Physiologically derived critical evaporative coefficients for protective clothing ensembles

Abstract

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 (he) which define this ability to evaporate sweat. We refined this approach by correcting the critical effective evaporative coefficient (K for sweating efficiency (Ke,eta') 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 (Pw) 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 he by solving the general heat balance equation, M - W +/- (R + C) = w.he.(Psk - Pw), 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 Psk is water vapor pressure of fully wet skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective Clothing in Hot Environments

“…Evaporative resistance of clothing may be directly measured with subjects 16,17) . It can also be determined on the basis of thermal insulation and the permeability index (i m ) for the fabric or for the ensemble.…”

Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment

“…8 In addition, exercising in heavy clothing or protective equipment increases the risk of heat related disorders as it interferes with heat loss through evaporation, radiation, convection, and conduction. [11][12][13] Compared with male participants in soccer, rugby, or lacrosse, and the individual sports of running or cycling, American footballers tend to be larger, not as aerobically fit, and have a higher percentage body fat. 14 15 In addition, as body surface area is larger in footballers, it is reasonable to expect that they have a greater number of sweat glands and therefore may be capable of sweating at higher rates than smaller athletes involved in cross country running.…”

Sustained increases in skin blood flow are not a prerequisite to initiate sweating during passive heat exposure

“…Stepwise increases in ambient vapor pressure elicited relatively stable values of mean body temperature, SkBF, CVC, and sweating until a critical ambient vapor pressure was attained where an upward inflection in each variable occurred (3). The critical ambient vapor pressure at which the inflection point occurred was determined separately for SkBF and sweating by using segmented regression analysis of the minute-averaged data (21,22,37). For CVC values, only one data point per ambient vapor pressure stage was used for segmented regression as blood pressure was measured at the end of each stepwise increment.…”