Examining the physiological strain with electric fans during high indoor heat stress

• Public health guidance from various agencies disagree on the limit for fan use. • Fans offset heat gained with evaporative efficiency, even at high air temperatures. • The onset of cardiovascular strain precedes rises in core temperature with fans. • Fans are likely safe up to 43°C in healthy adults, 3 – 11°C above current guidance. • Additional fluid intake at ∼250 mL/h offsets the sweating with a fan at 43°C. Fans have been positioned as a low-cost, sustainable, and accessible heat resilience solution during extreme heat. Many health agencies caution against fan use when air temperature exceeds skin temperature (e.g. ≥35°C) suggesting they will accelerate body heating compared to still air. However, the increased evaporative efficiency with a fan likely mitigates greater rises in core temperature and cardiovascular strain compared to still air when the air temperature is >35°C. The present study evaluated the physiological responses with and without fans when indoor air temperature exceeded skin temperature to elucidate the safe upper limit. In a randomized crossover design, 10 healthy adults (4 females, 24±4 y, 1.8±0.1 m, 75.8±10.1 kg) were exposed to a simulated indoor overheating scenario whereby air temperature increased linearly from 38°C to 47°C over 3 hours, with a fan (∼5.5 m/s) or still air (<0.2 m/s). Heart rate and core temperature were significantly greater with a fan compared to still air when air temperature was ≥44°C and ≥45°C, respectively. Mean skin temperature and skin blood flow were statistically higher with a fan. While a fan increased whole-body sweat rate, the additional sweating can be counterbalanced with increased fluid intake (∼250 mL/h at 43°C) to mitigate dehydration. In conclusion, fans result in a higher heart rate or core temperature compared to still air in healthy adults when indoor air temperature >43°C; ∼3-11°C greater than guidance from various health agencies globally. Future work in other age groups and heat-vulnerable populations is needed, including field-based evaluation.