The Influence of Ecogeographic Variation in Human Nasal Morphology on Thermal Conditioning of Inspired Air

To protect the lungs from desiccation and thermal damage, inspired air must be heated to core body temperature (37°C) and 100% saturated with water vapor upon reaching pulmonary tissues. The majority of air conditioning occurs in the nasal passages, where heat and moisture are transferred to inspired air from nasal mucosa via concurrent convection and evaporation. Given that physiological demand for air conditioning is largely dependent on the external environment, many studies have noted strong associations between climate and ecogeographic patterning of human nasal morphology. Specifically, these studies have shown that individuals indigenous to cold‐dry environments exhibit relatively longer/taller/narrower nasal passages than individuals from hot‐humid climates. These apparent climate‐mediated morphologies are assumed to reflect functional differences, with longer/taller/narrower nasal passages in cold‐dry climates enhancing respiratory heat and moisture exchange via increased relative mucosal surface area. However, few attempts to experimentally test these assertions have been made. Accordingly, the purpose of this study is to empirically test associations between nasal morphology and air‐conditioning function through both morphometric assessments of 3D morphology using Computed Tomography (CT) imaging and Computational Fluid Dynamics (CFD) analyses of nasal airflow. We assessed cranial CT scans of 2 individuals—one of European ancestry and one of West African ancestry. 3D models of the nasal passages were created using 3D Slicer and Mesh Mixer software. Airway models were artificially dilated in silico to simulate fully decongested nasal passages prior to collecting morphometric measurements of passage height/breadth/length dimensions, mucosal surface area, and airway volume. Using ANSYS fluent software, 3D models of each individual were then employed in CFD simulations to assess morphology‐mediated differences in intranasal heat and moisture transfer with ambient air conditions set at −5°C with 35% relative humidity. Consistent with previous research, the individual of European ancestry exhibited longer/taller/narrower nasal passages compared to the individual of West African ancestry. The individual of European ancestry had a higher mucosal surface area (SA=14.9 cm2) and lower airway volume (V=28.4 cm3) resulting in a higher surface‐area‐to‐volume ratio (SA/V=0.52) compared to the individual of West African ancestry (SA=14.8 cm2; V=29.9 cm3; SA/V = 0.49). Results from our CFD analysis similarly followed theoretical predictions. The higher SA/V ratio of the individual of European ancestry resulted in a slightly greater transfer of heat to inspired air entering the nasopharynx [32.6°C] compared to the individual of West African ancestry [32.4°C]. Our study thus provides support for assertions that ecogeographic variation in human nasal passages reflects climate‐mediated evolutionary demands for intranasal air‐conditioning.