Human thermoregulatory responses to cold air are altered by repeated cold water immersion

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Abstract
The effects of repeated cold water immersion on thermoregulatory responses to cold air were studied in seven males. A cold air stress test (CAST) was performed before and after completion of an acclimation program consisting of daily 90-min cold (18.degree. C) water immersion, repeated 5 times/wk for 5 consecutive wk. The CAST consisted of resting 30 min in a comfortable [24.degree. C, 30% relative humidity (rh)] environment followed by 90 min in cold (5.degree. C, 30% rh) air. Pre- and postacclimation, metabolism (M) increased (P < 0.01) by 85% during the first 10 min of CAST and thereafter rose slowly. After acclimation, M was lower (P < 0.02) at 10 min of CAST compared with before, but by 30 min M was the same. Therefore, shivering onset may have been delayed following acclimation. After acclimation, rectal temperature (Tre) was lower (P < 0.01) before and during CAST, and the drop in Tre during CAST was greater (P < 0.01) than before. Mean weighted skin temperature (.hivin.Tss) was lower (P < 0.01) following acclimation than before, and acclimation resulted in a larger (P < 0.02) Tre-to-.hivin.Tsk gradient. Plasma norepinephrine increased during both CAST (P < 0.002), but the increase was larger (P < 0.004) following acclimation. These findings suggest that repeated cold water immersion stimulates development of true cold acclimation in humans as opposed to habituation. The cold acclimation produced appears to be of the insulative type.