The biophysical and physiological basis for mitigated elevations in heart rate with electric fan use in extreme heat and humidity.

Electric fan use in extreme heat wave conditions has been thought to be disadvantageous because it might accelerate heat gain to the body via convection. However, it has been recently shown that fan use delays increases in heart rate even at high temperatures (42 °C) in young adults. We here assess the biophysical and physiological mechanisms underlying the apparently beneficial effects of fan use. Eight males (24 ± 3 y; 80.7 ± 11.7 kg; 2.0 ± 0.1 m2) rested at either 36 °C or 42 °C, with (F) or without (NF) electric fan use (4.2 m/s) for 120 min while humidity increased every 7.5 min by 0.3 kPa from a baseline value of 1.6 kPa. Heart rate (HR), local sweat rate (LSR), cutaneous vascular conductance (CVC), core and mean skin temperatures, and the combined convective/radiative heat loss (C+R), evaporative heat balance requirements (Ereq) and maximum evaporative potential (Emax) were assessed. C+R was greater with fan use at 36 °C (F 8 ± 6, NF 2 ± 2 W/m2; P = 0.04) and more negative (greater dry heat gain) with fan use at 42 °C (F -78 ± 4, NF -27 ± 2 W/m2; P < 0.01). Consequently, Ereq was lower at 36 °C (F 38 ± 16, NF 45 ± 3 W/m2; P = 0.04) and greater at 42 °C (F 125 ± 1, NF 74 ± 3 W/m2; P < 0.01) with fan use. However, fan use resulted in a greater Emax at baseline humidity at both 36 °C (F 343 ± 10, NF 153 ± 5 W/m2; P < 0.01) and 42 °C (F 376 ± 13, NF 161 ± 4 W/m2; P < 0.01) and throughout the incremental increases in humidity. Within the humidity range that a rise in HR was prevented by fan use but not without a fan, LSR was higher in NF at both 36 °C (P = 0.04) and 42 °C (P = 0.05), and skin temperature was higher in NF at 42 °C (P = 0.05), but no differences in CVC or core temperatures were observed (all P > 0.05). These results suggest that the delayed increase in heart rate with fan use during extreme heat and humidity is associated with improved evaporative efficiency.