Thermoregulation and the Human Nose: Balancing Climatic and Energetic Factors

Purpose: Studies have shown that indigenous individuals from cold-dry climates exhibit longer, taller, and especially narrower nasal passages compared to equatorial counterparts, enhancing inspiratory air-conditioning (heating and moisturizing) capacity and reducing susceptibility to respiratory tract infections. Concurrently, due to increased demand for thermogenesis, cold-dry climates are also metabolically more expensive than tropical environments, necessitating greater volumetric intake of oxygen. Accordingly, recent research has suggested that while a narrower nose enhances inspiratory air-conditioning, the accompanying restriction on volumetric intake may necessitate increased nasal height to maintain sufficient intake of oxygen. The purpose of this study was to examine the relationship between nasal dimensions, climate, and the metabolic demands. Methods: We employed 12 linear measurements collected from the nasal skeleton of 837 modern human crania from major geographic (Arctic Circle, Asia, Australia, Europe, Africa) and climatic (polar, temperate, hot-arid, tropical) zones. Anterior-posterior femoral head diameter (FHD) was further employed as a proxy for overall body size and metabolic requirements. Morphological, climatic, and geographic data were then employed in multivariate analyses. Results: Our results indicate that most breadth measurements of the nasal aperture and internal cavity are significantly correlated with climate (all significant R2 values between 0.29–0.51 with p-values 2 values between 0.67–0.78 with p-values 2 values between 0.36–0.56 with p-values 2 = 0.67, p = 0.0003), while nasal passage shape retained a significant relationship with climate (R2 = 0.66, p = 0.0004) with relatively tall/narrow airways associated with colder-drier environments. Conclusion: Collectively, these results support the assertion that physiological demands for temperature and moisture exchange are predominantly mediated by nasal passage breadth, with airway height representing a compensatory mechanism for ensuring a metabolically sufficient oxygen intake. Additional studies employing more direct measures of metabolic demands are accordingly warranted.