Use your fan in or next to a window. Box fans are best.” (City of Toronto, Heat Alerts and Extreme Heat Alerts, 2010)

Much controversy has surrounded the use of electric fans during an extreme heat event and whether their use contributes or impedes heat loss at high ambient temperatures, particularly when accompanied by high humidity. This may cause confusion and be a factor in any inconsistent messages regarding the use of fans during extreme heat.
During the 1995 Chicago heat wave, an elderly man was found dead in his sweltering apartment; sitting in front of a fan with the windows and doors closed.1 It was suggested that, due to high indoor temperatures, the fan contributed to the man’s death by blowing hot air on his body, increasing his internal body temperature and consequently causing heat stroke. In the last decade, little research has focused on investigating whether fan use is beneficial or detrimental during extreme heat events. Furthermore, the available evidence is not entirely coherent. Some of the findings are presented here.
To date, there has been little research on the physiological effects of fan use. Therefore, it is difficult to assess the associated risks and benefits of fan use by elderly and other vulnerable populations during periods of high heat and humidity. Also, epidemiological studies describe varied effects of fan use on severe morbidity and mortality. Some studies have found no association between use of electric fans and the onset of heat stroke2-4; in other cases, fan use has been found to be slightly protective.5 A meta-analysis conducted by Bouchama et al.5 found a protective association with the use of fans (OR 0.60, 95% CI 0.4 – 1.1), but this result was not statistically significant. Bouchama et al.5cautioned the use of fans until further research is completed.
When is a fan useful?
To date, little information exists regarding the effectiveness of fans during periods of extreme heat. For this reason, the information presented below is derived from knowledge of heat transfer and body thermoregulation.6,7 Further empirical evidence is needed before well-defined recommendations can be made.
Air is cooler than skin temperature (34-36ºC): When air temperature is cooler than a person’s skin temperature, sitting in the direct path of a fan’s air flow can promote both convective and evaporative heat loss. As long as the air temperature is below skin temperature, the body gives off heat to the moving airstream more quickly than in the case of stagnant air, through transfer by conduction / convection, and evaporation, thereby keeping the body temperature lower than in a stagnant air situation.
Air is warmer than skin temperatures (above 36ºC): When air temperature is higher than a person’s skin temperature, sitting in the direct path of the fan’s air flow can help with evaporative heat loss only if someone is sweating or misting themselves with water and the sweat/water is evaporating. Otherwise, the hot air leads to further warming through convection.   
The cautionary note is that if air temperature is very high (greater than skin temperature) and humidity is also high (as to impede the rate of sweat evaporation), the use of a fan can become counter-productive, i.e., actually increasing the thermal load on the body, compared with a stagnant air situation.4,8 It is important to note that the point at which this temperature/humidity combination is achieved will vary, depending on rate of fan-generated air flow, type of clothing, activity level (metabolic heat load), personal characteristics, such as age (e.g., older persons may have a decreased degree of skin wetness by reduced sweating), and other conditions which may inhibit or diminish the sweating mechanism, such as diabetes, obesity, vascular impairments, or anhidrotic conditions (e.g., as may be brought about by certain medications).
What are the other potential advantages of a fan?
The above information suggests that in certain situations a fan may be useful in preventing heat-related illness. There are a number of other situations and tools, suggested by practitioners, for increasing the effectiveness of fan use; however, empirical evidence is lacking. For example, common sense indicates that a fan placed in the window may help draw in cooler air from the outside and lower the temperature inside a room. This may be especially useful at night when temperatures are generally lower outdoors. To increase cooling, a bowl of ice can be placed in front of the fan which will blow colder air, increasing the temperature gradient between the subject and the surrounding air, thus increasing convective heat loss. There are also suggestions that misting the body with cool water in areas of normal to low humidity can be effective in increasing evaporative heat loss.9 There is little information available about the effectiveness of different types of fans (e.g., ceiling, standing or box style) for air circulation.
The use of a fan to direct air at a person can be counter-productive when the air temperature is greater than skin temperature and the evaporation of sweat is impaired by high humidity or by age, certain diseases or the effects of specific medications. During extreme heat, indoor temperatures can be much hotter than the temperature outside and so re-circulating extremely hot air can be dangerous.It is also possible that some older fans have poor air flow and are not as efficient, therefore having less of an effect. 
Gaps and Questions 
  • Is there a specific set of environmental conditions (combinations of temperature and humidity) in which a fan may actually increase a person’s chance of developing a heat related illness?
  • What is the relative value of different types of fans (e.g., ceiling, standing, box style)?
  • Are tools, such as directing the air stream across cool water or combining fan use with misting the body using cool water, useful for preventing heat-related illness?
  1. Klinenberg E. Heat wave: A social autopsy of disaster in Chicago. Chicago, IL: University of Chicago Press; 2002.
  2. Naughton MP, Henderson A, Mirabelli MC, Kaiser R, Wilhelm JL, Kieszak SM, et al. Heat-related mortality during a 1999 heat wave in Chicago. Am J Prev Med. 2002 May;22(4):221-7.
  3. Semenza JC, Rubin CH, Falter KH, Selanikio JD, Flanders WD, Howe HL, et al. Heat-related deaths during the July 1995 heat wave in Chicago. N Engl J Med. 1996;335(2):84-90.
  4. Kilbourne EM, Choi K, Jones TS, Thacker SB. Risk factors for heatstroke. A case-control study. JAMA. 1982;247(24):3332-6.
  5. Bouchama A, Dehbi M, Mohamed G, Matthies F, Shoukri M, Menne B. Prognostic factors in heat wave related deaths: A meta-analysis. Arch Intern Med. 2007;167(20):2170-6.
  6. Guyton AC, Hall JE. Textbook of medical physiology 10th ed. Philadelphia, PA: W.B. Saunders Company; 2000.
  7. Knochel JP, Reed D, editors. Disorders of heat regulation. 5th ed. New York, NY: McGram-III; 1994.
  8. Lee DH. Seventy-five years of searching for a heat index. Environ Res. 1980 Aug;22(2):331-56.
  9. Wolfe RM. Death in heat waves: beware of fans. BMJ. 2003 Nov 22;327(7425):1228.
March 2011