Longitudinal distribution of ozone and chlorine in the human respiratory tract: Simulation of nasal and oral breathing with the single-path diffusionmodel

In the single-path model of the respiratory system, gas transport occurs wi thin a conduit of progressively increasing cross-sectional and surface area s by a combination of flow, longitudinal dispersion, and lateral absorption . The purpose of this study was to use bolus inhalation data previously obt ained for chlorine (Cl-2) and for ozone (O-3) to test the predictive capabi lity of the single-path model and to adjust input parameters for applying t he model to other exposure conditions. The data, consisting of uptake fract ion as a function of bolus penetration volume, were recorded on 10 healthy nonsmokers breathing orally as well as nasally at alternative air flows of 150, 250, and 1000 ml/s. By employing published data for airway anatomy, ga s-phase dispersion coefficients, and gas-phase mass transfer coefficients w hile neglecting diffusion limitations in the mucus phase, the single-path m odel was capable of predicting the uptake distribution for O-3 but not the steeper distribution that was observed for Cl-2. To simultaneously explain the data for these two gases, it was necessary to increase gas-phase mass t ransfer coefficients and to include a finite diffusion resistance of O-3 wi thin the mucous layer. The O-3 reaction rate constants that accounted for t his diffusion resistance, 2 x 10(6) s(-1) in the mouth and 8 x 10(6) s(-1) in the nose and lower airways, were much greater than previously reported r eactivities of individual substrates found in mucus. (C) 2001 Academic Pres s.