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 (T _c ) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGT _crit ), a validated index that represents the overall thermal environment. Little is known about WBGT _crit at rest and during low-to-moderate intensity exercise, or sex differences in WBGT _crit , in unacclimated young adults. The following hypotheses were tested: 1 ) WBGT _crit progressively decreases as metabolic heat production (M _net ) increases, 2 ) no sex differences in WBGT _crit occur at rest, and 3 ) WBGT _crit 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 (V̇o _2max )] 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 T _c (rectal or esophageal temperature). Sex-aggregated WBGT _crit was lower during 10 W (32.9°C ± 1.7°C, P < 0.0001) and 30% V̇o _2max (31.6°C ± 1.1°C, P < 0.0001) exercise versus at rest (35.3°C ± 0.8°C), and lower at 30% V̇o _2max versus 10 W ( P = 0.01). WBGT _crit 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 10 W (31.9°C ± 1.7°C vs. 34.1°C ± 0.3°C, P < 0.01) and higher during 30% V̇o _2max (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 WBGT _crit decreases as M _net increases, no sex differences occur in WBGT _crit at rest, and sex differences in WBGT _crit during exercise depend on absolute versus relative intensities.