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J Appl Physiol 107: 379-388, 2009. First published April 30, 2009; doi:10.1152/japplphysiol.00089.2009
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Expanded prediction equations of human sweat loss and water needs

¹Biology Department, New Mexico State University, Las Cruces, New Mexico; and ²United States Army Research Institute of Environmental Medicine, Natick, Massachusetts

Submitted 2 February 2009 ; accepted in final form 28 April 2009

The Institute of Medicine expressed a need for improved sweating rate (_sw) prediction models that calculate hourly and daily water needs based on metabolic rate, clothing, and environment. More than 25 years ago, the original Shapiro prediction equation (OSE) was formulated as _sw (g·m^–2·h^–1) = 27.9·E_req·(E_max)^–0.455, where E_req is required evaporative heat loss and E_max is maximum evaporative power of the environment; OSE was developed for a limited set of environments, exposures times, and clothing systems. Recent evidence shows that OSE often overpredicts fluid needs. Our study developed a corrected OSE and a new _sw prediction equation by using independent data sets from a wide range of environmental conditions, metabolic rates (rest to 450 W/m²), and variable exercise durations. Whole body sweat losses were carefully measured in 101 volunteers (80 males and 21 females; >500 observations) by using a variety of metabolic rates over a range of environmental conditions (ambient temperature, 15–46°C; water vapor pressure, 0.27–4.45 kPa; wind speed, 0.4–2.5 m/s), clothing, and equipment combinations and durations (2–8 h). Data are expressed as grams per square meter per hour and were analyzed using fuzzy piecewise regression. OSE overpredicted sweating rates (P < 0.003) compared with observed _sw. Both the correction equation (OSE_C), _sw = 147·exp (0.0012·OSE), and a new piecewise (PW) equation, _sw = 147 + 1.527·E_req – 0.87·E_max were derived, compared with OSE, and then cross-validated against independent data (21 males and 9 females; >200 observations). OSE_C and PW were more accurate predictors of sweating rate (58 and 65% more accurate, P < 0.01) and produced minimal error (standard error estimate < 100 g·m^–2·h^–1) for conditions both within and outside the original OSE domain of validity. The new equations provide for more accurate sweat predictions over a broader range of conditions with applications to public health, military, occupational, and sports medicine settings.

thermoregulation; modeling; fluid balance; hydration; fluid replacement