Well, this blows.

A new study has found that using electric fans to cool down during extreme heat is only effective when the humidity is high.

When the humidity is very low, the fans can actually make you feel hotter — and can strain your heart by spiking core temperature, according to the study published in the Annals of Internal Medicine.

The researchers warned that current guidelines for the use of electric fans is poorly based on heat index and not on the actual temperature.

“A combined value such as heat index (HI) is not appropriate for advising whether fans should be used or not,” senior study author Ollie Jay told Reuters.

“In very hot, dry conditions the HI was relatively lower, yet fans were detrimental; but in hot-humid conditions the HI was relatively higher and fans were beneficial,” added Jay, director of the Thermal Ergonomics Laboratory at the University of Sydney in Australia.

Jay said that in the very hot-arid conditions, “the heart has to do extra work to maintain blood pressure because as one gets hot, blood is diverted to the skin to facilitate cooling and the heart must beat more times per minute.”

He added: “Another form of strain (on the body) is dehydration, which also contributes to increases in body temperature and cardiovascular strain.”

For the study, the researchers recruited 12 men who were not taking any medication and had no medical conditions to sit in a climatic chamber that simulated either very hot, dry conditions or hot, humid conditions.

The heat index for the humid tests was higher than for the arid ones even though the temperature was lower in the humid condition.

The researchers measured the men’s temperatures rectally and assessed their heart strain using three-lead electrocardiography and blood pressure. They measured dehydration by keeping track of sweat.

Fan use in the humid conditions (temperature at 104 degrees Fahrenheit, humidity at 50 percent and HI at 132.8) reduced core temperature and cardiovascular strain while improving comfort.



Meanwhile, in the arid conditions (temperature at 116.6 degrees, humidity at 10 percent and HI at 114.8) the fan use raised core temps and cardiovascular strain, and made the men feel hotter — even though the HI was lower than it was in the humid condition.

While the researchers used pedestal fans for the research, “the principles are the same for a ceiling fan,” Jay said.

The new study “is very interesting,” said Dr. Matthew Levy, an associate professor of emergency medicine at the Johns Hopkins School of Medicine.

“There are some significant takeaways: people in hot, humid conditions tend to do better with a fan. In situations where air conditioning is not possible, this means people would do better with a fan than nothing at all,” he told Reuters.

That’s because fans can help with heat loss through evaporation of sweat, Levy said.

“But in a dry environment, the fans just push more warm air toward you,” he added.