Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-003900, 2008 Oct; :1-43
Inhalation exposure of fiber aerosol may have serious health consequences including lung cancers. The deposition pattern in the respiratory tract as a function of fiber dimensions is the information critical to understanding respiratory dosimetry and defining the index of exposure for health protection purposes. Controlled studies of fiber deposition in human volunteers are not available because of ethical concerns. However, total and regional depositions of inhaled fibers have been estimated from post-mortem measurement, mathematical modeling, and animal toxicity studies. Increasingly, mathematical deposition models have been used to assess the dosimetry of inhaled man-made vitreous fibers. However, current lung dosimetric models for fibers in the human respiratory tract are based on theoretical equations, which have not been verified with experimental data. This project has three objectives: (1) to develop experimental information on the deposition of fibrous aerosols as a function of fiber diameter and length in realistic human respiratory tract replicas, (2) to verify and improve the prediction of fiber dose estimates in human lungs using both empirical data as well as computational fluid dynamic technique, and (3) to define a size-selective exposure index based on fiber penetration data. In the deposition experiment, a human nasal airway cast and an oral/upper tracheobronchial airway replica were used for the study. Carbon, glass fiber, and Ti0 2 fibers were used as the test material. Deposition patterns in the oral airway, nasal airways, and the tracheobronchial region were obtained as a function of fiber dimensions and inspiratory flow rate. For the test fibers, deposition efficiency increased with flow rate and square of aerodynamic diameter, indicating that impaction is the main deposition mechanism of these fibers. The computation fluid technique shows the transport of fiber in the airway and deposition mechanism. Deposition efficiencies of fibers in the nasal, oral, and tracheobronchial airways of fibers were smaller than spherical particles of the same aerodynamic diameter. This appears to be a result of the tendency of fiber to align with flow direction, resulting in lower drag in the axial flow direction. On the other hand, fibers were in the perpendicular direction with respect to the airway wall, resulting in higher drag force in that direction. The end result is that a fiber in alignment with the main flow more easily penetrates the upper respiratory tract. Lower deposition efficiencies in the oral and nasal airways implied higher penetration of fibers into the lower airway regions. Fiber deposition equations in the nasal, oral, and tracheobronchial airways were developed and can be used in the fiber lung deposition model. Because lung diseases caused by inhaled fibers occur in the bronchial, alveolar, and parachymal regions, a thoracic fraction defined as the fraction of particles penetrating the larynx and reaching the lung was defined from the experimental data obtained in this study. The experimental data show that the thoracic fraction of fiber aerosol is different from that of spherical particles obtained in this study. This research also produced essential information on the dosimetry of inhaled fibers in the human lung, data for an improved mathematical lung deposition model, and a definition of the thoracic fraction of fibers for exposure assessment. Sampling devices based on this size-selection definition can be developed in the future for improved assessment of worker exposure.
Air-samplers; Air-sampling; Air-quality-monitoring; Air-quality-measurement; Air-sampling-equipment; Air-sampling-techniques; Airborne-particles; Analytical-processes; Analytical-models; Fiber-deposition; Aerosols; Aerosol-sampling; Aerosol-particles; Respiratory-irritants; Nasal-cavity; Oral-cavity; Models; Lung; Lung-disease