Introduction
It is a sad but universal truth that humans are most gullible when faced with seemingly unsolvable problems. The terminal ill are willing to try any unscientific wonder cure, and there is an extensive industry taking advantage of their misfortune. The same applies to people who are healthy, but made to believe they are merely ‘still healthy’ and in need of expensive food supplements to avert medical disaster. This market had a size of almost 50,000 million USD in 2025 in Europe alone 1. There are medical conditions which cause vitamin uptake deficiencies and for affected people, an extra intake of the vitamin involved is beneficial. However, the National Institutes of Health declare2: “Some dietary supplements can help you get adequate amounts of essential nutrients if you don’t eat a nutritious variety of foods.” In other words, you only need to take care of yourself by buying supplements if you don’t take care of yourself by eating well.
The increasingly common extreme temperatures have opened up a large market for devices giving relief to uncomfortably hot customers. The scientific solutions to this problem are as follows. For the short term: keep out of the sun, don’t exert yourself, and insulate your home. The natural and very effective body temperature regulation mechanism is greatly helped by increasing air flow and drinking enough. For the long term, it is evident that any action which reduces the growth of the greenhouse effect is beneficial. The intuitive urge to install air conditioners is interesting, in this respect, as it does reduce the problem in the short term (though at a high cost), but it makes it worse on the long term; air conditioning consumes a lot of energy3.
This sets the stage for a circus of sellers of modest devices with immodest claims. There are countless ads and videos on YouTube (which I am not going to reference here; if you haven’t seen a dozen, count yourself lucky) which claim that ‘the industry’ has refused or been unable to solve the problem of the high cost of air conditioning. Luckily, some lone engineer (or even ex-engineer; is it possible to stop being an engineer?) has had either a flash of lightning or a long spell of prototyping and has come up with a unique product, which makes all other products redundant. Often, there is a warning that stocks are running low. A huge discount is nevertheless offered by the seller, who thereby appears altruistic, whilst instead selling garbage for gold (as explained below).
An example from a respected website shows how there is a subtle (?) play with words to keep the operation just within the limits of the law: “Draagbare Airco” (portable air conditioner) in the title, but with a warning in the text: “let op, dit is geen airco” (this is not an airconditioner). I think you must be quite overcome by the heat to order this product which isn’t what it is. It reminds me of ‘La trahison des images’ (“Ceci n’est pas un pipe”) by René Magritte. Art, not physics.
Analysis
As stated above, a flow of air helps the natural cooling mechanism of the skin (transpiration). This works because the air flow removes moist air and replaces it with drier air, so that sweat evaporates more easily (ideally instantly, so that you don’t feel sweaty). This is what a fan does.
The miracle air conditioner that isn’t an air conditioner does this too, in a modest way, but it has an extra trick, which is just enough to substantiate the claims in the documentation, but not nearly enough to justify the marketing suggestions. The trick is that it can be used to evaporate a small amount of water. The heat needed for the evaporation is taken from the air in the room. This means the energy in the room remains constant, but the temperature is lowered. At the same time, the humidity of the air is increased.
Let’s calculate the maximum effect of this process. Assume 0.15 litre of water is evaporated in a room of 4 x 4 x 2.5 m or 40 m3. The initial temperature is 27 °C, and the initial relative humidity is 41 %. The atmospheric pressure was 1007,7 hPa (these values occurred outside in the Netherlands on 25 July 2026; the date is not important).
The air in the room, at the start, contains 0.42 kg of water4. For this analysis, we assume that the water in the cooler is instantly evaporated and mixed with the air in the room (the marketing videos claim this takes 90 s; how 40 m3 can be treated in 90 s with the small power available, is another question). The temperature reducing effect works because each litre of water needs 2.26 MJ to evaporate, and this energy is extracted from the air in the room, which has a mass of 47 kg (40 m3 , with a density (at 27 °C) of 1.17 kg/m3). The specific heat of air is 1000 J/(kg K), so the evaporation of 0.15 litre of water cools 40 m3 of air by 0.15*2.26*106/(47*1000) = 7.2 °C. and the new temperature is therefore 19,8 °C. By the uptake of 0.15 litre of water, the air has become a lot more humid (0.42 to 0.57 kg), but the lower temperature means the relative humidity has increased more: from 41% to 86 %5. The same increase cannot be made again (if RH exceeds 100%, water vapour condensates), so much further cooling by evaporation is not possible (in particular as the vapour pressure of water reduces further as temperature drops. This means that this cooling trick can only be repeated after the temperature has increased, or the air is replaced by drier (hot?) air.
Evaporation is a process that can extract heat from the dry portion of the air, lowering the temperature, but this is not a continuous process. The room with the cooled air will attract heat from its surroundings (heat flows to the lower temperature level), and the low temperature cannot be maintained by evaporating more water. It may be cool for a time after the first 90 seconds, but then the temperature will start to rise. In fact, the heat will start to seep into the room even in the first 90 seconds, so the temperature may never reach the low level calculated in the optimistic approach. The device does not extract heat, as an air conditioner does, but it humidifies the air, and blows it around the room. Initially, it may lead to a lower temperature, but this is a temporary relief.
An important question remains: what is more comfortable, a lower temperature or a lower humidity? The airco that isn’t an airco does lower the temperature, at least initially, but soon becomes a simple fan.
Does this make the device useless? Perhaps not, but it certainly isn’t the miraculous solution the industry doesn’t want you to know about. For what it does, there are better products available, which do not claim to be what they are not.
If temperatures are high, keep your head cool, and do not fall for slick talk promising breakthroughs. They may not actually be lies, but they certainly suggest more than they can deliver.
1 Europe Dietary Supplements Market Size Report, 2033
2 Dietary Supplements: What You Need to Know - Consumer
3 in certain climate zones, it may be all but indispensable, just like heating in winter, and in combination with good insulation, the cost can be acceptable.
4 at 27 °C, the vapour pressure of water is 3568 Pa. 41 % of this is 1463 Pa, which is 1.45% of the air pressure (1007,7 hPa). In a room of 40 m3, this is 580 litres of water vapour, which amounts to 0.58*100770/8.314/300.18 moles of water (n=P*V/(R*T), or 23.4 mol. Water has a mass of 18.02 g/mol, so the air in the room contains 0.42 kg of water.
5 at 19.8 °C, the vapour pressure of water is 2310 Pa. Using the results from the previous footnote, the RH is now 0.57/0.42*3568/2310*41% = 86% (Note: this one-step calculation is approximate; a more accurate calculation is more complicated, but these values are accurate enough for now).
