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. Based on currently available information, the following is recommendation:
When 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.
When 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. 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. 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).
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.
See related @dvice Event: Heat and Extreme Heat Alerts.