Margen
The fan in your bedroom can spend a whole afternoon moving air without bringing the temperature of the room down by a tenth of a degree. It has no cooling circuit, no refrigerant gas, no compressor: it is a motor turning a set of blades. That motor adds heat to the room, and the draught it raises only cools you when it reaches your skin.
You bought it thinking of a small, cheap air conditioner. That is why, before going out, you leave it running with the door shut so the bedroom will be cool when you get back at eight. You walk in, you find air moving, and the first gust feels like proof that it worked. The thermometer on the wall says otherwise: the room is much as you left it, and on top of that it has taken in the heat the machine has produced for eight hours.
All the energy coming in through the plug ends up inside. Part of it turns the blades, part overcomes friction in the motor and part goes into noise, but movement and sound also end up dissipating as heat. A 50-watt pedestal fan left on for eight hours releases 400 watt-hours, the same as an old 50-watt bulb burning for that same stretch. How far the thermometer climbs depends on how well the room is insulated and on where that heat escapes; what never happens is that it goes down.
You feel relief because the machine works on your skin. Around your body you carry a boundary layer, a thin film of air your own skin has warmed and moistened. With still air, that layer behaves like a coat and it costs you more to shed heat. The fan keeps sweeping it away and replacing it with air that is less warm and less humid.
In doing so it speeds up the evaporation of your sweat, which is the real cooler. To turn from liquid into vapour, water needs energy, and it takes it from your skin: that departing energy is the coolness you feel. The fan does not need to manufacture cold air; it is enough for it to carry off the vapour piling up next to your body so that more sweat can evaporate.
Stand in front of it with slightly damp skin and the sensation shifts within seconds; step out of the airstream and you lose almost all of the effect. While you are in front of it, your body sheds heat faster; the moment you move away, the room is exactly the one it was before.
And even so, when you open the door at eight, the room does feel cooler to you. It is cooler, for about half a minute: you come in sweating from the street, the air hits your face and your arms and evaporation spikes right then. The sensation dies away as soon as you sit down out of the airstream, and it is what has kept the habit of leaving it running for nobody alive for years.
An ordinary hot day and a heatwave are not the same situation. On the ordinary day, a fan does less the more humid the air is, because sweat struggles to evaporate. In extreme heat it flips round: the trials that have measured both find harm when the air is very hot and very dry, because it blows air hotter than your skin at you and speeds up a dehydration that no longer pays off, and they still find benefit when the heat comes with high humidity. Even so, in older people, who sweat little and regulate their temperature less well, the margin is narrower.
The two bodies that give the practical rule do not give the same number. The Centers for Disease Control and Prevention, the United States public health agency, say to use a fan only with the house below 32 degrees. The World Health Organization sets the limit at 40. What they agree on is what comes next: during a heatwave a fan is no substitute for finding a cool place or wetting your skin, and if dizziness, confusion or severe weakness appear, turning it up solves nothing.
So, tonight: open the windows when the street is cooler than the room and set the fan pushing air out through one of them; close up and lower the blinds before the sun hits the front of the building; and when you sit down or lie down, point it at yourself, at the lowest speed you can feel, with damp skin.