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

Most air conditioning of inspired air occurs in the nasal passages and is largely dependent on external environments. Studies show strong associations between climate and ecogeographic patterning of human nasal morphology. Individuals indigenous to cold-dry environments exhibit relatively longer/taller/narrower nasal passages than individuals from hot-humid climates, and these morphologies are assumed to reflect functional differences. To test these associations between nasal morphology and air-conditioning function, we assessed cranial CT scans of 2 individuals"—one of European ancestry (EA) and one of West African ancestry (WA). 3D models of nasal passages were created using 3D Slicer software and were artificially dilated in-silico to simulate fully decongested nasal passages prior to collecting morphometric measurements, mucosal surface area (SA), and airway volume (AV). 3D models of each individual were then employed in Computational Fluid Dynamics (CFD) simulations, via ANSYS fluent software, to assess differences in intranasal airflow heat and moisture transfer. Ambient air conditions were set at -5°C, 35% relative humidity. As expected, the EA individual exhibited longer/taller/narrower nasal passages compared to the WA individual. The EA individual exhibited higher mucosal SA and lower AV resulting in a higher surface-area-to-volume (SA/V) ratio compared to the WA individual. Our CFD simulations also followed theoretical predictions. The higher SA/V ratio of the EA individual resulted in increased heat transfer compared to the WA individual. The results of our study provide support for assertions that ecogeographic variation in human nasal passages reflects climate-mediated evolutionary demands for intranasal air-conditioning.