Fans don’t always cool you downScott Barbour/Getty Images
With Europe in the grip of a mega heatwave and deaths already being reported, staying cool isn’t just about comfort – for many, it is a matter of life and death. Many people will be relying on fans rather than air conditioning, so it is important to be aware that, as it gets hotter, fans can flip from cooling you to warming you.
But how hot is too hot? The UK government warns fans may not prevent heat-related illnesses above 35°C (95°F), whereas the World Health Organization says the tipping point from cooling to heating occurs at 40°C (104°F). Some studies suggest even lower or higher values. The reality is that there is no simple answer, as several factors are involved.
The key thing to know is that you become less good at sweating as you age, so fans will start to warm you at a lower temperature than is the case for younger people. However, you can compensate for this by spraying yourself with water or wetting your clothes.
The situation would be very simple if we didn’t sweat. Our skin temperature in the shade is typically between 35°C and 37°C (99°F). If the air temperature is lower than our skin temperature, heat is transferred to the air, cooling us, but if it is higher, we gain heat from the air. Moving air over our skin with a fan speeds up this heat transfer, whichever way it is going.
