matter of months. Finally, white blood cells or macrophages roam the gas exchange regions and ingest particles deposited there for removal through the lymph system. Human macrophages are approximately 17 :m in diameter and can only ingest particles smaller than they are. Therefore, thin fibers are likely to deposit in the gas exchange region and, of these, the long insoluble fibers can remain in the lungs indefinitely.

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NIOSH Manual of Analytical Methods