Effect of water temperature on cooling efficiency during hyperthermia in humans

We evaluated the cooling rate of hyperthermic subjects, as measured by rectal temperature (T-re), during immersion in a range of water temperatures. On 4 separate days, seven subjects (4 men, 3 women) exercised at 65% maximal oxygen consumption at an ambient temperature of 39degreesC until T-re increased to 40degreesC (45.4 +/- 4.1 min). After exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14, or 20degreesC until T-re returned to 37.5degreesC. No difference in cooling rate was observed between the immersions at 8, 14, and 20degreesC despite the differences in the skin surface-to-water temperature gradient, possibly because of the presence of shivering at 8 and 14degreesC. Compared with the other conditions, however, the rate of cooling (0.35 +/- 0.14degreesC/min) was significantly greater during the 2degreesC water immersion, in which shivering was seldom observed. This rate was almost twice as much as the other conditions (P < 0.05). Our results suggest that 2°C water is the most effective immersion treatment for exercise-induced hyperthermia.