Effect of water temperature on cooling efficiency during hyperthermia in humans

1 Faculty of Health Sciences, University of Ottawa, Ottawa K1N 6N5; and 2 Defence R&D Canada-Toronto, Toronto, Ontario, Canada M3M 3B9 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 39°C until T re increased to 40°C (45.4 ± 4.1 min). After exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14, or 20°C until T re returned to 37.5°C. No difference in cooling rate was observed between the immersions at 8, 14, and 20°C despite the differences in the skin surface-to-water temperature gradient, possibly because of the presence of shivering at 8 and 14°C. Compared with the other conditions, however, the rate of cooling (0.35 ± 0.14°C/min) was significantly greater during the 2°C water immersion, in which shivering was seldom observed. This rate was almost twice as much as the other conditions ( P