Predicting sweat loss response to exercise, environment and clothing

Metabolic heat production (M), clothing heat transfer characteristics, and the environment dictate a required evaporative cooling (E_req) from the body to maintain thermal balance. However, the maximal evaporative capacity (E_max) is dictated by vapor transfer properties of the clothing and environment. Relationships between metabolic load, environmental conditions, clothing and sweat loss were studied in 34 heat-acclimatized males categorized into four groups (eight, eight, eight, and ten subjects) and exposed to various environmental conditions (ambient temperature, 20-54‡ C, and relative humidity, 10-90%), three levels of metabolic rate (resting; walking 1.34 m·s^-1, level; or walking 1.34 m·s^-1, 5% grade) while wearing various clothing ensembles (shorts and T-shirts, fatigues, fatigues plus overgarment, or sweat suit). Individual groups were not exposed to all combinations. Exposures lasted 120 min: either 10 min rest - 50 min exercise - 10 min rest - 50 min exercise, or 120 min at rest. Physiological measurements included heart rate, rectal temperature, mean skin temperature, energy expenditure and sweat loss (δm_sw). E_max and E_req were calculated from environmental conditions, metabolism, clothing insulation and permeability. The ratio E_req/δm_sw was found to correlate with E_max and not with M. The predictive equation for sweat loss was: δm_sw=18.7×E_req×(E_max)^-0.455 within the limits 50<E_req<360; W·m^-2 and 20<E_max<525; W·m^-2. This formula predicts sweat loss for specific work loads, climates and clothing ensembles.