100% oxygen breathing during acute heat stress: effect on sweat composition.

Twelve male Indian Air Force subjects were exposed on two occasions to a simulated hot environment (DB 57 degrees C; WB 35.5 degrees C; RH 25%) for a period of 50 min. On one occasion the subject breathed normal atmospheric air while on the other occasion he breathed 100% oxygen at ambient atmospheric pressure. Arm sweat collected at the end of the two experiments was analysed for Na+, K+, Mg++, C1-, and lactic acid. Arm sweat Mg++ was found to be much higher than that reported in the literature. Hyperoxia during heat stress improved arm sweating and showed significantly lower concentrations of Mg++, Na+, K+, and lactic acid. However, the total loss of these cations and lactic acid through the arm were not found to differ significantly for the two experiments. Based on arm sweat concentration, when the total body loss of these cations was worked out for the two runs, only whole-body Mg++ showed a significantly lesser loss during hyperoxia. The selective retention of Mg++ during the hyperoxic heat run and its association in lowering the heat-induced physiological strain are discussed.