With the heat wave hitting France and Europe, staying cool is a question of survival. Traditionally resistant to air conditioning, France generally relies on fans to cool itself, but be careful, beyond a certain temperature threshold, the device can in fact become your enemy, explains New Scientist.
What is this threshold exactly? That’s the whole problem. The UK government warns that fans no longer prevent heat-related illness above 35°C. The World Health Organization places the tipping point at 40°C. In truth, several factors come into play.
To understand why, we must start from a simple principle: our skin, in the shade, is naturally at a temperature between 35 and 37°C. As long as the surrounding air is cooler than our skin, heat dissipates naturally.
A fan accelerates this phenomenon, in one direction or the other. This is why the 35°C mark has long been presented as the limit beyond which the fan is no longer useful, explains George Havenith, researcher at Loughborough University in the United Kingdom. But this figure does not take into account an essential mechanism: sweating.
It all depends on the humidity level
When we sweat, the evaporation of sweat carries away a large amount of heat, cooling us even when the air temperature exceeds that of our skin. But the efficiency of a fan strongly depends on the humidity level.
In very dry weather, sweat evaporates as quickly as it is produced. Activating a fan then no longer helps: the hot air blown on the skin only makes the situation worse. Modeling shows that at 15% humidity and 45°C, a fan will definitely warm you up. Conversely, when the humidity is high and you are dripping with sweat, the fan becomes useful again: it can still cool you down to 38°C with a humidity level of 60%. Beyond that, even this mechanism reaches its limits.
The other big variable is age. As we age, sweating starts later and becomes less abundant. The threshold at which a fan heats more than it cools is therefore lower in older people. To compensate, wetting your clothes or spritzing your body with water regularly can help.
The conclusion is clear: if you live in a home that accumulates heat, the interior temperature could well exceed the fan’s efficiency threshold during a heatwave, even if you splash it with water. “At this point, you will probably need to leave your home and find somewhere coolerwarns George Havenith. Because the situation is getting really dangerous.”
