We know that heat can change the shape of hair. If someone with pin curls runs a flat iron through their hair — POOF! — their hair becomes straight. If someone with poker straight hair wraps it around a curling iron — BAM! — they have beautiful curls. But what is it about heat that can seemingly change the shape of hair? As it turns out, heat styling is a pretty simple technique that works quite well on a temporary level to change hair’s structure.
What Makes Hair Straight or Curly?
All hair, regardless of shape, has the same basic structure. In the skin is a complex mini-organ known as the hair follicle, which is responsible for producing the hair shaft. The system of the hair follicle, hair shaft, and sebaceous glands is known as the pilosebaceous unit. What we think of as “hair,” the shaft, is essentially made up of a protein called keratin and melanin, along with small amounts of metallic elements (Methods and Findings in Experimental and Clinical Dermatology, International Journal of Trichology).
The hair shaft can be broken into three parts. The outer section of the hair shaft, the cuticle, is protective exterior made of downward pointing scales. The next section, the cortex, is the bulk of what we think of as hair, including melanin granules and ovoid bodies, which give hair shape. The very center section, the medulla, is the central section made of scales (University of South Carolina).
It’s the follicle shape that causes hair to be straight, curly, or somewhere in between. The follicle dictates the shape of the cuticle, and thus the shape of the shaft. So, a circle, which is even on all sides, produces straight hair; while an oval, which is uneven, will produce a curly hair. This, as dermatologist Dr. Paradi Mirmirani explains, is reminiscent of curling ribbon by running scissors down one side to change the shape and make it curl because of the unevenness (NPR).
How Does Heat Change the Shape of Hair?
The keratin of the hair, which is part of the cortex or the part of the shaft that gives out hair its appearance, is held together by hydrogen bonds. These hydrogen bonds are the glue that holds the shape of the hair together, and they can be altered in one of two ways: by water or by heat. When these hydrogen bonds are weakened or broken, they allow the hair to be rearranged before reforming in that shape (Chemical and Physical Behavior of Human Hair).
This is why wet, straight hair can be put in curlers and will dry into curls. It’s also why a flat iron will make curly hair straight and a curling iron will make straight hair curly (Exploratorium, Cornell). But it’s also why the effects of curlers and curling and flat irons are temporary. Those hydrogen bonds that were easily altered to create the new style can be easily altered back to their normal state, usually by water. That’s why a rain or a hot, humid day can ruin carefully crafted heat styling.
Why Does Heat Styling Do Damage?
Heat styling creates cracks in the protective cuticle layer of hair and causes the scale to lift. This makes the hair easier to penetrate and more vulnerable to damage from things like penetration from chemicals and sunlight. Over time, this leads to the degradation of the cortex, which results in dry, lackluster, and otherwise damaged-looking hair (Chemical and Physical Behavior of Human Hair). It also causes hair to become more negatively charged, meaning that it will repel itself and cause more frizz (Spring Handbook of Nanotechnology).
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Heat styling can also cause a condition that sounds innocuous, but is very damaging, known as “bubble hair.” Using tools like blow dryers at high temperatures can cause water in the hair to turn into steam. This creates a bubble in the hair that result in a loss of the cuticle (protective) layer and can cause hair to break (American Academy of Dermatologists).
Using high temperatures to style hair can also increase free radical damage. Researchers found that heating hair to the boiling point of water — 100°C, or 212°F — resulted in more free radicals than UV-irradiation (Life Sciences).
How Can You Protect Hair AND Style?
Contrary to what some shampoo bottle claim, once hair is damaged, it can’t repair itself. Other than letting it grow out (without the damaging offenders), your best bet is to use products that help protect your hair and make it softer, smoother, and shinier. Using oils like Orofluido Beauty Elixir ($15, amazon.com) with argan, cyrpus, and linseed oils can help add moisture back to dry hair. Products with antioxidants will also help mitigate some of the free radical damage.
But to add protection to hair when heat styling — and to protect your style — the best place to look is silicones. Silicones help to keep moisture in, but form a protective layer to keep water from getting in. So they stop some of the drying of heat styling, but also prevent the water absorption that can ruin your style. They also make hair stronger against both dry and wet brushing force, which is particularly important because tugging and stretching when hair is wet and the hydrogen bonds can cause more damage to hair (Journal of the Society of Cosmetic Chemists, HAPPI).
[Read More: Are Silicones in Hair Products Good or Bad?]
But if your hair is already damaged, you might want something that also contains quaternary ammonium compounds (quats). Silicones don’t adhere as well to damaged hair, but quats both stick better, because they’re positively charged (Journal of the Society of Cosmetic Chemists).
The shape of your hair follicles is responsible for the shape of your hair and, subsequently, whether it’s curly or straight. By breaking or weakening the hydrogen bonds between the keratin in the cortex, you can change the shape of the hair, which the hydrogen bonds will maintain when they reform — unless, of course, you get your hair wet. However, heat styling can have some serious consequences for hair, so be sure to keep it healthy and protected with the right products.