Increasing humidity progressively reduces sweating efficiency similarly in males and females during exercise-heat stress.

This study compared the impact of increasing relative humidity (RH) on sweating efficiency (SE) in males and females exercising in the heat. Twenty recreationally active adults (10 females) completed four experimental trials (15, 35, 55, or 75% RH) in an environmental chamber (35°C) dressed in a T-shirt and shorts (and sports bra for females). Each study visit involved cycling for 60 min at a moderate intensity (Hprod = 220 W·m-2) over a pan of mineral oil, situated above a high precision scale. Evaporated sweat loss (ESL; g) was measured as the change in mass of the combined subject + ergometer + oil pan system during exercise; total sweat loss (TSL; g) was measured as the change in nude mass from beginning to end-exercise. SE (%) was calculated as ESL/TSL × 100. The effects of RH and sex were assessed by two-way ANOVA; there was no significant RH × sex interaction for any variable. ESL was not different among 15, 35, and 55% RH, but was lower in 75% RH (P < 0.001). In contrast, TSL increased in each RH condition (all P < 0.001). Consequently, in both sexes SE progressively decreased (all P < 0.001) with increasing RH (males: 91.0 ± 3.1, 86.1 ± 4.8, 71.0 ± 9.3, and 46.4 ± 11.8%; females: 86.6 ± 3.7, 79.9 ± 5.7, 67.6 ± 6.7, and 44.3 ± 5.8%). These findings demonstrate that SE during exercise is progressively reduced by increasing RH and this effect is similar in males and females.NEW & NOTEWORTHY This study provides the first direct measurements linking the efficiency of sweating to the total fluid lost to evaporative cooling in upright, clothed, exercising subjects. These findings demonstrate that sweating efficiency during exercise is progressively reduced by increasing relative humidity, and this effect is similar in males and females. Decrements in sweating efficiency were related to sweat retained in the clothing.