Introduction

Extreme heat is the primary weather-related cause of death in the United States.¹ The frequency of extreme heat events is projected to increase. For example, the National Oceanic and Atmospheric Administration Future Heat Events tool estimates that in Hinds County, MS, the number of days with temperatures above 95ºF will rise to 58 days by 2030, 64 days by 2040, and 75 days by 2050.² Exposure to extreme heat causes illness and death from heatstroke and hyperthermia, and can exacerbate a range of other medical conditions, particularly cardiovascular and respiratory disease.³ The most vulnerable groups are the very young, the elderly, those with chronic medical and mental health conditions, persons on medications that inhibit the body’s ability to initiate a thermoregulatory response (e.g., anticholinergics and antihistamines), outdoor workers, undomiciled persons, persons living alone and/or without social support, and low-income people.⁴ The latter are susceptible because they may not be able to afford air conditioning, or may use it intermittently to save money. They may also live in energy-inefficient housing that does not maintain cool ambient temperatures.

The elderly are particularly vulnerable because thermoregulatory and cardiovascular responses are impaired, with reduced physiological capacity to secrete sweat and an increased likelihood of impairment of renal function which could be exacerbated by dehydration.^5–9 Certain medications (e.g., anticholinergics, antihistamines, cyclic antidepressants, medications for Parkinson’s disease, and antipsychotics including phenothiazines, butyrophenones, and thioxanthenes) impair the body’s ability to initiate a thermoregulatory response.^10,11 Diuretics increase the risk of heat-related illness by exacerbating dehydration.

Electric cooling fans are a low-cost, low-energy demand cooling method that may be suitable for use by low-income residents and the elderly. We conducted a scoping review of the use of electric cooling fans for reducing adverse health effects during heatwaves, particularly in the elderly and low-income populations, with specific reference to Mississippi and the southern United States, where summer humidity is high. We also reviewed articles on secondary cooling methods including skin wetting (or self-dousing), foot immersion, and wetting (or soaking) of clothing.

Methods

Using PubMed, we conducted an online literature search of the published English-language literature for the period 01/01/2000-04/26/2022 to obtain relevant publications on human studies of the health effects of electric fans and other simple, low-cost cooling strategies during extreme heat events (heat waves/heatwaves). The search query included the terms: “electric fan” OR “cooling fan” OR “skin wetting” OR “self-dousing” OR “foot immersion” OR “wetting of clothing” AND “heat wave” OR “heatwave” OR “extreme heat”. We also checked the reference lists of relevant articles and the websites of relevant national and international organizations.

Results

Of the 352 articles retrieved, 14 were judged to be relevant. Three hundred and thirty-eight articles were excluded because they were general articles on climate change; climate modeling studies; reviews of heat-related illness and mortality and the clinical management of heat stroke and hyperthermia; specific clinical studies on athletes, pregnant women, or outdoor workers; population studies on surveillance for heat-related illness, extreme heat warning indexes and systems, epidemiology and economic costs of heat-related illness and mortality; health education about the risks of extreme heat; physiological studies; or ecosystem studies on freshwater or other natural systems.

The 14 relevant studies included 7 clinical trials, 4 biophysical modeling studies, 2 occupational studies, and 1 Cochrane review; the 2 occupational studies were not reviewed further as we wanted to focus on cooling methods suitable for use at home. All the clinical trials were small, the number of participants varying from 8-18, and, with one exception, lacked a control group; 6 were judged to be level 4 evidence because they were a case-series, case-control study, or historically controlled study. One study was determined to be a level 3 because it was mechanism-based reasoning.¹² Based on the results of the reviewed clinical trials and modeling studies, we present conservative recommendations by age group, with particular attention to cooling strategies that are suitable for low-income people living in the southern United States.

Recommendations by Age Group

Adults Less than 65 years of Age

The effect of fan use depends on the specific combination of ambient temperature and humidity. Normal average skin temperature is about 91°F (33°C). In conditions of high ambient temperature, when air temperature exceeds skin temperature, the skin-to-air temperature gradient is reversed and dry heat transfers toward the body by convection. This convective heat gain increases the rate of sweat evaporation. Relative humidity, the percentage of water vapor in the air at a given temperature, must also be taken into account. At moderate levels of relative humidity around 30% - 50%, electric fan use is likely to provide beneficial cooling effects if the temperature is 102.2ºF (39ºC) or lower. If the temperature is greater than 102.2 (39^oC), electric fan use may exacerbate heat stress, leading to a rise in the body’s core temperature.^13–15 At higher levels of relative humidity (above about 70%), sweat evaporation decreases, and fan use may be harmful as convective heating outpaces evaporative cooling. It should be noted that in very hot, dry conditions, that is, RH around 10%, sweat evaporates readily, even without the use of a fan. When RH is very low, fans provide no additional benefit and can even accelerate body heating and exacerbate cardiovascular strain and thermal discomfort.¹⁵

As additional beneficial measures, both skin-wetting (or self-dousing) with a wet sponge or spray bottle and foot immersion (e.g., submersion of the lower legs to mid-calf into cool water for about 20 minutes, followed by 10 minutes out, then repeated) can reduce physiological heat strain, dehydration, and thermal discomfort at temperatures up to 116.6°F (47°C), irrespective of humidity.¹⁶ No studies have been conducted in this age group on combining fan use with wearing water-soaked clothing.

Adults 65 years of age and older not taking medications that may impair the body’s thermoregulatory response

At moderate levels of RH (about 30% - 50%), electric fan use is likely to provide beneficial cooling effects if the temperature is 100.4ºF (38ºC) or lower. Electric fan use may exacerbate heat stress if the temperature is greater than 100.4ºF (38ºC).¹³ The temperature threshold is lower because of the reduced sweating capacity at older ages. Additionally, both skin-wetting (or self-dousing) and foot immersion, as described for younger adults, are even more useful in this age group as they may help to compensate for the reduced sweating capacity in this age group.¹⁶ Combining fan use with wearing a water-soaked t-shirt, however, provides no additional benefit.¹⁷

Adults 65 years of age and older taking medications that may impair the body’s thermoregulatory response

At moderate levels of RH (about 30% - 50%), electric fan use is likely to provide beneficial cooling effects if the temperature is 98.6ºF (37ºC) or lower. Electric fan use may exacerbate heat stress if the temperature is greater than 98.6ºF (37ºC).¹³ The temperature threshold is further lower because of the combined effect of reduced sweating capacity at older ages and the impairment of the body’s ability to initiate a thermoregulatory response by medications. As described for elderly adults not on medications impairing thermoregulation, both skin-wetting (or self-dousing) and foot immersion are useful additional measures but wearing a water-soaked t-shirt provides no additional benefit.^16,17

Discussion

The cooling effects of electric fans and various other simple cooling strategies depend on the prevailing combination of temperature and humidity and the interaction of these climatic factors with a number of individual factors, such as age, comorbidities, medication use, and the ability to sweat. Although the studies we have reviewed here are few in number and small, the results are consistent and indicate that electric fans, when used carefully, are a low-cost cooling method suitable for use by low-income residents in regions with moderate relative humidity during periods of extreme heat.¹⁸ Our recommendations for fan use are shown below in graphic form for quick reference (figure); in setting the temperature limits we have erred on the side of caution to avoid the possibility of exacerbating heat stress. The recommendations will also be made available as a free trifold brochure for the general public (contact the first author for more information).

Figure 1.Recommendations for use of electric cooling fan, by age group and ambient temperature.

The most recent (2012) Cochrane review of this topic “does not support or refute the use of electric fans during a heatwave.”¹⁹ However, at the time of the Cochrane review no reports of laboratory or clinical trials of the use of electric fans had been published. Existing guidelines from the World Health Organization, Environmental Protection Agency, and Centers for Disease Control and Prevention, summarized in the table, also do not reflect the recent research.^20–22 The findings of the clinical trials and modeling studies reviewed here, all conducted since 2015, support the use of electric fans during a heatwave in hot regions with moderately high RH, such as the southern United States, as an effective and affordable way of keeping cool for those without (or with limited access to) air conditioning, provided consideration is taken of ambient temperature, age and use of medications.^13,18 Skin wetting (or self-dousing) and foot immersion, but not wetting (soaking) of clothing, are useful additional measures, and, of course, staying well hydrated is important.