August 15, 2025, 12:52 pm | Read time: 5 minutes
A fan can provide relief from extreme heat—or have the opposite effect. Whether the device truly helps or can even become dangerous largely depends on how well the body is hydrated. A new study from Australia has now examined how using a fan in extreme heat can strain your circulatory system, affect body temperature, and personal temperature perception—and the role dehydration plays.
What Was Studied and Why?
Heatwaves are an increasing health threat worldwide. In the summer of 2022, over sixty thousand people in Europe died due to extreme temperatures.1 People without air conditioning are particularly at risk—many instead use simple fans. However, this seemingly harmless helper can actually harm the body under certain conditions.
Previous studies showed that fans can help reduce physical strain at ambient temperatures up to about 39 to 40 degrees Celsius. If it gets hotter, the effect can reverse: The air movement can cause the body to absorb more heat than it can release through the skin–especially if sweating is impaired.
This is where the new study comes in: A research team from the University of Sydney investigated how using a fan in high heat and humidity (just under 40 degrees Celsius and nearly 50 percent humidity) affects the body, both in well-hydrated and dehydrated individuals. The goal was to determine whether dehydration changes the fan’s effect on circulation, body temperature, sweating, and temperature perception.2
Participants Spent Hours in Climate Chamber
The study was conducted with 20 healthy adults. All participated in four three-hour heat stress tests in a climate chamber—each with and without a fan, once well-hydrated and once dehydrated.
For the well-hydrated trials (technical term: “euhydrated,” balanced water balance in the body), participants drank enough water the day before and also consumed warm water (about 37 degrees Celsius) during the stress. In the dehydration trials, they avoided water-rich foods for 24 hours and drank nothing during the heat exposure.
During the tests, they sat still, wearing shorts and a cotton undershirt. A fan with an airspeed of two and a half meters per second was placed one and a half meters away. Measurements included core body temperature (rectal), pulse, sweat amount (calculated by weight loss), as well as subjective heat perception, discomfort, and thirst.
What Were the Results?
The effect of the fan strongly depended on whether the test subjects were adequately hydrated or not.
With Good Hydration: Noticeable Relief
In a well-hydrated state, the fan did not strain the circulatory system. The heart rate remained stable, and temperature perception even improved: The heat was perceived as significantly less uncomfortable—on a scale, the value dropped by an average of nine millimeters. Discomfort also noticeably decreased (by seventeen millimeters on the scale).
With Dehydration: Additional Circulatory Strain
It was different with dehydration. Here, the pulse increased even further due to the fan—by an average of five beats per minute compared to dehydrated individuals without a fan. This difference was statistically significant—meaning it is very likely due to the fan. No noticeable relief in heat perception or discomfort was observed. The cooling effect was absent.
More Sweat–But Not Always an Advantage
Regardless of hydration status, the fan significantly increased sweat production—in dehydrated individuals, the increase was about 125 grams per hour. With good hydration, this is not a problem—but with dehydration, it exacerbates the water deficit and thus the circulatory strain.
Dehydration Severely Strains the Body
In general, it was shown: Dehydration significantly worsens all measured values–regardless of fan use. Core body temperature increased by an average of three-tenths of a degree, and heart rate increased by up to 17 beats per minute. Subjective discomfort and thirst were also significantly more pronounced.
A Fan Is Not Always Beneficial
The study makes it clear: A fan can only be effectively used for cooling in extreme heat if the body is well-hydrated. If not, the fan can further strain the circulatory system—especially because it increases sweating without actually providing cooling.
In everyday life, this means: Anyone using a fan on hot days should ensure they drink enough. Even slight dehydration can cause the airflow to be unhelpful or even harmful—especially for older people or those with conditions that limit sweating.
The results also show: Thirst is a serious warning signal. Even at the same heat, pulse and body temperature were significantly higher in dehydrated individuals, and subjective discomfort was greatly intensified.
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Context and Limitations
The study was conducted with young, healthy adults in a controlled laboratory environment. This ensures very precise and comparable data. Through the so-called crossover design—where each person went through each test condition—individual differences were well balanced.
However, the transferability to other groups is limited. Older people, children, or individuals with pre-existing conditions might react differently to heat or fan use. Medications that affect sweating or thirst were also not considered. Additionally, only a sitting situation was tested–how the body behaves during movement or physical work remains open.
Other cooling methods, such as targeted dousing with water (“self-dousing”), which have been promising in other studies, were not examined.
Conclusion
Fans can help in extreme heat—but only if you are well-hydrated. If not, the fan can further strain the circulatory system. Those relying on airflow should ensure they drink enough. The new study provides important insights for the sensible and safe use of fans—especially in times of increasingly frequent heatwaves.