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) cannot be maintained at equilibrium. Those limits can be represented as critical wet-bulb globe temperature (WBGT), a validated index that represents the overall thermal environment. Little is known about WBGT at rest and during low-to-moderate intensity exercise, or sex differences in WBGT, in unacclimated young adults. The following hypotheses were tested: ) WBGT progressively decreases as metabolic heat production (M) increases, ) no sex differences in WBGT occur at rest, and ) WBGT 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)] 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 (rectal or esophageal temperature). Sex-aggregated WBGT was lower during 10 W (32.9°C ± 1.7°C, < 0.0001) and 30% V̇o (31.6°C ± 1.1°C, < 0.0001) exercise versus at rest (35.3°C ± 0.8°C), and lower at 30% V̇o versus 10 W ( = 0.01). WBGT was similar between sexes at rest (35.6°C ± 0.8°C vs. 35.0°C ± 0.8°C, = 0.83), but lower during 10 W (31.9°C ± 1.7°C vs. 34.1°C ± 0.3°C, < 0.01) and higher during 30% V̇o (32.4°C ± 0.8°C vs. 30.8°C ± 0.9°C, = 0.03) exercise in women versus men. These findings suggest that WBGT decreases as M increases, no sex differences occur in WBGT at rest, and sex differences in WBGT during exercise depend on absolute versus relative intensities.