Critical environmental limits, defined as those above which heat balance cannot be maintained for a given metabolic heat production, have not been determined for unacclimated subjects. To characterize critical environmental limits and to derive evaporative heat exchange coefficients (K_e') for unacclimated young men (n = 11) and women (n = 10), subjects of average aerobic fitness walked at 30% maximal aerobic capacity in an environmental chamber. Critical environmental conditions were defined as the psychrometric loci of dry-bulb temperature and water vapor pressure at which core (esophageal) temperature was forced out of equilibrium (heat gain exceeded heat loss). Compared with the men in our study, the women had significantly higher critical environmental limits (P < 0.001) in warm (34-38°C), humid (>60%) environments, a function of their lower absolute metabolic heat production at the fixed relative exercise intensity. Isotherms constructed from biophysical models closely fit the data in this range of environments but underestimated empirically determined critical limits in hotter, drier environments. Sex-specific values of K_e' were derived by partial calorimetry in the critical water vapor pressure environments, in which full skin wettedness occurred. There were no sex differences for K_e' (men = 17.4, 15.5, and 14.2 W · m² · Torr¹ and women = 16.8, 15.5, and 14.2 W · m² · Torr¹ at 34, 36, and 38°C, respectively). These K_e' values were lower than those previously published for fully heat-acclimated men (18.4 W · m² · Torr¹ at 36°C) and women (17.7 W · m² · Torr¹ at 36°C and 15.5 W · m² · Torr¹ at 38°C) and may be used to model heat balance responses for unacclimated men and women working in hot environments.