Critical environmental limits are those above which human heat balance cannot be maintained for a given metabolic heat production. These limits, and associated critical evaporative coefficients (Ke=) that can be used to model responses in hot environments, have not been determined for older subjects. The present paper graphically characterizes psychrometric limits and environmental isotherms and derives Ke= values for a group of unacclimated older (n = 10; age 62 - 80 yr) women exercising at 30% V· O2max. Uniquely, we compare and contrast these data with published data from young, unacclimated and young, heat-acclimated women tested across a four-decade span using the same protocol in the same environmental chamber. These loci are presented graphically on a psychrometric chart (with confidence intervals). Isotherms constructed from biophysical modeling and sweating capacity closely fit the data but underestimated empirically derived data points in hotter, drier environments. Compared with the young (age 19-26 yr) women previously tested, the older women had significantly constrained (lower) critical environmental limits, in part due to lower sweating rates. Age-specific values of the critical evaporative coefficient, Ke=, derived by partial calorimetry in the more humid environments (in which skin wettedness approached 1), were likewise lower for the older women (overall mean = 9.1 W·m-2·mmHg-1; P < 0.05) vs. unacclimated (15.4 W·m-2·mmHg-1) and acclimated (17.0 W·m-2·mmHg-1) young women. Constrained psychrometric limits and lower critical evaporative coefficients lend biophysical clarity to decreased abilities of older women for prolonged exercise in the heat. NEW & NOTEWORTHY This study is the first to describe, graphically and quantitatively, critical environmental limits for women between the ages of 62 and 80 yr based on the biophysics of heat exchange. These psychrometric limit lines define combinations of ambient temperature and humidity above which human heat balance cannot be maintained for a given metabolic heat production. These limits, and associated critical evaporative coefficients (Ke=), can be used to model low- to moderate-intensity exercise responses in hot environments and have directly translatable data that can be used for evidence-based policy decisions, to prepare for impending heat events, and for implementation of other safety interventions.
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