Autonomic thermoregulatory responses and subjective thermal perceptions upon the initiation of thermal behavior among resting humans in hot and humid environment

Background While thermoregulatory behavior is critical for maintaining homeostasis, our knowledge of behavioral thermoeffectors in humid heat is limited compared to the control of autonomic thermoeffectors. The predictions that the frequency and duration of intensified humid heat events are expected to increase in the coming years underline this shortcoming. Therefore, this study aims to elucidate the activation of autonomic thermoregulatory responses and subjective thermal perceptions upon deciding to initiate thermal behavior in a hot and humid environment. Methods Ten young male adults participated in an experimental trial where local cooling was permitted at any time during the behavioral assessment during passive exposure to humid heat. The air temperature and relative humidity were kept at 33 ^∘ C and 80 % , respectively. Skin temperatures, core body temperature (T _core ), and skin blood flow (forearm, upper arm, and upper back) were obtained 120 s preceding thermal behavior. Local sweat rate (forearm and upper arm) and subjective thermal perceptions (neck and whole-body) upon thermal behavior initiation were also recorded. Results Mean skin temperature ( T_sk ) and T _core increased prior to thermal behavior initiation ( p = 0.002; p = 0.001). An increase in mean body temperature ( T_body ) was also observed ( p < 0.001). However, the initiation of thermal behavior is not preceded by an increase in skin blood flow ( p ≥ 0.154) and local sweat rate ( p ≥ 0.169). An increase in thermal discomfort and skin wetness perception was observed ( p ≤ 0.048; p ≤ 0.048), while thermal sensation did not differ from the baseline ( p ≥ 0.357). Conclusion These findings suggest that when given the opportunity to behaviorally thermoregulate in a hot and humid environment, changes in skin blood flow and sweat rate are not required for thermal behavior to be initiated in resting humans. Moreover, an increase in T_sk and T _core , which appears to cause an increase in thermal discomfort, precedes thermal behavior. In addition, an increase in T_body leading up to thermal behavior initiation was observed, suggesting that changes in T_body rather than T_sk and T _core alone mediate thermal behavior in humid heat. Collectively, the results of this study appear to support the hypothesis that the temporal recruitment of autonomic thermoeffectors follows an orderly manner based on their physiological cost.