Why Does a Futon Get Fluffy When You Air It in the Sun?
― The Mites Aren't Dying in the Sun
Hang a futon out on a clear day and it comes back in looking wonderfully puffed up. People often say "sunlight kills the mites" or "the sun's power revives the fibres", but neither is the main story. What is leaving the futon is the water you soaked into it the night before.
In the morning, you lift the covers and find that only the part your body lay on is pressed flat. You pat it with your hands, but it barely springs back.
You carry it out to the balcony, leave it for a few hours, then bring it back in. The moment you pick it up, it feels lighter. The thickness is back too.
It is the same futon, yet it is so different before and after. What did the sun do to it?
Two things decide how puffy it is
A person is said to sweat about a glassful overnight. Much of it moves into the futon and settles in the gaps of the cotton or down. Water is heavy, and it sticks the fibres together and stops them from puffing up.
The thickness of a futon is not the fibres themselves but the air the fibres hold. Once the water is gone, the fibres can bend freely and spring back to their original shape.
So the fluffiness is not something sunlight newly gives the futon. The weight of the water is lifted, and the futon simply returns to the shape it always had. What about the familiar claim that sunlight wipes out mites? We will look at that in detail later.
The puffiness is air, not fibre
Most of the volume of any duvet or mattress is air. Cotton and down are made of fine fibres that tangle together to form gaps, and those gaps trap air. Because this air stays still, heat escapes slowly and you feel warm.
Once water soaks in, things change. Water creeps into the narrow gaps and pulls the fibres together by surface tension. It is the same thing that makes wet hair lie in clumps, and it happens inside the futon. Look at the left of Figure 1. The fibres are squashed and the air pockets are small.
Put the futon in the sun and its temperature rises. The warmed water turns to gas and leaves. The force pulling the fibres together disappears, and the fibres stand back up, as on the right of Figure 1. This is the moment the thickness returns.
"Sunlight kills mites": how true is it?
The mites living in futons are mostly a group called house dust mites. They eat human dander and skin debris, and leave behind substances that cause itching and sneezing. These mites are said to die when kept above roughly 50°C for a certain time.
The question is how deep into the futon that temperature reaches. Even on a clear day, only the surface facing the sun gets that hot. The inside and the underside often rise only a little above the outside air temperature.
And mites can move. They simply go from the hot side to the cooler underside or deeper inside. As Figure 2 shows, the futon always keeps some "escape route". That is why airing in the sun does not greatly cut the number of mites.
That does not mean it does nothing. Mites are said to multiply well in damp places. Drying out the water makes it harder for them to thrive. Think of it as making the futon a less comfortable home, rather than killing them directly, and you are closer to the truth.
Some people air a futon under a large black bag. The black surface soaks up sunlight and gets hot, and the air inside the bag is trapped. The heat is more likely to reach the inside, not just the surface, so water leaves more easily than with plain airing.
Beating it hard breaks the fibres into fine dust that stays in the futon. Mite droppings and dead bodies are also crushed into fine particles and thrown into the air. Brushing the surface lightly and going over it with a vacuum cleaner is said to make more sense.
Summary
A futon's fluffiness is not something the sun newly creates. The water soaked in overnight leaves, and the fibres regain their natural spring. Sunlight's job is to drive that water out. As for mites, it is closer to the truth to say the sun does not kill them so much as turn the futon into a dry place where they do not thrive.
What leaves in the sun is a glassful of water.
What puffs up is not the fibres but the air that has come back.
The way water carries away a great deal of heat as it turns to gas is covered in more detail in Why do dogs stick out their tongues and pant? The way fibres change shape as they dry is the same kind of phenomenon as in Why do pinecones close when wet and open when dry?
- Pick one portable piece of bedding, such as a pillow or cushion, and weigh it first thing in the morning. A kitchen scale is fine.
- Air it in the sun for 2 to 3 hours on a clear day and weigh it again right after bringing it in. If it is a few tens of grams lighter, that is the water that left.
- Hang the same item in a shaded, well-ventilated spot and compare how much weight it loses. It takes longer than in the sun, but it does still lose weight.
If it loses weight even in the shade, then what matters is not "sunlight itself" but "conditions that let water escape". On rainy or very humid days, it hardly loses any.
For those who want to know more ― terms, formulas and links to textbooksFrom middle-school science to university specialist courses, each part is marked with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics / Basic Chemistry"
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (heat transfer engineering, transport phenomena)
- ResearchNot taught as settled fact even at university; researchers are still investigating it
Middle schoolTerms: this phenomenon has a name
- Evaporation: a liquid turning into gas from its surface even when it has not reached its boiling point. This is what removes the water from a futon.
- Latent heat of vaporisation: the heat needed to turn a liquid into gas at the same temperature. It is heat that disappears without raising the thermometer reading.
- Thermal conductivity: a measure of how easily heat is conducted. Still air has a small value, which is the source of a futon's warmth.
- Capillary force: the force by which water enters narrow gaps and pulls nearby objects together. It is why wet fibres clump.
Middle schoolHigh schoolChecking with a formula: how many minutes of sunshine to evaporate one night's water?
The main character is water. So let us work out how many minutes of sunshine it takes to turn all the water soaked into the futon into gas. What we want is the time itself.
| In symbols | t ＝ (m × L) ÷ (S × A) |
| In words | Time needed ＝ (weight of water × heat needed to evaporate it) ÷ (sunlight intensity × area receiving it) |
| Where it comes from | Conservation of energy. It sets the total heat received from above equal to the heat needed to turn the water into gas |
| t | Time needed. Unit: seconds |
| m | Weight of water soaked into the futon. Unit: kilograms |
| L | Heat needed to turn 1 kilogram of water into gas. Unit: kilojoules per kilogram |
| S | Sunlight intensity. Heat received by 1 square metre in 1 second; unit: watts |
| A | Area of the futon facing the sun. Unit: square metres |
| Water moving into the futon overnight (m) | About 0.2 kilograms (said to be about a glassful) |
| Latent heat of vaporisation of water (L) | About 2400 kilojoules per kilogram (near 30℃) |
| Sunlight at midday on a clear day (S) | About 700 watts per square metre |
| Area of futon facing the sun (A) | About 2 square metres |
| Heat needed to turn the water into gas | 0.2 × 2400 ＝ 480 (kilojoules) |
| Convert to joules | 480 × 1000 ＝ 480000 (joules) |
| Heat the futon receives per second | 700 × 2 ＝ 1400 (watts) |
| Time needed | 480000 ÷ 1400 ≒ 343 (seconds) |
| In minutes | 343 ÷ 60 ≒ 5.7 (minutes) |
Going by the amount of heat alone, about 6 minutes would be enough. Yet real airing in the sun takes 2 to 3 hours. What is lacking is not heat. It takes time for the water vapour to travel from deep inside the futon to the surface. Much of the sunlight received warms the futon and the air, or simply escapes to the outside. What decides the fluffiness is not the strength of the sun so much as the "ventilation" and the "dryness of the air".
High schoolHigh school+The temperature doesn't rise, so where did the heat go?
High schoolHeat that warms an object is called sensible heat, and heat that only changes its state without moving the temperature is called latent heat. What the water in the futon takes in is mainly latent heat. That is why the futon's temperature does not rise as much as you might expect while it is airing. The value of 2400 kilojoules per kilogram used in the calculation above is this latent heat.
High school+How much water can leave depends on the state of the air. Air at a given temperature can hold only so much water vapour, and this upper limit is called the saturation vapour pressure. The equation linking temperature and saturation vapour pressure is known as the Clausius–Clapeyron equation. The difference between the amount of water vapour next to the futon and that in the surrounding air sets the speed of evaporation. On a humid day the futon does not dry because this difference is small.
UniversityDrying splits into two periods
The drying of powders, foods and fibres is treated in chemical engineering under transport phenomena. While the surface is wet enough, the amount of water leaving per unit time stays nearly constant. This is called the constant-rate drying period. Once water no longer reaches the surface, the rate drops and the falling-rate drying period begins. When airing a futon for a long time, the effect fades toward the end because it has entered this second stage. Here heat transfer and mass transfer proceed together, and a dimensionless quantity called the Lewis number, the ratio of the two, is used.
📖 Derivations and further reading: Heat of vaporisation (latent heat of evaporation)
ResearchWhat is still not clear
- How much of the water actually leaves a household futon. There are too many combinations of material, thickness and airing time, and there are still few measurements that can be offered as a shared guideline.
- The details of how mites avoid heat. Reports are said to vary by species and dampness on how big a temperature difference makes them move, and how fast.
- How far allergen levels can be reduced. Reducing the number of mites and reducing the substances that cause symptoms are separate problems, and the effect of measures people can take at home is still being studied.
In other words, this article too is "an explanation within what is known now". The contents of a futon differ in character by material, so please read the numbers as rough guides.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: changes of state and heat | Water taking heat away as it turns to gas |
| High school | Basic Physics: heat quantity and latent heat | The whole calculation in "Checking with a formula" |
| High school+ | Basic Earth Science: saturation vapour pressure | Why it won't dry when humidity is high |
| University | Chemical engineering: transport phenomena | Constant-rate and falling-rate drying periods |
| Research | Housing environment science, allergology | Testing how well anti-mite measures work |
| ― | Links to daily life | Choosing when to air, handling without beating |
- Heat of vaporisation (Japanese Wikipedia) ― definition of latent heat of vaporisation and the value for water
- Reports on moisture absorption and release of bedding, including in the Journal of Japan Society of Home Economics (日本家政学会誌)
- Explanatory material on "living-environment measures for allergic diseases" from the Ministry of Health, Labour and Welfare (厚生労働省)
- The Society of Chemical Engineers, Japan (化学工学会), ed., Handbook of Chemical Engineering (『化学工学便覧』), chapter on drying (the constant-rate / falling-rate distinction)
*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. For measures related to health, such as allergies, please follow the advice of a doctor or specialist body according to your symptoms.