In this blog post, we will take a closer look at the structure of hair, the scientific principles behind hair dyeing, the effects of hair dye ingredients on hair, and precautions to take when dyeing your hair.
The history of life began in the oceans and entered a new phase when it moved onto land. In order to survive in an environment different from that of the water, living organisms underwent evolution, and in the process, their skin gradually became harder and hair began to grow. Some organisms’ hair became long and thick to focus on insulation, while others’ hair became light and smooth to facilitate gliding.
However, humans began to lose their hair as they evolved larger brains, which required efficient heat dissipation. Most body hair disappeared, except for a few important areas. The hair remaining on the head evolved to serve functions such as protecting the brain, regulating body temperature, and conveying individual characteristics and messages. In modern society, this meaning has become even more prominent. For example, women or students shaving their heads may indicate personal resolve or determination, while the bright yellow or pink hair of punk subcultures reflects their anti-establishment tendencies. People do not hesitate to invest time and money in hairstyles as a means of self-expression.
The importance of hair as a means of expression can also be seen throughout history. In ancient Egypt, royalty and nobility wore different wigs to indicate their status, and in medieval Europe, hair styles were used to express social status. Hair has transcended its role as a mere part of the body to become an important element revealing social and cultural identity. In modern times, with the development of the fashion industry, various hair styling methods such as dyeing, perming, and cutting have been developed, making hair a major means of expressing individuality and creativity.
So, what characteristics of hair make these activities possible? And what principles are used in dyeing? First, let’s understand the structure of hair.
Hair is broadly divided into three parts. These are the medulla (innermost part), cortex (middle part), and cuticle (outermost part). First, let’s talk about the medulla. The medulla is the central part of the hair, consisting of a structure of polygonal cells filled with cavities arranged in the direction of the hair’s length. These cavities contain air, which helps insulate the hair. The more air they contain, the shinier the hair appears. However, this structure is not present in all hairs. It is typically found in hairs with a diameter of 0.09 mm or more, but is barely visible in hairs with a diameter of 0.07 mm or less.
Next is the cuticle. The cuticle is the outermost part of the hair, accounting for approximately 10–15% of the hair. It is composed of keratin, a type of protein, and has a structure where 5–15 layers of transparent, thin cells overlap like roof tiles. It has strong chemical resistance and protects the inner part of the hair. If the cuticle is damaged, it is difficult to regenerate and can affect the cortex and medulla, leading to hair breakage.
Finally, let’s look at the cortex. The cortex is the part between the medulla and the cuticle, accounting for 85–90% of the hair. The medulla is hollow, and the cuticle is composed of transparent, thin cells, so it does not affect the color of the hair. However, the cortex is composed of cortex cells (keratin protein) and intercellular binding substances, and it contains a small amount of melanin pigment in the cortex cells, which determines the natural color of the hair. Melanin pigment is primarily divided into two types. One is eumelanin, which primarily produces colors ranging from brown to black. The other is pheomelanin, which produces red or yellow colors. The ratio of these two pigments determines the hair color, ranging from black to brown to gold. In addition to hair color, the medulla and cortex also determine the overall characteristics of hair, such as elasticity, strength, and texture, making them the core components of hair.
In summary, hair is composed of the medulla, cortex, and cuticle. Among these, the medulla and cuticle primarily serve to anchor the hair shaft and protect it from external factors, but they do not significantly influence hair color or other characteristics. These characteristics are determined by the components of the cortex, which constitutes the majority of the hair shaft. Chemical treatments are performed by destroying or altering these components. Let us now explore the principle behind hair dyeing based on this understanding.
There are three main types of hair dyeing. The first is temporary dyeing, where dye is applied to the surface of the cuticle to temporarily change the color. This type of dyeing uses products like hair color spray or color mousse, and the effect disappears immediately when washed with water. The second is semi-permanent dyeing, where dye is slightly absorbed into the cortex and forms a thin layer on the surface of the cuticle to create color. Common examples include hair coating, manicure, and waxing. These methods do not significantly affect the hair cortex, so they do not cause hair damage. However, since they only coat the surface of the cuticle, the color fades with each wash, and the hair returns to its original color within about 1-2 months.
Finally, there is permanent dyeing, which destroys melanin pigments and fixes the desired color in the hair shaft. Unlike the previous two types of dyeing, this method results in permanent color as long as the hair is not cut. However, since it directly affects the hair shaft, there is a risk of hair damage. Three main substances are used in permanent dyeing: ammonia, hydrogen peroxide, and dye pigments. The cuticle is composed of keratin protein, a highly stable protein, making it resistant to chemicals. To deposit pigment into the cortex, the cuticle must be removed or lifted to allow the pigment to penetrate. Ammonia plays the role of lifting the 5–15 layers of thin cells that make up the cuticle. This allows hydrogen peroxide and dye, which must act inside the hair shaft, to penetrate effectively. Hydrogen peroxide destroys melanin pigment within the hair. Since melanin, the pigment responsible for hair color, remains in the hair, it is difficult to achieve the desired color and for dye to penetrate. Therefore, melanin must be destroyed. Once the melanin pigment is destroyed and the desired dye settles in its place, the dyeing process is complete. The newly settled dye settles into the hair cortex (where the dye molecules undergo a chemical reaction to form larger pigment molecules), and after using a hair treatment, the raised cuticle flakes settle back down, preventing the dye from fading easily. This is why the color does not fade even after washing and remains permanent.
There are important considerations to keep in mind before proceeding with this type of dyeing. These pertain to the hair dye used during the process. Synthetic hair dyes, which are generally widely used, are known for their excellent color development and fixation. Synthetic hair dyes are categorized into oxidative dyes, direct dyes, metallic dyes, and natural dyes based on their dye form. Direct dyes and metallic dyes are used for temporary or semi-permanent coloring. They do not penetrate deeply into the hair or contain ingredients that are sensitive to the skin, so there are no major precautions except for thoroughly rinsing them out after use. However, oxidative dyes used for permanent coloring contain substances that can cause allergic reactions. These include diaminoderivatives (p-phenylenediamine (PPD), toluene-2,5-diaminesulfate (TDS)), 2-methyl-5-hydroxy ethylaminophenol (MHEAP), N, N’-bis(2-hydroxyethyl)-p-phenylenediaminesulfate (2HEPPD), p-toluenediamine (PTD), among others. Among these, p-phenylenediamine is particularly allergenic, potentially causing allergic contact dermatitis, scalp disorders, and hair loss. Therefore, it is essential to perform a patch test before dyeing to select a suitable hair dye for each individual.
Finally, proper care is necessary after dyeing to protect the hair. After dyeing, hair becomes dry and prone to damage, so it is important to use hair masks or treatments regularly to maintain moisture. Additionally, using hair products with UV protection to shield hair from sunlight is crucial. Dyed hair is more sensitive and prone to damage than natural hair, so it is advisable to use a soft brush and avoid high-temperature styling tools.
We have now looked at the structure of hair and how it is dyed. Although the ratio of these components varies from person to person, hair is mainly composed of 10-15% cuticle, 85-90% cortex, and a small amount of medulla. The color, thickness, and shape of hair are determined by the ratio of substances contained in the cortex. Among these, color is determined by the ratio of melanin pigments, and to dye hair, it is necessary to destroy these melanin pigments. Therefore, hair dye contains hydrogen peroxide, and ammonia is used to help the hydrogen peroxide and dye penetrate the cortex. However, ammonia is a strong alkaline substance that can have adverse effects on hair and scalp. Additionally, the hair dyes commonly used in dyeing are synthetic organic compounds, which can cause allergic reactions in people with sensitive skin. Therefore, it is essential to perform an allergy test before dyeing and ensure that no dye residue remains after dyeing.
