Role of the nasal cavity as a perception site of environmental humidity via a menthol-sensitive cold reception mechanism.

Humans can sense environmental humidity; however, the mechanism underlying this ability remains unclear. This study aimed to clarify whether the nasal cavity is involved in humidity sensation. Experiments were conducted on healthy young adults, in which controlled nasal inhalation was conducted under four air conditions (i.e., temperature of 25 °C or 34 °C with relative humidity [RH] of 30 % and 70 %). The temperature of the nasal mucosa and the thermal and humidity sensations of the inhaled air were assessed in the first experiment. In the second and third experiments, sublimated l-menthol was added to the inhaled air, and the sensations of the inhaled air were evaluated. The temperature of the nasal mucosa was higher in inhaled air at 34 °C with 30 % RH than at 25 °C. The rating for thermal sensation (warm sensation) was the highest at 34 °C and 70 % RH. At both temperatures, the humidity sensation rating was higher in air with 70 % RH than in air with 30 % RH. Linear relationships were observed between the ratings for thermal and humidity sensations under the four conditions (R2 = 0.339-0.516). The thermal sensation rating decreased when air at 34 °C was inhaled with l-menthol. A linear relationship was also observed between the ratings for thermal and humidity sensations under this condition (R2 = 0.665). These findings suggest that the nasal cavity is critical for the sensation of humidity in an environment in which cold reception via the TRPM8 channel is involved.