Influence of human thermal adaptation and its development on human thermal responses to warm environments

The human thermal adaptation has been documented while the underlying mechanisms of the adaptive processes remain incompletely understood. This study was to evaluate human thermal adaptation and take a step further to reveal the development via a repeated heat exposure design. Two series of experiments were conducted in a well-controlled climate chamber, one in summer and the other, in autumn. Sixteen subjects were selected in Series I (temperature: 26 °C/29 °C/32 °C, relative humidity (RH): 15%/50%/85%) and twenty-four were engaged in a 7-day repeated experiment in Series II (32 °C/20% RH vs 32 °C/80% RH). Results show that subjects' mean skin temperatures (MST) and thermal sensation votes (TSV) were higher at 32 °C in Series I. In Series II, subjects who were de-acclimated showed quicker and stronger responses with regards to MST during the first 30 min and higher change rates of MST (32 °C/80% RH: peak value of 0.49 °C/min, 32 °C/20% RH: 0.16 °C/min) during the first exposure. This was accompanied by a higher skin blood flow (SBF) and total water loss (32 °C/80% RH: 292 g ± 63 g, 32 °C/20% RH: 307 g ± 47 g) on Day 1. The physiological regulations for MST, SBF, and total water loss attenuated continuously over the seven days, especially at 32 °C/20% RH. The repeated exposure consequently ameliorated subjects' thermal and sweating sensations. The findings indicate the mechanism of human physiological and psychological adaptation, which provides deeper understandings of people's dynamic thermal adaptation and application in thermal environment designs.