Relatively minor influence of individual characteristics on critical wet-bulb globe temperature (WBGT) limits during light activity in young adults (PSU HEAT Project)

Critical environmental limits are temperature-humidity thresholds above which heat balance cannot be maintained for a given metabolic heat production. This study examined the association between individual characteristics [sex, body surface area (A_D), aerobic capacity (V̇O_2max), and body mass (m_b)] and critical environmental limits in young adults at low metabolic rates. Forty-four (20 M/24 F; 23±4 yrs) subjects were exposed to progressive heat stress in an environmental chamber at two low net metabolic rates (M_net); minimal activity (MinAct; M_net = ~160 W) and light ambulation (LightAmb; M_net = ~260 W). In 2 hot-dry (HD; ≤25% rh) environments, ambient water vapor pressure (P_a = 12 or 16 mmHg) was held constant and dry-bulb temperature (T_db) was systematically increased. In 2 warm-humid (WH; ≥50% rh) environments, T_db was held constant at 34 or 36 °C and P_a was systematically increased. The critical wet-bulb globe temperature (WBGT_crit) was determined for each condition. During MinAct, after entry of M_net into the forward stepwise linear regression model, no individual characteristics were entered into the model for WH (R²_adj = 0.01, p = 0.27) or HD environments (R2adj = -0.01, p = 0.44). During LightAmb, only m_b was entered into the model for WH environments (R²_adj = 0.44, p < 0.001), whereas only V̇O_2max was entered for HD environments (R²_adj = 0.22; p = 0.002). These data demonstrate negligible importance of individual characteristics on WBGT_crit during low-intensity non-weight-bearing (MinAct) activity with a modest impact of m_b and V̇O_2max during weight-bearing (LightAmb) activity in extreme thermal environments.