Here's When Using a Fan Will Actually Make You Hotter

Image: haru__q (Flickr (CC BY-SA 2.0))

Electric fan fans are in for some bittersweet news. A new study out Monday suggests that while fans can help people cool down on hot and humid days, in hot and arid environments they may actually make you more uncomfortable—and may even be unsafe.

The researchers, based at the University of Sydney in Australia, recruited 12 healthy college-aged men for their experiment. The shorts-wearing men were asked to sit in a room for two hours at a time in four very unpleasant scenarios.

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For one pair of scenarios, the room was heated to 104 degrees Fahrenheit, with a relative humidity of 50 percent, amounting to a heat index (the temperature it actually feels like) of 132.8 degrees Fahrenheit. In the other scenarios, the room was heated to 116.6 degrees Fahrenheit, with a relative humidity of 10 percent, making for a lower heat index of 114.8 degrees Fahrenheit. Half the time, the men sat in front of a typical electric fan.

Before and after the experiments, the men had their heart rate, sweat level, and body temperature measured (rectally, unfortunately), and they also reported how generally comfortable they felt.

As you might expect, it wasn’t a great experience no matter which scenario it was. But the men did report feeling twice as comfortable in the hot, humid room when the fan was on than when it wasn’t. Their body temperature and level of cardiac stress was slightly lower when the fan was on, though they did sweat more, increasing the risk of dehydration.

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But when the fan was on in the hot and dry room, the men fared worse on every measure compared to the no-fan scenario, despite the room “feeling” less hot than the high-heat-and-humidity version.

Currently, organizations like the Environmental Protection Agency advise that people not use fans when the heat index is over our body temperature, or 99 degrees Fahrenheit. The logic goes that fans can actually cause more strain on the body at that temperature and above, by blasting us with air warmer than our skin and making us hotter through convection.

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But the study’s results, published in the Annals of Internal Medicine, seem to show that relying on heat index alone might not be the best way to tell if using a fan is helpful, according to the authors.

“These findings highlight issues that may arise when [heat index] values are used to recommend fan use during heat waves,” they wrote.

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The study’s conclusions should be taken with some guarded skepticism, as even the authors admit. Twelve volunteers is not a lot of people for a study, and two hours isn’t a lot of time to base a comparison on. We also can’t be sure these effects would be seen, and to what extent, in other groups of people. Perhaps for people who are taking certain medications, or who just came back from a workout, fans might affect them differently during either kind of hot day.

Still, there might be some room for comfort for people living in areas where hot and humid summers are the norm, but air conditioners are not. In areas such as the South Central United States, South Australia, and the Middle East, where heat waves tend to be relatively dry, fans might not be advisable, the authors wrote.

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“However, in much of the remainder of the United States, Southeast Asia, South America, and Europe, temperatures rarely exceed [104 degrees Fahrenheit] but are accompanied by moderately high humidity,” they added. “In these regions, fans could be encouraged as a cheaper and accessible alternative to air conditioning that also limits net electricity demand and carbon pollution.”

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About the author

Ed Cara

Science writer at Gizmodo and pug aficionado elsewhere