Physiological responses to changes in relative humidity under thermally neutral, warm and hot conditions

• Four hypotheses on change in mean skin temperature (Tsk) were proposed when relative humidity (Rh) changes in thermally neutral, warm (TW) and hot (H) conditions.
• T¯sk) (E¯sk) T¯skE¯sk Under TW conditions, Tsk upon increase in Rh, decrease, increase and no change in Tsk upon decrease in Rh were predicted. Under H conditions, increase in Tsk upon increase in the Rh and decrease in Tsk upon decrease in the Rh were predicted.
• T¯skE¯skT¯skThe experimental results partly validated the hypotheses.

Four hypothetical thermophysiological responses to changes in relative humidity (Rh) under thermally neutral, warm, and hot conditions were proposed for a person at rest. Under thermally neutral and warm conditions, the first hypothetical response to an increase in Rh was a decrease in mean skin temperature (T¯sk) due to increase in mean evaporation rate (E¯sk), and the second hypothetical response to a decrease in Rh was a decrease, an increase, or no change in T¯sk, depending on changes in the E¯sk. Under hot conditions, the third hypothetical response to an increase in the Rh was an increase in T¯sk or decrease in T¯sk upon decrease in the Rh due to changes in E¯sk, and the forth hypothetical response to an increase in Rh was an increase in T¯sk due to increase in the peripheral blood flow rate (SkBF). To test these hypotheses, the T¯sk and E¯sk of four young male volunteers were measured at 28 °C, 30 °C, or 32 °C while the Rh was maintained at 40% or 80% Rh for 60 min after 20 min exposure at 60% Rh (control condition). In a second experiment, the T¯sk, E¯sk, and SkBF of five young male volunteers were measured at 34 °C-40% Rh or 36 °C-40% Rh, or 34 °C-70% Rh or 36 °C-70% Rh for 60 min after 20 min exposure at 28 °C-60% Rh (control condition). The first hypothesis was partly supported by the findings that the T¯sk was lower than the control values at 28 °C-80% Rh and the E¯sk was higher than the control values at 80% Rh at any tested temperature. The second hypothesis was partly supported by the findings that the T¯sk was lower than the control values at 28 °C-40% Rh, and there were small changes in both T¯sk and E¯sk at 30 °C-40% Rh. The third and fourth hypotheses were supported by the findings that the T¯sk at 36 °C-70% Rh was significantly higher (p<0.01) than at 36 °C-40% Rh, the E¯sk was significantly higher (p<0.01) at 70% Rh than at 40% Rh, and SkBF was positively correlated with T¯sk.