On a scorching day, a fan can feel like the simplest answer to rising temperatures. Yet science shows that this familiar device does not always cool the body -- and in extreme heat, it can sometimes add to the strain.
Why the Same Fan Can Help or Hurt
A fan does not lower room temperature. It only moves air across the skin, and its effect depends on a balance between two processes: heat entering the body and sweat evaporating from it. When the air is cooler than skin, the moving air helps release heat. When the air is hotter, it can do the opposite.
That is why many public-health guidelines place caution around the 35°C mark. The UK government advises using electric fans only when air temperatures stay below that level. The U.S. Centers for Disease Control and Prevention is even more conservative, while the World Health Organization notes that fan use becomes less reliable as temperatures climb.
Humidity Changes the Equation
Humidity can completely change how a fan feels. In dry heat, sweat may evaporate quickly already, so extra airflow may offer little benefit. In humid conditions, however, a fan can help strip away the damp air around the body and support evaporation.
Research published in JAMA and JAMA Network Open found that younger, healthy adults could benefit from fan use in some hot conditions, including higher humidity. But studies involving older adults showed a different pattern: in very hot, dry environments, fans could increase body heat strain rather than reduce it.
Age, Hydration and Skin Wetting Matter
Age plays a major role in heat response. Older adults often sweat less efficiently and may regulate body temperature more slowly. Hydration also matters: when the body is already low on fluids, a fan can increase sweat loss without improving comfort.
One practical way to improve cooling is to combine airflow with water. Wetting the skin, using a damp cloth, or wearing light, moist clothing can boost evaporation and make a fan more effective. This approach is especially useful in humid heat, where sweat alone may not evaporate well.
The key takeaway is simple: a fan is useful in many situations, but not all. Below 35°C it is usually a reasonable option, while above that threshold its value depends on humidity, age, hydration and indoor conditions. In very hot, dry rooms, it is better to look for deeper cooling strategies.
As heatwaves become more common, smarter cooling habits may shape how homes, cities and public-health systems adapt in the years ahead.