How to dry your hair, according to science | Lab Muffin Beauty Science (2024)

What’s the best way to dry your hair, according to science? Is it hair dryers, hair straighteners, towels, or just leaving it to dry naturally?

I have fragile, expensively damaged hair and try a lot of hair products, so I have a special interest in this. Here’s the science of drying your hair.

The video is here, keep scrolling for the text version…

Hair and water

First we need to talk about what happens when hair gets wet. Hair has a complicated structure, with lots of different parts. Most of these like water, especially the inside (the cortex).

When your hair is properly wet, like when you’re washing it, it soaks up water and swells. Undamaged hair can absorb about 1/3 of its own weight in water. Damaged hair has more gaps for water to fill, like a bubbly sponge, so it holds onto even more water.

Water makes hair weaker

The most important impact of water on hair is that it breaks hydrogen bonds.

Your hair is held together by lots of bonds, which give it its strength and shape. Some of these (disulfide and ionic bonds) are pretty permanent, and only really break with chemical treatments.

But hydrogen bonds are temporary – they break and reform all the time.

When your hair is dry, hydrogen bonds stick hair proteins to hair proteins. It’s kind of like they’re holding hands. They’re essentially glued together at a molecular level.

But water can also form hydrogen bonds. So when your hair is wet, your hair proteins start bonding with the water instead of with each other:

These are a bit like you and your dog at a dog park. You have a strong bond, he comes when you call him.

But adding water is like adding a whole bunch of really friendly puppies. You’re both having interactions with all these new puppies, so you and your dog have a temporarily weaker bond.

When these other water/puppies leave the hair/park, you and your dog have a stronger bond again.

The problem: water isn’t attached to anything.The hair proteins are less glued together, so it’s like a ladder with whole bunch of broken rungs (I will never run out of analogies for this).

Your hair is weaker, because there are less hydrogen bonds holding it together. If you’ve ever noticed that your hair is stretchier when it’s wet, this is why.

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Water makes hair rougher

On top of this, the cuticle scales on the surface of your hair (those overlapping roof tiles) don’t absorb water evenly.

When dry, the cuticle scales of the hair are lying down relatively flat and compact:

But when wet, the underside of each scale (endocuticle – yellow) and the glue between the scales (cell membrane complex or CMC – purple) absorbs more water than the resilient top (exocuticle – brown). They expand unevenly, so the cuticle scales stick up, away from the hair:

This means the scales get snagged and chipped more easily.

So water itself isn’t very damaging to hair. But wet hair is more fragile, so any physical trauma will generally cause magnified damage. (There’s one big exception, detangling curly hair – check out this post that explains this.)

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You might’ve heard about “hygral fatigue”, the idea that water itself damages hair – the evidence doesn’t really support this. We’ll revisit this point a bit later.

Drying hair and technique

What this means for wet hair? With any method, technique makes a massive difference.

Towel drying: Rubbing your hair like you’re trying to start a fire is very different from blotting and squeezing it gently – you can imagine the tips of the cuticle scales bashing against each other.

Blow drying:Yanking your hair violently with a round brush is very different from gently spreading out the hair with your fingers and moving it around.

Air drying:Sleeping on wet hair and smooshing it into your pillow with the full weight of your galaxy brain, or jamming your hair between your back and the couch as you watch a stressful movie can be very damaging.

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Hair and heat

The other big thing we need to talk about is heat. Heat can cause a lot of different types of hair damage:

• Proteins in your hair can change shape (denature) or break down, which makes your hair weaker.
• Substances inside your hair can oxidise and change colour, often to yellow. Dyes can also break down and change colour.
• Heat can cause microscopic cracks and cavities in your hair, especially at the surface, and cuticle scales can lift. This makes your hair rougher and duller. On the bright side, it can make your hair seem less oily, because the gaps can hold onto oil (one of the rare upsides to damage!)

Another potential problem is bubble hair. If you heat wet hair above ~90 °C (194 °F), the water inside can evaporate faster than it can escape. It boils explosively and punches its way out, leaving bubbles inside your hair. Your hair is basically popcorn.

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For all of these types of damage, more heat causes more damage. There are also some changes that don’t occur until particular temperatures – damage is particularly severe from around 180 °C (356 °F).

Using a standard hair straightener on dry hair can cause a lot of damage if you aren’t careful. But using one on wet hair is one of the most damaging things you can do – if you hear snaps and crackles, that’s water boiling quickly, and some of that might be turning your hair into Rice Bubbles.

Blow dryers don’t make your hair anywhere near as hot, so you probably don’t need heat protecting products for them. The water evaporating cools down your hair so it doesn’t heat up as much, and air movement also helps hair dry. It’s best to move the dryer around to dry your hair evenly, so you’re not heating up strands that are already dry for no good reason.

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The distance between the dryer and your hair also makes a big difference, as seen in this study. The researchers washed and dried hair samples every day for 30 days with different drying methods. They left some to air dry, and used a hair dryer at different distances. The further away the hair dryer, the lower the temperature, and the longer it took to dry:

• Air drying at 20 °C (room temperature) – takes 2 hours for hair to dry
• Hair dryer at 15 cm distance – hair reaches 47 °C, takes 60 seconds to dry
• Hair dryer at 10 cm distance – hair reaches 61 °C, takes 30 seconds to dry
• Hair dryer at 5 cm distance – hair reaches 95 °C, takes 15 seconds to dry

All of the blow-dried samples had damage to the cuticle scales on the surface (lifting and cracking), with higher heat causing more damage. There were also signs of bubble hair at a distance of 5 cm (hair temperature 95 °C).

These findings agree with other studies. Cuticle cracking likely happens because heat makes the outside of the hair dry and shrink faster than the inside – the inside essentially Hulks out, cracking the outer cuticle.

Bubble hair with hair dryers has largely been reported when they’re held close, so the hair gets really hot – most of the time, it’s curling irons and straighteners that cause Swiss cheese hair.

Does air drying cause more damage?

This study does have a weird finding:the air-dried hair had some bulges in the CMC (the “glue” sticking cuticle scales together). The authors suggest the CMC might be swollen and damaged when it’s wet for too long, and they concluded that using a hair dryer is less damaging than air drying.

This hasn’t been replicated as far as I know, and I don’t think their explanation makes sense.

The study used 20 cm long human hair, which is about 1.5 years of growth. You’d expect all the hair to have been washed and air-dried at least 30 times before they were collected, but only the air-dried sample had bulges. I suspect there’s another explanation – for example, that sample might’ve come from someone with more sun exposure, or something unusual happened to the sample during the study.

So I don’t think this study really supports the idea that air drying is more damaging than blow drying. However, air drying does leave your hair wet and fragile for longer, which can be bad in some situations (sleeping, stressful movies).

(I think I’ve quoted this study before on this, before I knew as much about hair science and before I had the confidence to interpret studies more critically, so I’ve changed my mind. Sorry, I’m human, I get stuff wrong!)

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Other benefits from different methods

Hair damage isn’t the only aspect to think about – sometimes, damage is worth it! Bleach is one of the most damaging things you can do to your hair, but I’ve gone through extreme bleaching from black to white three times, and I’m going to keep doing it. However, it’s important to know the benefits and risks of different drying methods, so you can work out whether it’s “worth it” for you.

Styling

You’ve probably noticed that washing resets your hair’s shape – like hair strength, it’s because of bonds.

Hair is glued into a particular shape by permanent and temporary bonds. When hair gets wet,water comes in and breaks a whole bunch of temporary bonds (mostly hydrogen bonds). If you keep your hair in a particular shape as it’s drying, the temporary bonds that re-form hold your hair in the new shape. (Heat can also break temporary bonds, and cooling allows new ones to form.)

When you wash your hair, temporary bonds break. Permanent bonds pull hair back into its original shape, and that’s what you get when it dries unless you hold it in a different shape. Hair also slowly reverts to its original shape over time without washing, because heat and humidity also break temporary bonds.

Different techniques work best to get particular styles, and some work better when you use both heat and water (e.g. blow drying).

Dandruff

If your scalp is prone to dandruff, it might be a good idea to dry it quickly – which means heat.

One of the main contributors to dandruff is overgrowth of a fungus, Malassezia. It’s naturally on your scalp, but causes issues when there’s too much. Air drying leaves your scalp damp for longer, which is perfect for fungal growth. It’s even worse if the moisture is trapped with a hat or a helmet.

You might want to consider heat drying even if you don’t have dandruff, but your scalp is irritated – fungal overgrowth can cause irritation before any visible dandruff shows up.

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Time, effort, routine

There’s also time and effort, and where things fit into your routine.

Microfibre hair towels and turbans can speed up drying with minimal effort and damage. These are made of a water absorbent material that can form hydrogen bonds. An extrusion process creates small fibres with a much larger surface area.

(The supramolecular/physical chemistry explanation of how these towels work: When you touch the towel to wet hair, the water has two choices – it can stick to your hair, or stick to the towel. If the towel has more hydrogen bonding sites that aren’t already holding onto water, it’ll stick to the towel. Bigger surface area means more exposed hydrogen bonding sites – basically, more hands for water to grab onto. Hydrogen bonds are really underrated – I’m half convinced they’re secretly running the world.)

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Blow drying is more effort and potentially more damaging (depending on technique), but it’s faster. On the scale of things that damage hair, it’s quite a few levels below oxidative chemical treatments like bleach and permanent hair dye.

“Wet-to-dry straighteners” combine drying and styling in the one step. Many of these claim to be less damaging (some even claim no damage), boasting features likelower temperature plates (e.g. 120-140 °C), hot air, and sensors that adjust the temperature in real time. In my opinion, some do seem a lot less damaging than using a regular straightener on wet hair. I don’t think they’re likely to be less damaging than gentle blow drying or air drying, but they’re faster and more convenient – it’s a compromise.

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Takeaways

Air drying, drying hair with low heat, and gentle towel drying are the least damaging ways to dry your hair.

Wet hair is much more prone to mechanical damage. If you’re going to rub your hair against something (pillowcase, couch), it’s probably best to dry it as much as possible first.

As well as damage, you might want to also consider hair style, scalp irritation/dandruff, time, and effort.

My personal compromise:

• I try to wash my hair a bit earlier in the day (before 7 pm). Bleached hair holds onto water better and needs more time to dry properly – if I go to bed with damp hair, it dries rough and bent, and I see more split ends the next morning.
• I blot it with a normal towel, then squeeze and wrap it with an Aquis towel for about 5 minutes (if I leave it longer, my fringe dries in a weird way, and I cannot restyle it for the life of me without properly wetting it again).
• In winter it takes longer for my hair to dry, and dandruff is generally worse in winter for everyone – I don’t get much dandruff anymore, but my scalp does get irritated. I dry the top of my hair, near the scalp with a hair dryer – I’m still using the Panasonic Nanoe dryer (link is for the new model, I talked about the older model in this post), which has a scalp mode that’s just cool enough to not hurt my scalp.
• Then I let my hair air dry. If it’s still damp a few hours before bed, I’ll blow dry it again. The Panasonic Nanoe has really good heat control, and I move it around a lot to dry it evenly and avoid hot spots. I try to do this early so the inside still has lots of time to dry properly. If it still isn’t properly dry at bedtime, I blast it with the blow dryer again.

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References

Lee Y, Kim YD, Hyun HJ, Pi LQ, Jin X, Lee WS. Hair shaft damage from heat and drying time of hair dryer. Ann Dermatol. 2011;23(4):455. doi:10.5021/ad.2011.23.4.455

Gamez-Garcia M. The effects of lipid penetration and removal from subsurface microcavities and cracks at the human cuticle sheath.J Cosmet Sci. 2009;60(2):85-95.

Gamez-Garcia M. The cracking of human hair cuticles by cyclical thermal stresses.J Cosmet Sci. 1998;49(3):141-153.

Gamez-Garcia M. Void and pore formation inside the hair cortex by a denaturation and super-contraction process occurring during hair setting with hot irons,J Cosmet Sci2001,62, 109-120.

Crawford RJ, Robbins CR, Curran J, Chesney K. A hysteresis in heat dried hair. J Soc Cosmet Chem. 1981;32:27-36.

Krasnoff J, Glusac E, Bolognia JL. Bubble hair – a possible explanation for its distribution. Int J Dermatol. 1998;37(5):380-382. doi:10.1046/j.1365-4362.1998.00463.x

Mehregan DR, Sayoc L, Ramos D, Misra‐Higgins I. “Bubble hair,” a report of two cases. Int J Dermatol. 2006;45(11):1319-1320. doi:10.1111/j.1365-4632.2006.02699.x

Albers LN, Maley AM, MacKelfresh JB. Blowing bubbles: Dermoscopy of bubble hair. Int J Trichol. 2017;9(3):122-123. doi:10.4103/ijt.ijt_11_17

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