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

Abstract

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/m²; 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.

Keywords: Inflammation; Passive heat; Rectal temperature; Skin temperature; Thermal comfort.