Electric Fan Use in Replicated 8-Hour Extreme Heat Event: Sex Differences in Systemic Biomarkers and Thermoregulation

Health agencies, including the Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO), recommend different temperature thresholds for electric fan use during heat events (CDC: 32.2℃, WHO: 40℃). Nonetheless, these guidelines do not account for the fan’s physiological effects on immune and inflammatory responses in males and females. This study evaluated the efficacy of electric fan use in mitigating immune function, inflammation, and organ function during an eight-hour simulated extreme heat event replicating conditions in Hangzhou, China, on the August 3, 2024 (semi-hourly temperature and humidity fluctuations; average temperature: 39.9℃ [37.6-41.1℃], RH: 47.1% [40-57%]). Thirty-two young adults (16 males and 16 females) underwent three eight-hour trials: (1) no electric fan with limited fluid intake (500mL; Con), (2) fan use with limited fluid intake (500mL; Fan) and (3) fan use with sufficient fluid intake (3L; Fan+Fluid). Core temperature, cardiovascular responses, plasma electrolytes, stress hormones, inflammatory markers, and organ function biomarkers were assessed. Fan+Fluid significantly reduced core temperature, stress hormone levels, inflammatory responses, and organ function biomarkers in both sexes (all p <0.05). Notably, Fan+Fluid greatly reduced IL-6, IL-1β, ALT and BUN by 23.9%, 32.5%, 15.9% and 23.6%, respectively, compared to CON, despite the core temperature difference is marginal. Females exhibited consistently higher stress hormone levels, inflammatory responses, and organ function markers than males across all trials (all p <0.05). These findings highlight the benefits of electric fan use in prolonged extreme heat and suggest that females may require more intense cooling interventions due to heightened inflammatory and organ stress responses.