Quantification of Head Sweating in Desert and Tropic Conditions

Abstract

Protective helmets can interfere with air flow to the head and limit the capacity for evaporative heat loss. The degree to which ventilation under a helmet can increase evaporative heat loss depends on sweating rate over the surface area covered. PURPOSE: To quantify head sweating rate (msw) during rest and exercise in desert (D; 45°C, 20% RH) and tropic (T; 35°C, 70%RH) conditions (matched at 31.6°C WBGT). METHODS: Six males completed 20 min each of rest and exercise at three metabolic rates (337 W; 492 W; 624 W) in D and T, which were counterbalanced. Heads were shaved before each trial, and surface area (485±42 cm2) was measured by 3D scanner. For measurement of msw, dry air was passed over the head through a sealed helmet. Forearm msw was measured with a capsule (15.9 cm2) ventilated with ambient air. Whole body sweat loss was calculated from change in nude body weight plus fluid intake. RESULTS: Head msw increased (p=0.007) with metabolic rate, and overall was lower (p=0.048) in D (0.3±0.2 mg·cm-2·min-1 at rest to 0.8±0.4 mg·cm-2·min-1 at 624 W), compared to T (0.4±0.2 to 1.2±0.7 mg·cm-2·min-1). Forearm msw also increased (p<0.001) with metabolic rate, but was higher (p=0.013) in D (0.5±0.3 to 1.5±0.7 mg·cm-2·min-1) than T (0.1±0.1 to 1.1±0.4 mg·cm-2·min-1, respectively). Esophageal temperature did not differ between trials (D: 36.8±0.1 to 37.7±0.3°C; T: 36.8±0.2 to 38.0±0.2°C). Mean skin temperature (Tsk) was initially higher (p=0.01) in D (36.3±0.3°C) than T (35.1±0.8°C), but at 624 W was similar between trials (D: 36.7±1.0°C; T: 36.5±0.5°C). The increase in heart rate (HR) with exercise was lower (p=0.022) during D (80±14 to 135±15 b·m-1) than T (81±17 to 150±19 b·m-1). Whole-body sweat loss was higher (p=0.03) in D (563±123 g·m-2·h-1) than T (470±171 g· m-2·h-1). For comparison, peak head sweat loss was 504±243 g·m-2·h-1 in D, and 696±447 g·m-2·h-1 In T. CONCLUSIONS: The higher head msw in T despite similar core and skin temperatures at 624 W suggests an improved sweat capacity at the head in compensation for suppressed whole-body evaporative heat loss. The results from this study suggest that improved ventilation through a helmet could increase heat loss by as much as 24 W. The opinions or assertions contained herein should not to be construed as official or reflecting the views of the Army or the Department of Defense.