The physiologic determination of the vapor permeation efficiency of clothing

Two calculations of the coefficient for vapor permeation efficiency (F_pcl) have been proposed for use in the rational (heat balance) approach to heat stress problems caused by wearing protective clothing-one proposed by Candas et al. (1987) and a second (derived from K_eη′) proposed by Kenney et al. (1987). Both of these coefficients are based on the concept of sweating efficiency and each is used in the calculation of E_max, the maximal evaporative capacity of the environment. This paper compares these 2 approaches based on data collected across several clothing ensembles common to industry. In general, the lighter and more permeable the ensemble, the closer the agreement between the two coefficients. However, as clothing becomes impermeable (e.g., vapor-barrier clothing made of plastic, coated Tyvek®, etc.), use of the former F_pcl significantly overestimates E_max. It is suggested that either calculation can be used for lightweight clothing (although Candas F_pcl is simpler to calculate) but that the F_pcl calculated by Kenney and co-workers is preferable for heavier ensembles.