Metabolism- and sex-dependent critical WBGT limits at rest and during exercise in the heat

Critical environmental limits are environmental thresholds above which heat gain exceeds heat loss and body core temperature (Tc) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGTcrit), a validated index that represents the overall thermal environment. Little is known about WBGTcrit at rest and during low-to-moderate intensity exercise, or sex differences in WBGTcrit, in unacclimated young adults. The following hypotheses were tested: 1) WBGTcrit progressively decreases as metabolic heat production (Mnet) increases, 2) no sex differences in WBGTcrit occur at rest, and 3) WBGTcrit is lower during absolute-intensity exercise but higher at relative intensities in women than in men. Thirty-six participants [19 men (M)/17 women (W); 23 ± 4 yr] were tested at rest, during light, absolute-intensity exercise (10 W), or during moderate, relative-intensity exercise [30% maximal oxygen consumption (VO2max)] in an environmental chamber. Dry-bulb temperature was clamped as relative humidity or ambient water vapor pressure was increased until an upward inflection was observed in Tc (rectal or esophageal temperature). Sex-aggregated WBGTcrit was lower during 10W (32.9°C± 1.7°C, P < 0.0001) and 30% VO2max (31.6°C± 1.1°C, P < 0.0001) exercise versus at rest (35.3°C± 0.8°C), and lower at 30% VO2max versus 10W (P = 0.01). WBGTcrit was similar between sexes at rest (35.6°C± 0.8°C vs. 35.0°C± 0.8°C, P = 0.83), but lower during 10W (31.9°C± 1.7°C vs. 34.1°C± 0.3°C, P < 0.01) and higher during 30% VO2max (32.4°C± 0.8°C vs. 30.8°C± 0.9°C, P = 0.03) exercise in women versus men. These findings suggest that WBGTcrit decreases as Mnet increases, no sex differences occur in WBGTcrit at rest, and sex differences in WBGTcrit during exercise depend on absolute versus relative intensities.