Health-based multifactorial criteria for electric fan use in extreme heat

Extreme heat events are becoming more frequent and intense, posing severe health risks, particularly for vulnerable populations. Current public health recommendations on electric fan use rely predominantly on core temperature responses and cardiac strain parameters such as heart rate and blood pressure.1,2 Although these measures are crucial, they do not capture the broader physiological and systemic effects of fan cooling, particularly its influence on inflammation, hydration status, and perceptual responses. A growing body of research underscores the need to move beyond singular outcomes to assess cooling strategies.

A 2024 study has questioned the benefit of fan use in the heat when it produces only marginal reductions in core temperature (eg, 0·2–0·3°C).3 However, this narrow interpretation underestimates the cumulative benefits of fan use. A randomised crossover trial of 58 older adults (mean age, 66–70 years) showed that fan use significantly reduced cardiovascular strain, as measured by rate-pressure product (RPP), during hot−humid exposure.4 Importantly, fan use offers benefits beyond thermoregulation. Controlled trials conducted during the simulated August, 2024 Hangzhou extreme heatwave event showed that fan use reduced circulating levels of inflammatory markers, such as interleukin-6 and interleukin-1β, by over 20%, even when the core temperature was only modestly lowered.5 These findings align with the understanding that systemic inflammation plays a central role in heat-related pathology and can exacerbate cardiovascular strain. Elevated cytokine levels, including those of interleukin-6 and tumour necrosis factor-α, are associated with poor heat tolerance and greater risk of a systemic inflammatory response syndrome (SIRS)-like state.5

To evaluate the effects of electric fan use during extreme heat more holistically, a multifactorial framework based on four physiological and perceptual domains is proposed (figure): thermoregulation, systemic inflammation, hydration status, and perceptual response. If fan use satisfies any one of these domains under specific environmental conditions, its use should be encouraged as a viable and accessible heat mitigation strategy.