Electric fans aren’t always best for cooling off. Here’s why.

When it’s hot out but dry, an electric fan is not your friend: It won’t cool you off, and can make your body strain harder to fight the heat.

A small study from Australia, published Monday in the Annals of Internal Medicine, looked at what goes on in the body when a person tries to cool off with a fan in different climate conditions.

The researchers, from the University of Sydney, measured the vitals of 12 healthy young men while they sat in a climate chamber — a closed room that replicates different environmental conditions — and found that fans were more effective at cooling the body in hot, humid conditions compared with hot, dry conditions.

The men had four sessions in the climate chamber, with the temperature raised to either a humid 104 degrees Fahrenheit or a dry 117 degrees, for two hours at a time. Under the humid conditions, the heat index reached 133 degrees, and under the dry conditions, it reached 115 degrees.

Each condition was tested with and without an electric fan placed about 4 feet away. Sensors measured the men’s vitals — including heart rate and rhythm, core temperature, whole-body sweat rate, and blood pressure — while they were in the chamber.