Physiologically derived critical evaporative coefficients for protective clothing ensembles

When work is performed in heavy clothing, evaporation of sweat from the skin to the environment is limited by layers of wet clothing and air. The magnitude of decrement in evaporative cooling is a function of the clothing's resistance to permeation of water vapor. A physiological approach has been used to derive effective evaporative coefficients (h(e)) which define this ability to evaporate sweat. We refined this approach by correcting the critical effective evaporative coefficient (K) for sweating efficiency (K(e,η)') since only a portion of the sweat produced under such conditions is evaporated through the clothing. Six acclimated men and women walked at 30% maximal O₂ consumption (150-200 W·m^-2) at a constant dry bulb temperature as ambient water vapor pressure was systematically increased 1 Torr every 10 min. Critical pressure was defined as the partial pressure of water vapor (P(w)) at which thermal balance could no longer be maintained and rectal temperature rose sharply. Each test was performed in various clothing ensembles ranging from cotton shirt and pants to 'impermeable' suits. This approach was used to derive h(e) by solving the general heat balance equation, M - W ± (R + C) = w·h(e)·(P(sk)* - P(w)), where M is metabolic heat production, W is external work, R is radiant heat exchange, C is convective heat transfer, w is skin wettedness, and P(sk)* is water vapor pressure of fully wet skin. Critical effective evaporative coefficients calculated from h(e) and corrected for air movement and the portion of sweat produced necessary to maintain thermal balance (K(e,η)') were 14.5 ± 1.0 (cotton shirt and pants), 9.4 ± 0.4 (cotton outerwear), 8.8 ± 1.7 (Gore-Tex outerwear), 8.5 ± 0.8 (double cotton outerwear), and 4.2 ± 0.3 W·m^-2·Torr^-1·(m/s)^-0.6 (cotton outerwear + coated vaporbarrier suit). These coefficients can be used in the rational approach to setting safe environmental limits or for selecting appropriate protective clothing in industrial applications involving heat stress.