Quantification of head sweating during rest and exercise in the heat

Abstract

The head’s capacity for evaporative heat loss is important for design of protective helmets for use in hot environments. This study quantified head sweating rate (m _sw) in eight males during rest and exercise at three metabolic rates (338 ± 36, 481 ± 24, 622 ± 28 W) in hot-dry (HD: 45 °C, 21 % RH) and hot-wet (HW: 35 °C, 69 % RH) conditions (matched at 31.6 °C WBGT), which were counterbalanced. Heads were shaved, and surface area was (458 ± 61 cm²) measured by 3D scanner. For measurement of head m _sw, dry air was passed through a sealed helmet, whereas for forearm m _sw a capsule (15.9 cm²) was ventilated with ambient air. Evaporation rate was determined from the increase in vapor pressure in the exiting air. Whole-body sweat loss was calculated from the change in nude weight plus fluid intake and corrected for respiratory fluid losses. Head m _sw increased (p = 0.001) with metabolic rate, and was lower (p = 0.018) in HD (0.4 ± 0.2 mg cm⁻² min⁻¹ at rest to 1.1 ± 0.6 mg cm⁻² min⁻¹ at 622 W), compared to HW (0.5 ± 0.3–1.4 ± 0.8 mg cm⁻² min⁻¹). Forearm m _sw increased (p < 0.001) with metabolic rate, but was higher (p = 0.002) in HD (0.4 ± 0.3–1.4 ± 0.7 mg cm⁻² min⁻¹) than HW (0.1 ± 0.1–1.1 ± 0.3 mg cm⁻² min⁻¹). Whole-body sweat loss was not significantly different (p = 0.06) between HD (647 ± 139 g m⁻² h⁻¹) and HW (528 ± 189 g m⁻² h⁻¹). This study demonstrates the importance of the head for evaporative heat loss, particularly for populations who wear protective clothing which can impair vapor transfer from the skin.