Electric fan use for cooling during hot weather: a biophysical modelling study

Abstract

Background In hot weather, electric fans can potentially provide effective cooling for people, with lower greenhouse gas emissions and cost than air conditioning. However, international public health organisations regularly discourage fan use in temperatures higher than 35°C, despite little evidence. We aimed to determine humidity-dependent temperature thresholds at which electric fans would become detrimental in different age groups.Methods We used biophysical modelling to determine the upper humidity-dependent temperature thresholds at which fan use would become detrimental (ie, worsen heat stress) for healthy young adults (aged 18-40 years), healthy older adults (aged ≥65 years), and older adults taking anticholinergic medication. We also obtained hourly environmental data for the period Jan 1, 2007, to Dec 31, 2019, for 108 populous cities to determine the number of days fan use would be effective for cooling, standardised to a 31-day hot weather month. We established simplified temperature thresholds for future fan use recommendations on the basis of temperatures below which fan use would never have been detrimental between Jan 1, 2007, and Dec 31, 2019, across all prevailing levels of ambient humidity. FindingsAccording to our model, fan use would have been beneficial on 30•0 (96•6%) of 31 hot weather days for healthy young adults and 29•4 (94•9%) of 31 hot weather days for both older adults and older adults taking anticholinergic medication between Jan 1, 2007, and Dec 31, 2019. Adherence to the current WHO recommendation of fan use below temperatures of 35°C only, fan use would have been recommended on 27•2 days (87•7%) of 31 hot weather days. According to our simplified thresholds for fan use (at temperatures <39•0°C for healthy young adults, <38•0°C for healthy older adults, and <37•0°C for older adults taking anticholinergic medication), fan use would have been recommended on 29•6 (95•5%) of 31 hot weather days in healthy young adults, 29•4 (94•8%) days in healthy older adults, and 28•8 (93•0%) days in older adults taking anticholinergic medication between Jan 1, 2007, and Dec 31, 2019. Interpretation Electric fan use, particularly for healthy young adults, would not have worsened heat stress on the majority of study days between 2007 and 2019. Our newly proposed thresholds for fan use provide simple guidelines that improve future heatwave fan use recommendations.

Application of Gagge’s energy balance model to determine humidity-dependent temperature thresholds for healthy adults using electric fans during heatwaves

“…Increasing air movement is a personalized cooling strategy that bypasses the issues associated with refrigerant gases and is more efficient than compressor-based air conditioning [8]. Electric fans are relatively inexpensive (for example, a basic model can be purchased for 10 USD in India), energy-efficient, some (e.g., pedestal and desk) do not have any installation cost, and with direct current motors, they now consume single-digit watts and provide substantial air flows [9].…”

“…Electric fans are relatively inexpensive (for example, a basic model can be purchased for 10 USD in India), energy-efficient, some (e.g., pedestal and desk) do not have any installation cost, and with direct current motors, they now consume single-digit watts and provide substantial air flows [9]. The relatively low electricity consumption and cost of fans could help mitigating additional negative compounding health factors such as socioeconomic inequality or poor access to air conditioning [8]. Electric fans can be used either as an alternative cooling technology or in combination with a reduced level of compressor-based air conditioning [10][11][12].…”

“…Excessive heat can compromise the ability to maintain core temperature within safe limits and can worsen the health of those with pre-existing health conditions [1]. For example, people with cardiovascular disease are at an elevated risk of catastrophic collapse due to an exacerbation of cardiovascular strain associated with the maintenance of central blood pressure in the face of rising skin blood flow sometimes accompanied by decreases in blood volume due to dehydration [8].…”

“…Based on this evidence, advising healthy adults not to use fans when 𝑡 𝑑𝑏 exceed 𝑡 𝑠𝑘 or 35 • C could increase their risk of suffering from heat strain and would prevent them from using an effective, energy-efficient, and low-cost cooling technology. To better understand under which environmental conditions (i.e., 𝑡 𝑑𝑏 , 𝑅𝐻) the use of fans would be beneficial, both Jay et al [19] and Morris et al [8] developed a simplified heat balance model. They then used their model to develop a chart that can be used to assist public health messaging in explaining to young and older adults when the use of electric fans is beneficial to cool the human body.…”

“…Jay et al [19] chose a value of 4.5 m/s based on measured values of air speed at a distance of 1.0 m of an 18" pedestal fan set at maximum speed [19]. Morris et al [8] chose instead a value of 3.5 m/s. Their results may be difficult to generalize since such high air speeds cannot be easily achieved by most ceiling fans [24], pedestal, or desk fans [9] available on the market.…”