Absolute humidity and the human nose: A reanalysis of climate zones and their influence on nasal form and function
Objectives: Investigations into the selective role of climate on human nasal variation commonly
divide climates into four broad adaptive zones (hot-dry, hot-wet, cold-dry, and cold-wet) based on
temperature and relative humidity. Yet, absolute humidity—not relative humidity—is physiologically
more important during respiration. Here, we investigate the global distribution of absolute humidity
to better clarify ecogeographic demands on nasal physiology.
Methods: We use monthly observations from the Climatic Research Unit Timeseries 3 (CRU TS3)
database to construct global maps of average annual temperature, relative humidity and absolute
humidity. Further, using data collected by Thomson and Buxton (1923) for over 15,000 globallydistributed
individuals, we calculate the actual amount of heat and water that must be transferred
to inspired air in different climatic regimes to maintain homeostasis, and investigate the influence
of these factors on the nasal index.
Results: Our results show that absolute humidity, like temperature, generally decreases with latitude.
Furthermore, our results demonstrate that environments typically characterized as “coldwet”
actually exhibit low absolute humidities, with values virtually identical to cold-dry environments
and significantly lower than hot-wet and even hot-dry environments. Our results also
indicate that strong associations between the nasal index and absolute humidity are, potentially
erroneously, predicated on individuals from hot-dry environments possessing intermediate (mesorrhine)
nasal indices.
Discussion: We suggest that differentially allocating populations to cold-dry or cold-wet climates
is unlikely to reflect different selective pressures on respiratory physiology and nasal morphology
—it is cold-dry, and to a lesser degree hot-dry environments, that stress respiratory function. Our
study also supports assertions that demands for inspiratory modification are reduced in hot-wet
environments, and that expiratory heat elimination for thermoregulation is a greater selective pressure
in such environments.
