As Europe battles a severe heat wave, keeping cool is critical for survival. Many individuals rely on fans instead of air conditioning, but it’s essential to recognize that at extreme temperatures, fans can transition from cooling to warming your body.

How hot is too hot? The British government cautions that fans may not prevent heat-related illnesses above 35°C (95°F), while the World Health Organization states that the shift from cooling to heating occurs at 40°C (104°F). Some studies suggest varying thresholds, but various factors complicate the situation.

Importantly, as you age, your ability to sweat diminishes, causing fans to become less effective at lower temperatures than they do for younger individuals. You can counteract this by spraying yourself with water or dampening your clothes.

In the shade, typical skin temperature ranges from 35°C to 37°C (99°F). When the air temperature is lower than this, heat is dissipated into the air, promoting cooling. Conversely, when the air temperature exceeds skin temperature, heat is absorbed from the air. Using a fan accelerates this heat transfer.

Historically, it was believed that 35°C was the temperature where fans ceased their cooling effects, according to George Havenith, Loughborough University. However, this figure overlooks the benefits of evaporative cooling.

When water evaporates, it efficiently removes heat from the skin, helping keep you cool even when the air temperature is higher than your skin temperature. However, in overly dry or humid conditions, a fan may not effectively cool your skin.

In dry heat, sweat evaporates quickly, and a fan’s airflow can actually transfer more heat to your skin. Research indicates that at 15% humidity and 45°C, turning on a fan may increase discomfort.

Conversely, elevated humidity can slow evaporation, which affects the cooling efficiency of a fan. When sweat drips, it indicates that it’s coming out faster than it can evaporate, making a fan useful in such conditions, up to around 60% humidity and 38°C.

As humidity levels rise further, evaporation slows down, diminishing the fan’s effectiveness.

“Humidity plays a significant role,” Hebnis explains. “That’s why we often analyze the climate in various regions.”

Your age remains a critical factor, too. Aging decreases both body and skin temperatures, leading to delayed sweating and diminished output. Thus, fans may begin to warm you at lower temperatures if you’re older.

Wearing damp clothing or using a spray can mitigate these challenges by promoting sweating without increasing dehydration risks.

Many variables, including clothing type and body acclimatization to heat, influence cooling. Ultimately, if you’re in a hot building during a heat wave, temperatures can surpass the threshold at which fans are helpful, even with water sprays.

“At that point, finding a cooler environment outside is likely your best option,” Hebnis warns. “Because situations like this can be dire.”