When it’s humid and the air temperature is higher than your core temp (97-plus degrees Fahrenheit), fans make your sweat evaporate, thereby cooling you down, says study author Nathan Morris, Ph.D., a postdoctoral researcher at the University of Copenhagen in Denmark.
But if you’re in dry heat (think: Nevada or Arizona) your perspiration evaporates on its own. In that case, fans simply blow hot air on you, forcing your body to sweat even more to lower your body temp, Morris explains. In turn, you’re at a higher risk of dehydration.
The bottom line:
“A good rule of thumb is if you can see or feel moisture on your skin, use a fan,” Morris says. If you live in a desert environment characterized by dry heat, use air conditioning instead or spritz cold water on your skin when you’re outdoors.