Why does a futon get so fluffy after airing in the sun?
― Sunlight doesn't kill the mites
Air a futon out on a sunny day, and when you bring it back in, it looks completely different — noticeably puffier. People often say "the sun kills the mites" or "sunlight revives the fibres," but neither is really the main story. What's actually leaving the futon is the water you soaked into it the night before.
In the morning, you fold back the futon and see the spot where your body lay is flattened. Patting it doesn't bring it back.
You hang it on the balcony and leave it for a few hours before bringing it in. The moment you lift it, it feels lighter. The thickness has come back too.
It's the same futon, yet it's so different before and after. What exactly did the sun do to it?
Two things decide how fluffy it gets
A sleeping person is said to sweat roughly a cupful over a night. Much of it moves into the futon and settles between the cotton or down fibres. Water is heavy, and it sticks fibres together, getting in the way of fluffiness.
What actually makes a futon thick isn't the fibres themselves — it's the air they trap. Once the water leaves, the fibres can bend freely again and spring back to their original shape on their own.
In other words, "fluffiness" isn't something the sunlight newly creates. It's simply the futon returning to the shape it already had, once the weight of the water is gone. So what about the well-known claim that "sunlight kills mites"? We'll look at that in detail shortly.
What makes it fluffy is air, not fibre
In both a duvet and a mattress-style futon, most of the volume is air. Cotton and down are made of thin fibres tangled together, forming gaps that trap air. Because that trapped air doesn't move, heat escapes slowly, and the futon feels warm.
Once water soaks in, though, things change. Water gets into the narrow gaps and, through surface tension, pulls the fibres toward each other — the same thing that happens when wet hair clumps into strands. Look at the left side of Figure 1. The fibres have collapsed, and the air pockets have shrunk.
Out in the sun, the futon's temperature rises. The warmed water turns to vapour and leaves. The force pulling the fibres together disappears, and as shown on the right of Figure 1, the fibres spring back up. That's the moment the thickness returns.
How true is "sunlight kills the mites"?
The mites that live in futons are mainly dust mites (Dermatophagoides). They eat skin flakes and dander and leave behind substances that cause itching and sneezing. These mites are generally thought to die when kept above about 50°C for a certain length of time.
The question is how far that temperature actually reaches into the futon. Even on a clear sunny day, only the surface facing the sun gets very hot. The inside and the underside often rise only a little above the outdoor air temperature.
What's more, mites can move. They simply shift away from the hot surface to the cooler underside, or deeper in. As Figure 2 shows, there's always somewhere to escape to inside a futon. That's why sun-drying doesn't greatly reduce the number of mites on its own.
That doesn't mean it's useless, though. Mites are said to multiply well in damp conditions. Airing the futon out and removing the water makes the environment harder for them to thrive in. Think of it less as killing them directly, and more as making the place a worse place to live — that's closer to what's really happening.
There's a method of airing a futon inside a large black bag. The black surface absorbs sunlight well and gets very hot, and the air inside the bag stays trapped too. Because the temperature rises not just at the surface but further inside, water escapes more readily than with plain airing.
Beating it hard breaks the fibres, leaving fine powder inside the futon. Mite droppings and remains also get pulverised and sent airborne. Lightly brushing the surface and vacuuming is thought to make more sense.
Summary
A futon's fluffiness isn't something the sun newly creates. It's simply the water that soaked in overnight leaving, and the fibres regaining their original springiness. Sunlight's job is to drive that water out. As for mites, the more accurate description isn't "killing them" but "turning the futon into a drier place where they struggle to multiply."
What airing removes is about a cupful of water.
What puffs up isn't the fibre — it's the air coming back.
The mechanism by which water carries away a large amount of heat as it turns to gas is covered in more detail in Why do dogs pant with their tongues out?. The way drying changes a fibre's shape belongs to the same family of phenomena as Why do pine cones close when wet and open when dry?.
- Pick a portable piece of bedding, like a pillow or cushion, and weigh it first thing in the morning. A kitchen scale is fine.
- Air it in the sun for 2–3 hours, then weigh it again right after bringing it in. If it's lighter by tens of grams, that's the water that left.
- Air the same item in a shaded, well-ventilated spot and compare how much weight it loses. It takes longer than in direct sun, but the weight still drops.
If it still loses weight in the shade, that tells you what matters isn't "sunlight itself" but "conditions that let the water escape." On rainy or humid days, it barely loses any weight at all.
Want to go deeper? ― Terms, formulas, and textbook linksClearly labelled from middle-school science up to university-level courses
- MSCovered in middle-school science
- HSCovered in high-school "Basic Physics/Chemistry"
- HS+Advanced high-school content, or textbook sidebar material
- UnivNot taught in high school — university-level content (heat transfer, transport phenomena)
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has names
- Evaporation: a liquid turning into gas from its surface, even below boiling point. This is what happens as the futon's water leaves.
- Latent heat of evaporation: the heat needed to turn a liquid into gas at the same temperature. Heat that disappears without raising the thermometer reading.
- Thermal conductivity: a measure of how easily heat travels through something. Still air has a low value here, which is the basis of a futon's warmth.
- Capillary force: the force by which water entering narrow gaps pulls surrounding material toward it. The reason wet fibres clump together.
MSHSCheck it with a formula: how many minutes of sunlight does it take to drive off a night's worth of water?
Water is the main character here. So let's work out "how many minutes of sunlight it takes to turn all the water that 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) ÷ (strength of sunlight × area receiving it) |
| Where it comes from | Conservation of energy. We set 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 | Strength of sunlight. Heat received per square metre per second, in watts |
| A | Area of the futon facing the sun. Unit: square metres |
| Water moved into the futon overnight (m) | About 0.2 kilograms (roughly a cupful) |
| Latent heat of evaporation of water (L) | About 2400 kilojoules per kilogram (around 30°C) |
| Strong midday sunlight 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 to gas | 0.2 × 2400 = 480 (kilojoules) |
| Converting to joules | 480 × 1000 = 480000 (joules) |
| Heat the futon receives per second | 700 × 2 = 1400 (watts) |
| Time needed | 480000 ÷ 1400 ≒ 343 (seconds) |
| Converted to minutes | 343 ÷ 60 ≒ 5.7 (minutes) |
Looking only at the amount of heat, about 6 minutes would be enough. But actual sun-drying takes 2–3 hours. What's lacking isn't heat. It's time for the water vapour to travel from deep inside the futon out to the surface. Much of the sunlight received goes into warming the futon and the surrounding air, or simply escapes outward. It turns out that fluffiness depends less on the strength of the sunlight than on "airflow" and "how dry the surrounding air is."
HSHS+If the temperature doesn't rise much, where does the heat go?
HSHeat that warms an object is called sensible heat; heat that only changes its state without changing its temperature is called latent heat. What the futon's water mostly receives is latent heat. That's why the futon's temperature doesn't rise as much as you'd expect while it airs. The 2400 kilojoules per kilogram used in the calculation above is this latent heat.
HS+How much water can leave depends on the state of the surrounding air. There's an upper limit to how much water vapour air at a given temperature can hold, called the saturation vapour pressure. The relationship between temperature and saturation vapour pressure is known as the Clausius–Clapeyron equation. The difference between the water vapour concentration near the futon and that in the surrounding air determines how fast evaporation proceeds. That's why airing a futon on a humid day doesn't dry it well — the difference is small.
UnivDrying happens in two stages
The process of drying powders, food, or fibres is studied in chemical engineering's transport phenomena. While the surface stays sufficiently wet, the amount of water leaving per unit time stays roughly constant — this is called the constant-rate drying period. Once water stops reaching the surface fast enough, the rate falls off and the process enters the falling-rate drying period. If you notice diminishing returns from airing a futon for a very long time, that's because it has entered this later stage. Here heat transfer and mass transfer proceed together, and their ratio is expressed by a dimensionless quantity called the Lewis number.
📖 For the derivation of the formula and further reading: Heat of evaporation (latent heat of vaporisation)
ResearchWhat's still not fully understood
- What fraction of the water actually leaves a household futon. There are too many combinations of material, thickness, and airing time, so there isn't yet much measurement data offering a common benchmark.
- Exactly how mites behave to avoid heat. How large a temperature difference triggers movement, and how fast they move, is reported to vary by species and moisture conditions.
- How much airing reduces allergens. Reducing the number of mites and reducing the substances that trigger symptoms are two different problems, and how effective household measures actually are remains under study.
So the content of this article, too, reflects "the current state of understanding." Because futon materials vary, treat the figures here as rough guides.
Links to the textbook curriculum (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science ― states of matter and heat | How water absorbs heat as it turns to gas |
| HS | Basic Physics ― heat quantity and latent heat | The whole "check it with a formula" calculation |
| HS+ | Basic Earth Science ― saturation vapour pressure | Why humidity prevents drying |
| Univ | Chemical engineering ― transport phenomena | Constant-rate and falling-rate drying periods |
| Research | Housing/environmental science, allergy research | Verifying how effective mite countermeasures are |
| ― | Everyday connections | Choosing when to air bedding, and handling it without beating |
- Heat of evaporation (Japanese Wikipedia) ― definition of latent heat of evaporation and the value for water
- Journal of the Japan Society of Home Economics (日本家政学会誌) and other reports on moisture absorption and release in bedding
- Explanatory material from the Ministry of Health, Labour and Welfare (厚生労働省) on "Living Environment Measures for Allergic Disease"
- 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 (classification of constant-rate and falling-rate drying)
※This article is a general-audience science explainer. The figures given are approximate, meant to aid understanding of the underlying mechanism. For health-related matters such as allergies, please follow the advice of a doctor or specialist appropriate to your symptoms.