Deposition of Ultrafine Aerosols in the Head Airways During Natural Breathing and During Simulated Breath Holding Using Replicate Human Upper Airway Casts

Abstract

Natural breathing and simulated breath-holding techniques have been used to measure inspiratory and expiratory head deposition of inhaled particles in human subjects. Because the simulated breath-holding path, in which the aerosol is drawn through the nose and mouth, differs from the natural path where inhaled particles enter the nose and penetrate through the larynx and trachea, the present study was undertaken to compare the deposition of ultrafine aerosol between these two experimental methods. Two replicate human upper airway casts containing a nasal airway, an oral passage, and a laryngeal-tracheal section were used to measure the head deposition efficiencies of monodisperse silver or polystyrene latex particles. Particles whose thermodynamic diameters ranged from 3.6 to 150 nm were drawn through the casts at constant flow rates ranging from 4 to 30 L/min. For the inhalation study, test aerosols were drawn into the nasal airway and directed either through the laryngeal-tracheal section or through the oral passage; these flow patterns were reversed for the exhalation study. Results indicated that the difference in ultrafine aerosol deposition was not statistically significant at the 95% confidence level between the nose-mouth and nose-trachea paths for inhalation (p = 0.10) and exhalation (p = 0.33). For the range of particle sizes and flow rates studied, this finding suggests that the simulated breath-holding method, where test aerosols are drawn through the nose and mouth, is appropriate for estimating the inspiratory and expiratory deposition efficiency of ultrafine particles in the nose-trachea path.