Underlying mechanism of diurnal change in thermal sensation response at high relative humidity.

Abstract

Diurnal changes in physiological and behavioral responses to constant relative humidity (RH) were investigated in summer to validate a hypothesis that thermal sensation responses (TSR) vary with the diurnal cycle while maintained at stable RH's of 60, 70, or 80%. Seven lightly clothed male subjects were exposed to one of three RH while air temperature (Ta) was held at 28°C from 9:00-18:30. Mean skin temperature (T‾sk) and tympanic temperature (Tty) were monitored at 2-min intervals throughout the experimental period. Cutaneous warm and cool sensation thresholds and stratum corneum water content (SCWC) on the anterior forearm, posterior forearm, and anterior thigh, finger blood flow rate (SkBF) were measured by a thermal stimulator controlled by a Peltier element (Intercross-230; Intercross, Co., Tokyo, Japan), a water content of stratum corneum monitor (Corneometer CM825 & MPA 5; Integral Co., Tokyo, Japan), and a laser flowmeter (ALF21; Advance Co., Tokyo, Japan), accordingly at the end of three periods: 9:30-10:30, 13:30-14:30, and 17:30-18:30. The TSR and the thermal comfort response were also recorded using subjective scales of thermal sensation and comfort thresholds at the end of three periods. The Tty and mean skin temperature (T‾sk) remained unchanged during the day under all RH conditions. Temperature difference between warm and cold sensation thresholds and SkBF decreased slightly towards the evening under all RH conditions whereas SCWC increased from the morning to the evening at 60% and 80% RH. Behaviorally, the subjects responded more than "slightly warm" at 70%, and 80% RH, and then the TSR changed significantly (p<0.05) to less than "slightly warm" in the evening, although T̅sk remained unchanged at 34.0°C. The results from the previous study in winter was reconfirmed, and the data verify the hypothesis that TSR changes diurnally even though subjects are exposed to a constant and high RH's and stable Ta. It was confirmed in summertime as previously shown in wintertime that the combined reduction in SkBF to increases SCWC when exposed to high RH explains the altered TSR although Ta are unchanged.