Electric fan use in heat waves: Turn on or turn off?

Heat waves have been responsible for more deaths worldwide than all other natural disasters combined. In Europe, this “silent killer” caused 70000C excess deaths in 2003, and more recently 3500C people died during 2 separate heat waves in India (May 2015) and Pakistan (June 2015) (Fig. 1). Groups among the most vulnerable include the elderly, poor, and people with cardiovascular disease. Identifying simple and cost-effective cooling strategies are therefore an urgent priority. In this Discovery article, we highlight our study which was the first to assess the efficacy of the humble electric fan for mitigating cardiovascular and thermal strain in humans during simulated heat wave conditions. Despite the high cooling capacity of air conditioners (AC), the millions of tonnes of CO2 they generate annually potentially contribute to a vicious cycle of worsening future heat waves. 3 Mass ACuse has also led to electricity blackouts or brownouts during heat waves, and in some cases catastrophic exacerbations in morbidity and mortality. By contrast, electric fans have an electricity requirement that is 50-fold lower than conventional AC units (55-100 W vs 1500-5000 W). In South Korea, fans are equipped with an off-timer due to the unsubstantiated belief held since the mid-1920s that prolonged fan use can lead to asphyxiation. Meanwhile, all major international public health agencies warn against fan use during heat waves due to a similarly unsupported belief that they paradoxically increases the risk of heat related illness. Specifically, the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) state that electric fan use above »35 C will critically exacerbate dehydration and “speed the onset” of heat exhaustion. In stark contrast, a 2012 Cochrane review identified that no empirical evidence exists to support or refute the use of fans in heat waves. In order to maintain body temperature, the human body must balance the rate of internal heat production with the rate of heat dissipation to the surrounding environment. One avenue of heat dissipation is sensible heat transfer, which in indoor environments primarily occurs via convection, following the temperature gradient between the skin surface and ambient air. When ambient air is less than skin temperature (»35 C), heat flows away from the body, but when the ambient air exceeds skin temperature this gradient is reversed and heat is instead added to the body. Since convective heat transfer increases sharply with increases in air velocity, more heat flows in to the body with forced air movement (e.g. with a fan).