Hot water immersion is associated with higher thermal comfort than dry passive heating for a similar rise in rectal temperature and plasma interleukin-6 concentration.

PURPOSE: To compare the perceptual responses and interleukin-6 (IL-6) concentration following rectal temperature-matched dry heat exposure (DH) and hot water immersion (HWI). METHODS: Twelve healthy young adults (BMI 23.5 ± 3.6 kg/m(2); age: 25.8 ± 5.7 years) underwent 3 trials in randomised order: DH (air temperature 68.9 °C), HWI (water temperature 37.5 °C), and thermoneutral dry exposure (CON, air temperature 27.3 °C). Blood samples to determine IL-6 plasma concentration were collected; basic affect and thermal comfort, rectal and skin temperature (T(skin)) were assessed throughout the intervention. RESULTS: Rectal temperature (T(rec)) did not differ between DH (end temperature 38.0 ± 0.4 °C) and HWI (37.9 ± 0.2 °C, P = 0.16), but was higher compared with CON (37.0 ± 0.3 °C; P ≤ 0.004). Plasma IL-6 concentration was similar after DH (pre to post: 0.8 ± 0.5 to 1.4 ± 1.5 pg·ml(-1)) and HWI (0.5 ± 0.2 to 0.9 ± 0.6 pg·ml(-1); P = 0.46), but higher compared with CON (0.6 ± 0.5 to 0.6 ± 0.4 pg·ml(-1); P = 0.01). At the end of the intervention, basic affect and thermal comfort were most unfavourable during DH (Basic affect; DH: - 0.7 ± 2.9, HWI: 0.8 ± 1.9, CON 1.9 ± 1.9, P ≤ 0.004; Thermal comfort; 2.6 ± 0.8, HWI: 1.4 ± 0.9 and CON: 0.2 ± 0.4; P ≤ 0.004). Mean T(skin) was highest for DH, followed by HWI, and lowest for CON (DH: 38.5 ± 1.3 °C, HWI: 36.2 ± 0.5 °C, CON: 31.6 ± 0.7 °C, P < 0.001). CONCLUSION: The IL-6 response did not differ between DH and HWI when matched for the elevation in T(rec). However, thermal comfort was lower during DH compared to HWI, which may be related to the higher T(skin) during DH.