In hot, arid conditions with a relatively low heat index, electric fans may be detrimental to health and are not advisable. However, in hot, but very humid weather conditions with a much higher heat index, fans lowered core temperature and cardiovascular strain and improved comfort. Findings from a brief research report are published in Annals of Internal Medicine.
Researchers from the University of Sydney recruited volunteers on campus to examine the effect of electric fan use on thermal strain (rectal temperature), cardiovascular strain (heart rate and blood pressure), the risk for dehydration (whole-body sweat rate), and thermal comfort (assessed using 120-mm visual analog scale). Volunteers were monitored during a 2-hour exposure to simulated peak conditions of two types of heatwaves: One that was very hot and dry (mimicking the peak conditions of the California heatwave in July 2018, and the Ahmedabad heatwave in May 2018) and one that was cooler but more humid and with a higher heat index (representing the peak conditions reported during the Chicago heatwave in July 1995, and the Shanghai heatwave in July 2017).
The researchers found that in a hot, humid condition with a heat index of 56 °C, fans lowered core temperature and cardiovascular strain, and improved thermal comfort. However, fans were detrimental for all measures in the very hot, dry condition, despite a lower heat index of 46 °C.
According to the researchers, these findings highlight the issues that may arise when heat index values are used to recommend fan use during heat waves - as presently done by the US Environmental Protection Agency (EPA). The effectiveness of fans depends on the climate, but climate dependency is not adequately captured by heat index values. Fans may be a cheaper and more accessible alternative to air conditioning for some people, such as those living in parts of the United States that have higher humidity, or Southeast Asia, South America, and Europe, where temperatures rarely exceed 40 °C but are also accompanied by moderately high humidity.