Deconstructing adaptation in the human nasal airway: An iterative assessment of intranasal airflow and nasal morphology in two and three dimensions.

INTRODUCTION: Nasal geometries have been established as being strongly correlated with climate. In cold-dry climates geometries tend to be tall/narrow, while in hot-humid climates geometries are short/broad. This pattern suggests that taller/narrower nasal airways functionally enhance intranasal air-conditioning (i.e., heat and moisture transfers) by increasing nasal surface area relative to intranasal volume, but this hypothesis has not been empirically tested. METHODS: This study employs 3D models of decongested nasal passages generated from cranial CT scans of individuals of predominantly European (5 males, 6 females) and African (5 males, 5 females) ancestry to directly investigate proposed form-function relationships. Each 3D model then underwent an airflow simulation using computational fluid dynamics to collect measurements of intranasal air temperature, humidity, and airflow velocity across consecutive planes of the 3D models. Morphological measurements of nasal passage area, perimeter, and circularity (i.e., shape) were subsequently collected from the same cross-sectional planes for which airflow measurements were collected. spanning the length of the passage. Multivariate regressions were then performed to assess relationships between morphological and physiological variables at each 2D plane and cumulatively across the entire 3D passage. RESULTS: Regression results indicate that nasal passage morphology, particularly turbinate chamber circularity was significantly associated with intranasal air-conditioning (R2 =0.54, p=0.029). Consistent with proposed hypotheses, individuals with taller/narrower (i.e., less circular) turbinate chamber cross-sectional shapes were found to be associated with higher total inspiratory heat and moisture transfers. CONCLUSIONS: This study provides support for climatic pressures selecting for intranasal air conditioning promoted adaptive changes in nasal morphology during human evolution, likely accounting for the observance of taller/narrower nasal passages in Pleistocene and Holocene Homo sapiens following migrations into colder and/or drier environments.