Comparison of responses of men to immersion in circulating water at 40.0 and 41.5°C

Background: The recommended maximum water temperature for public hot tubs has been set at 40.0°C, but no research has been published on human immersion in hot water at higher temperatures. Hypothesis: We hypothesized that thermoregulatory and cardiovascular responses at two water temperatures would be proportional to the water:blood temperature gradients. Methods: Six healthy men were immersed for 21 min in circulating hot water at 40.0 and 41.5°C in separate trials in random order 1-3 wk apart. Measurements included heart rate, systolic BP, esophageal, rectal, and non-immersed skin temperatures, sweat rate, and perceived comfort. Results: The rise in all body temperatures, sweat rate, and heart rate were significantly greater in the 41.5 vs. 40.0°C water. Peak esophageal temperatures were 38.3 ± 0.2°C vs. 37.8 ± 0.03°C, peak sweat rates were 0.48 ± 0.05 vs. 0.32 ± 0.03 kg · m^-2 · h^-1, and peak heart rates were 123 ± 7 vs. 108 ± 5 bpm, respectively. Systolic BPs followed different patterns of response in each trial, whereas diastolic pressures were not different between trials. Comfort at each level of immersion was reduced during the 41.5°C trial compared with the 40.0°C in excess of that predicted by difference in esophageal temperature between the trials. Conclusions: These results suggest that risks of hyperthermia or adverse cardiovascular effects in hot tubs may not be greater in water above 40.0°C unless perceptual judgment is impaired. Hypotension when standing to exit the tub occurred in both trials and may represent a potential hazard to hot tub use.