We think that the fan saves us from the heat, but above 35 degrees it achieves exactly the opposite effect.

With the arrival of high temperatures, the fan becomes a great ally in many homes that cannot have the air conditioning on all day long. When you turn it on, it blows the air into your face and you feel almost immediate relief from the heat, but the reality is that we do not have this effect at any temperature, since there is a point beyond which it not only stops being useful, but can become our worst enemy.

It doesn’t cool the air. To understand why a fan can be dangerous, you first have to understand how it works. Just like point out the guidelines of the According to the WHO on indoor temperatures, fans do not lower the temperature of the room even a single degree, since all they do is move the air.

The relief we feel is due to a purely physiological mechanism which is the evaporation of sweat. By moving the air around us, the fan accelerates the evaporation of moisture from our skin, which “steals” heat from our body and cools us down. But the problem comes when the air in the room is hotter than our own body.

The critical point. The normal temperature of human skin is around 35ºC and thermoregulation systems try to keep it that way. The problem arises when the ambient temperature of the room exceeds that figure, since the fan will begin to move air that is hotter than the temperature of our skin, and instead of dissipating body heat, it will be injecting heat from the environment directly.

And it is something that is supported, for example, by the Basque Government’s Heat Plan, which point that fans stop being effective and useful when the air exceeds 35-36 °C. But in the United States they are even more demanding, since the CDC advise against its use above 32.2 ºC.

Beyond temperature. Although the general rule says that above 35 ° C you should turn off the fan, science suggests that humidity matters as much or more than temperature. Thus, in dry heat conditions the fan is harmful, since if the air is hot and dry, the sweat evaporates on its own without needing help from the fan. But turning on the fan only pushes hot air against the skin.

In the case of being in a humid environment, sweat does not evaporate as well, and that is why we feel ‘sticky’, especially in a coastal environment. In this scenario, the fan does help to break up that layer of humidity, and the Sydney studies showed that they can continue to be beneficial even up to 42°C.

The most vulnerable. Despite these data, the highest authority in medical literature review, Cochrane, posted a review in which it points out that there is no high-quality evidence that unequivocally demonstrates that fans reduce the adverse impacts on heat waves. The reason is precisely in the mix of factors such as humidity, age and, above all, the state of health with which we face these high temperatures.

But we must focus above all on age, which is a critical factor, since the body’s ability to sweat and thermoregulate decreases with age or with certain medications. This is why a fan can save a young person from trouble, but it can dehydrate and cause heat stroke in an older person under the same conditions.

Images | Dương Nhân

In Xataka | What science says about the “trick” of two crossed fans to cool the house: it works, but with limits