High-temperature effects on the mucus layers in a realistic human upper airway model

Abstract

Firefighters and workers in some industries often inhale (polluted) air at high temperatures, eg, 50–200°C, and very low relative humidity, say, 5–10%. Such severe environmental conditions may cause health problems or further aggravate existing ones. As in vivo tests are very complex, potentially harmful and costly, and in vitro experiments often lack high resolution and predictive capability, in silico studies become a priority. They may include computer simulations of the air-particle dynamics in human lung-airway models to understand the convection heat transfer as well as pollutant transport, deposition and uptake at high inlet temperatures and low relative humidity. In this study, detailed modeling, simulation and analysis focuses on two-phase flow in a human upper lung-airway model with a realistic 3D mucus lining. Especially modeling of the upper airway mucus layers was of interest in order to simulate the vapor mass transfer to the airflow because of its primary function for airway humidification. Naturally, the humidification causes water loss in the mucus layer, which leads to its reduction or even depletion when exposed to relatively high temperatures. Accounting for the changes in thickness and the rise in temperature in the mucus layer allows for the determination of locations of thermal injury in the human airways due to continuous exposure of such abnormal inhalation conditions. Different temperature profiles and local changes in mucus layer thickness were studied for ranges of severe inlet temperature conditions at a representative flow rate of 20 LPM (liters per minute). For inlet temperature reaching 100°C, mucus-layer thinning was observed in the upper airways. Interestingly, as a confirmation of the Reynolds analogy, the areas of significant wall heat flux and associated wall shear stress coincided with the regions of highest mucus evaporation, resulting in the humidification of the air with low relative humidity. Model development and mucus layer generation were done using C++ programming. All computer simulations were carried out using the open-source computational fluid dynamics toolbox OpenFOAM.