Why Electric Fans Aren’t Always Your Hot Weather Friend

Fans don't help when it's hot and dry out

August 15, 2019 

Humidity makes a difference in whether or not fans work

It seems pretty obvious: when you are outside and it’s hot, turning a fan on to blow the air around will cool you off. However researchers from the University of Sydney say that’s not always the case. In fact, the study found some truth to that saying “it’s not the heat it’s the humidity.”

In a study that was published in the Annals of Internal Medicine, scientists say when it’s HOT and DRY, an electric fan is not your friend. In fact, the study says it won’t cool you off and your body will strain harder to fight the heat.

Researchers measured the vital signs of 12 healthy young men while they sat in a climate chamber. That’s a closed room that can be programmed to replicate different environmental conditions. The study subjects experienced four sessions:

  1. 104 degrees-hot and humid-heat index 133 degrees ---with fan. 
  2. 104 degrees-hot and humid-heat index 133 degrees ---without fan.
  3. 117 degrees-dry-heat index 115 degrees---with fan.
  4. 117 degrees-dry-heat index 115 degrees---without fan.


 Sensors measured the men’s vital signs-including heart rate, core temperature, whole-body sweat rate and blood pressure.

Those vital signs provided a good snapshot of how much physiological strain each person was experiencing. The measurements show the session with a fan in hot, dry conditions was clearly worse for everyone. The bodies were struggling.

So, what's happening?

Think about how the body cools itself: evaporation. Sweat evaporates off of the skin. Fans increase the rate of evaporation by moving more air over the skin. More air means a faster evaporation rate and that results in cooler skin.

But when the air is hot and dry, the body wants to hold onto its water so it doesn’t sweat as much. Without sweat on the skin there is nothing to evaporate, so the fan is simply blowing hot air over the body. That actually hurts the cooling process. It’s why the U.S. Environmental Protection Agency only recommends using fans when the temperature is below 95-degrees. Ironically, the study found fans are effective above that temperature, but only in hot and humid conditions.

—Frank Graff 

 Frank Graff is a producer/reporter with UNC-TV, focusing on Sci NC, a broadcast and online science series.