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Fiber inhalability and head deposition in rats and humans.
Citation:
Asgharian, B., A. M. JARABEK, S. H. GAVETT, AND O. T. Price. Fiber inhalability and head deposition in rats and humans. Presented at 29th Annual Meeting of the American Association for Aerosol Research, Portland, OR, October 25 - 29, 2010.
Impact/Purpose:
The authors describe a model of fiber inhalability into the human and rat respiratory tract and deposition in the nasal cavity. Calculations of fiber inhalability, deposition, and retention in respiratory tract regions of both rats and humans are crucial, both to assess the health risk of fiber exposures and to facilitate inferences from rat inhalation studies.
Description:
Due to their dimensions and long durability, inhaled asbestos fibers clear slowly from lung airways. Retained fibers may injure the epithelium, interact with macrophages, or translocate to the interstitium to result in various respiratory diseases. Therefore, calculations of fiber inhalability, deposition, and retention in respiratory tract regions of both rats and humans are crucial, both to assess the health risk of fiber exposures and to facilitate inferences from rat inhalation studies. Rat inhalation experiments are underway at the EPA and NIEHS. A model of fiber inhalability and initial deposition in the human and rat nasal cavity was developed. Existing models for particles were extended to fibers by replacing particle diameter with an equivalent fiber diameter. Since fiber inhalability into the respiratory tract and deposition in the extra thoracic airways depended mainly on its inertia, equivalent impaction diameters were derived and substituted in expressions for spherical particle diameter to determine fiber inhalability and nasal losses. Fiber impaction diameter depended strongly on its orientation in the air. Highest inhalability was obtained when fibers were aligned perpendicular to the flow streamlines in the inhaled air. However, detailed calculations of fiber transport in slow moving air such as that in the atmosphere and in lung airways showed that fibers stayed primarily aligned (parallel) to the flow. Therefore, for inhalability calculations, fibers were assumed to align to the flow. For nasal deposition calculations, since the inhaled air underwent rapid mixing, fiber orientation tended to be random. Deposition model of spherical particles in nasal passages were extended to fibers by using an equivalent impaction parameter for fibers in random orientation. Model output will provide input concentration and size distribution to the lower respiratory tract for calculating lung deposition and retention of inhaled fibers. (These views are the authors and do not represent US EPA policy).