Woman straightening her hair with a flat iron

Low Heat Styling: Gentle or Damaging?

Heat styling has long been an accessible method to achieve a new hair style, whether it’s a carefully curled look or a glossy straight one.

Unfortunately, subjecting hair to the high temperatures of blow drying, curling, or flat ironing can have disastrous effects on its health and beauty. This is especially true for delicate, curly hair with its tendency to be particularly vulnerable to structural damage and breakage.

For this reason, many naturally curly haired people avoid using heat on their hair at all and rely upon air drying, scrunching or pineappling, and strategic placement of clips to impart body and shape to their tresses. However, the occasional yearning for a temporarily smooth, flat hairdo is felt by many, and the allure of the flat iron is ever present.

Low Heat Styling Tools

Many styling tools offer options to operate the equipment at lower temperatures. Recently, one company (CoolWay™, The Low heat Revolution“> has been marketing a low heat flat iron styling system as a way to safely achieve straight tresses without all the damage.  Among other things, their appealing claims boast that their system reduces drying time, increases hair strength by 300%, reduces breakage by 75%, and reduces frizz by 50%.

But are lower temperatures truly safer for your hair, and if so, what is the temperature threshold for safe usage?  Answers to that question can be found in a deeper understanding of how thermal damage occurs, the role water plays, and finally the mechanism and efficacy of thermal protection serums in its prevention.

What does heat do to hair?

Breaks hydrogen bonds

Application of heat to the hair breaks hydrogen bonds, and the use of tension or pressure allows hair to be re-shaped so that the new hydrogen bonds form to support the new shape (straight or curled”>.  These bonds generally remain in place until the hair is washed again or until they slowly revert to their preferred conformation, which means hair straightened via flat ironing can be silky and straight for a few days at a time.

Unfortunately, the temperatures required to achieve this effect can cause permanent structural damage to hair, especially when coupled with the relatively high mechanical forces used to change the shape of the hair. Flat irons are the ultimate players in the heat styling field, with temperatures easily reaching 350°F and even approaching (or surpassing”> 400°F.

Structural defects

Hair exposed to the extreme conditions of flat irons has been observed via optical microscopy and scanning electron microscopy and both radial (outward from the shaft”> and axial (along the length of the strand”> cracking have been observed on hair strands, as well as fusion of cuticle scales.  These structural defects become weaknesses that can result in frizz, tangling, and ultimately breakage.  Loss of curl pattern is also a common effect of repeat exposure to this method of straightening.

Bubble hair

Perhaps even more disconcerting has been the presentation of what has been labeled “bubble hair” in the industry, where hair exposed to the high temperatures of flat iron straightening develops voids and bubbles along its length. Hair is naturally filled with miniscule voids that hold air, which can also absorb and retain water.

When the local temperature of this water exceeds its boiling point, which can happen very easily with exposure to high temperatures in heat styling and blow drying, it can vaporize quickly and boil out of the hair, expanding the voids to form large, vacant bubbles in the structure of the hair. These bubbles give the hair strand an irregular, knobby shape, which is not only unattractive, but also creates stress concentration sites and multiple sources subsequent of breakage.

For this reason, it is extremely important to never flat iron hair that is not absolutely dry. Any residual dampness from washing puts the hair at greater danger for extreme, irreversible damage from rapid boiling of the water molecules. Since hair is never 100% free of water (which would be an undesirable state anyway”>, this risk is never completely eliminated.

Tonya McKay

Tonya McKay Becker is a curly-haired polymer scientist and cosmetic chemist whose academic and industrial research experience have provided her with expertise in the fundamentals and applications of polymer science and colloid chemistry. She has long had a fascination with the structure-property relationships of the complex solutions used in hair and skin care products, and how they interact with and impact these remarkable biological substrates. Ever curious, Tonya has dedicated herself for more than a decade to honing her expertise on the science of curly hair, how it differs from straight hair, and how product ingredients used on curly hair affect its health and beauty. Her passion for sharing this knowledge with others has led to her current career of educating people from all backgrounds who share an interest in this exciting field.

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