Elevated Humidity Impairs Evaporative Heat Loss and Self‐Paced Exercise Performance in the Heat

This study investigated the effects of absolute humidity on heat dissipation and subsequent thermal, cardiovascular, and performance responses during self-paced exercise in the heat. Twelve trained male cyclists performed a 700-kJ time trial in four different humidity conditions (Low: 1.6 kPa, Moderate: 2.5 kPa, High: 3.5 kPa, and Very high: 4.5 kPa) in 33°C. The gradient in partial water vapor pressure at the saturated skin surface and in air, which determines sweat evaporation, decreased significantly with increasing humidity (Low: 3.53 ± 0.30 kPa, Moderate: 2.74 ± 0.24 kPa, High: 1.99 ± 0.20 kPa, Very high: 1.19 ± 0.16 kPa; p < 0.001). The maximum evaporative capacity of the environment (E_max) also decreased with greater humidity (Low: 309 ± 26 W m^-2, Moderate: 240 ± 21 W m^-2, High: 175 ± 18 W m^-2, Very high: 104 ± 14 W m^-2; p < 0.001), as did sweating efficiency (S_eff) (Low: 0.50 ± 0.13, Moderate: 0.39 ± 0.10, High: 0.28 ± 0.09, Very high: 0.16 ± 0.04; p ≤ 0.003). Power output was similar between Low (260 ± 33 W) and Moderate humidity (257 ± 27 W; p = 0.999), but lower in Very high (222 ± 37 W) than in all other conditions (p < 0.001) and lower in High (246 ± 31 W) than in the Low and Moderate humidity (p < 0.001). Peak core temperature was higher in Very high (39.49°C ± 0.56°C) than in Low (38.97°C ± 0.44°C; p < 0.001), Moderate (39.04°C ± 0.39°C; p = 0.002) and High humidity (39.12°C ± 0.47°C; p = 0.010). Mean skin temperature was higher with elevated humidity (p < 0.001) and mean heart rate was not significantly different between conditions (p ≥ 0.056). These data indicate that reductions in evaporative potential and efficiency with elevated humidity exacerbate thermal and cardiovascular strain during self-paced cycling in the heat, resulting in marked performance impairments.