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 yr) 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 two 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 two warm-humid (WH; ≥50% rh) environments, T_db was held constant at 34°C 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 (R²_adj = -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 nonweight-bearing (MinAct) activity with a modest impact of m_b and V̇o_2max during weight-bearing (LightAmb) activity in extreme thermal environments.NEW & NOTEWORTHY Our laboratory has recently published a series of papers establishing the upper ambient temperature-humidity thresholds for maintaining heat balance, termed critical environmental limits, in young adults. However, no studies have investigated the relative influence of individual characteristics, such as sex, body size, and aerobic fitness, on those environmental limits. Here, we demonstrate the contributions of sex, body mass, body surface area, and maximal aerobic capacity on critical wet-bulb globe temperature (WBGT) limits in young adults.