Heart rate and body temperature responses to extreme heat and humidity with and without electric fans

Patz et al1 described the projected effects of more prolonged and severe heat waves on human health. A simple, low-cost cooling device is an electric fan. A Cochrane review2 concluded “no evidence currently exists supporting or refuting the use of electric fans during heat waves” for mortality and morbidity. However, public health guidance typically warns against fan use in hot weather. Recommended upper limits range from 32.3°C (90°F) at 35% relative humidity (RH) to the high 90s (96-99°F; 35.6-37.2°C, no RH stated2). The skin-to-air temperature gradient reverses with rising environmental temperature, causing dry heat transfer toward the body via convection rather than away from it. Fan use would increase this dry heat transfer, potentially accelerating body heating3,4; however, the efficiency of sweat evaporation from the skin would be simultaneously increased. Thus, fans could still improve net heat loss. Sweat evaporation declines with increasing humidity, so in more humid environments fans may not prevent heat-induced elevations in cardiovascular (heart rate, HR) and thermal (core temperature) strain. This study examined the influence of fan use on the critical humidities at which hot environments can no longer be physiologically tolerated without rapid increases in HR and core temperature.