Thermal sensation and comfort responses during repeated exposure to mild heat

Since psychological and physiological responses to repeated exposure to mild heat has not been fully studied, the present study was designed to confirm overshooting responses in thermal sensation after repeated exposure to mild heat (i.e., the cooling period), the manner of change in the thermal sensation responses (TSRs) and the thermal comfort responses (TCRs) during the cooling period, and effect of short-term heat acclimation during repeated exposure to mild heat. In the summer, eight young adult male subjects (a mean age of 21.1 ± 1.4 years; a mean height of 173.1 ± 5.6 cm and a mean weight of 58.8 ± 7.5 kg) with clothing insulation (I_cl, clo) of 0.3 clo first stayed in the control room at 26 °C for 15 min, then moved to the main testing room at 33 °C for 10 min (condition 1), 15 min (condition 2), or 20 min (condition 3), and finally returned to the control room for 15 min. The exposure was repeated five times. TSR and TCR were recorded in a 5-min interval from the beginning of the first exposure. The tympanic temperature (T_ty), skin temperatures at the chest, forearm, front of the thigh, and front of the shin, and ECG and heart rate were continuously monitored. Local sweat rates at the same sites of skin temperature were monitored at the end of each exposure. Changes in T_ty and mean skin temperature ( ${\overline{T}}_{\mathrm{sk}}$ ) indicated no significant difference between conditions and no indication of short-term heat acclimation. Since the subjects voted nearly "cold" when ${\overline{T}}_{\mathrm{sk}}$  remained high at the beginning of the cooling period, overshooting responses in thermal sensation were repeatedly observed in all conditions. The subjects voted "slightly cool" at the end of cooling period while ${\overline{T}}_{\mathrm{sk}}$  kept decreasing during the cooling period. The thermally neutral ${\overline{T}}_{\mathrm{sk}}$  was then estimated to be 0.3 °C-4.2 °C lower than ${\overline{T}}_{\mathrm{sk}}$  observed prior to the first exposure. Thus, a residual effect on TSR during the cooling period was confirmed. Changes in the mean sweat rate, TSR and TCR showed significant differences between conditions but no indication of short-term heat acclimation. However, change in heart rate and ECG analysis implied the effect of short-term heat acclimation.