Longitudinal Distribution of Ozone and Chlorine in the Human Respiratory Tract: Simulation of Nasal and Oral Breathing with the Single-Path Diffusion Model

In the single-path model of the respiratory system, gas transport occurs within a conduit of progressively increasing cross-sectional and surface areas by a combination of flow, longitudinal dispersion, and lateral absorption. The purpose of this study was to use bolus inhalation data previously obtained for chlorine (Cl2) and for ozone (O3) to test the predictive capability of the single-path model and to adjust input parameters for applying the model to other exposure conditions. The data, consisting of uptake fraction 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, gas-phase dispersion coefficients, and gas-phase mass transfer coefficients while neglecting diffusion limitations in the mucus phase, the single-path model was capable of predicting the uptake distribution for O3 but not the steeper distribution that was observed for Cl2. To simultaneously explain the data for these two gases, it was necessary to increase gas-phase mass transfer coefficients and to include a finite diffusion resistance of O3 within the mucous layer. The O3 reaction rate constants that accounted for this diffusion resistance, 2 × 106 s−1 in the mouth and 8 × 106 s−1 in the nose and lower airways, were much greater than previously reported reactivities of individual substrates found in mucus.