Heat exposure limits for young unacclimatized males and females at low and high humidity.

Little is known about the separate and combined influences of humidity conditions, sex, and aerobic fitness on heat tolerance in unacclimatized males and females. The purpose of the current study was to describe heat tolerance, in terms of critical WBGT (WBGT crit ), in unacclimatized young males and females in hot-dry (HD) and warm-humid (WH) environments. Eighteen subjects (9 M/9F; 21 ± 2 yr) were tested during exercise at 30% V̇O 2max in a controlled environmental chamber. Progressive heat stress exposures were performed with either (1) constant dry-bulb temperature (T db ) of 34 and 36 °C and increasing ambient water vapor pressure (P a ) (P crit trials; WH); or (2) constant P a of 12 and 16 mmHg and increasing T db (T crit trials; HD). Chamber T db and P a , and subject esophageal temperature (T es ), were continuously monitored throughout each trial. After a 30-min equilibration period, progressive heat stress continued until subject heat balance could no longer be maintained and a clear rise in T es was observed. Absolute WBGT crit and WBGT crit adjusted to a metabolic rate of 300 W (WBGT 300 ), and the difference between WBGT crit and occupational exposure limits (OEL; ΔOEL) was assessed. WBGT crit , WBGT 300 , and ΔOEL were higher in WH compared to HD ( p  < 0.0001) for females but were the same between environments for males ( p  ≥ 0.21). WBGT crit was higher in females compared to males in WH ( p  < 0.0001) but was similar between sexes in HD ( p  = 0.44). When controlling for metabolic rate, WBGT 300 and ΔOEL were higher in males compared to females in WH and HD (both p  < 0.0001). When controlling for sex, V̇O 2max was not associated with WBGT 300 or ΔOEL for either sex (r ≤ 0.12, p  ≥ 0.49). These findings suggest that WBGT crit is higher in females compared to males in WH, but not HD, conditions. Additionally, the WBGT crit is lower in females, but not males, in HD compared to WH conditions.