Anthropologists have long posited that geographic‐mediated associations between human nasal morphology and climate evince climatic adaptation. These arguments overwhelmingly focus on the prominent role of the nose in respiratory air‐conditioning, as intranasal heat and moisture exchange in different climates is governed anatomically via the amount of nasal mucosa surface area relative to the volume of air passing through each nasal passage. Yet, the inability to quantitatively account for the nasal cycle (a physiological process in which the left and right nasal passages reciprocally alternate in their mucosal congestion levels) has limited investigation into the adaptive influence of nasal soft‐tissues. Accordingly, the goals of this study were to 1) develop protocols for accurately modeling the three‐dimensional (3D) anatomy of nasal airways with in silico controlled variation in mucosal congestion, and 2) test the hypothesis that mucosal surface area‐to‐volume ratios (SA/V) remain constant throughout the nasal cycle. A computed tomography (CT) scan of one male human head was selected for use in the development of protocols for controlling congestion levels via digital expansion/contraction of the nasal mucosa in Amira‐Avizo. These protocols were then used to generate a fully decongested (left/right = 0/0%) nasal airway model for use as an anatomical baseline comparator. Models were then generated for two different phases of the nasal cycle: asymmetrical (left/right = 90/10%) and mid‐cycle (left/right = 50/50%). Nasal passage surface areas and volumes were collected for each model to permit comparisons of SA/V ratios across different mucosal congestion levels. Following theoretical expectations, the decongested model exhibited a substantially lower SA/V (0.57) than the mid‐cycle (0.72) and asymmetrical (0.74) models. Unilateral analyses also met anatomical expectations, with the highly congested left nasal passage of the asymmetrical model demonstrating a higher SA/V (1.06) compared to the same left passage in the mid‐cycle (0.84) and decongested (0.60) models. Cumulatively, these results suggest that the developed 3D digital methods permit reliable in silico modeling of nasal soft‐tissues, allowing future studies to control for mucosal congestion while investigating the role of nasal morphology on respiratory airflow (using computational fluid dynamics analysis, etc.). Moreover, the similar overall SA/V ratios of the two nasal cycle models appear consistent with the hypothesis that, despite morphological asymmetry, the nose’s overall SA/V and air‐conditioning capacity likely remains relatively stable throughout the nasal cycle. Thus, our study suggests that, rather than morphological variability within the nasal cycle, it is the distinction between the semi‐congested nasal cycle versus complete mucosal decongestion (as seen during strenuous exercise) that may confound functional interpretations of ecogeographic variation in nasal morphology. Consequently, further research is needed to determine how these disti...

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.