What’s the ideal frequency for hair dyeing?
From a health perspective, the recommended hair dyeing cycle is 2-3 times a year, aligning with the natural pace of hair growth and the metabolic cycle of dye within the body. Therefore, it’s advisable to wait around four months to six months before dyeing your hair again.
However, it’s crucial to remind everyone that if there are any wounds or scars on the scalp, it’s best to avoid dyeing, as it can easily lead to inflammation. Additionally, women should refrain from dyeing during pregnancy and menstruation.
Looking at it from a trichological perspective, human hair consists of three layers: the cuticle, cortex, and medulla. Typically, hair dye affects the cortex layer.
As a trichologist, let me be straightforward: from a health standpoint, the recommended hair dyeing cycle is 2-3 times a year, which aligns with the natural hair growth rate and the metabolic cycle of hair dye within the body. Thus, it’s best to wait between four months to six months before the next dyeing session.
However, it’s essential to caution everyone that dyeing should be avoided on a wounded scalp as it can lead to inflammation. Furthermore, women should refrain from dyeing during pregnancy and menstruation.
From a trichological standpoint, human hair consists of three layers: the cuticle, cortex, and medulla. Typically, dyeing affects the cortex layer.
Cuticle Layer: This layer comprises about 15% of the hair structure. It has a scaly appearance, offering protection to the hair against external damage. When exposed to water or alkaline substances, the scales open up, while acidic substances cause them to close. Intense physical or chemical contact can lead to scale loss and porosity. Dyeing requires opening these scales to allow color penetration into the hair, completing the coloring process.
Cortex Layer: Occupying roughly 75% of the hair structure, it’s a crucial component composed of fibrous proteins and natural pigment particles. It contains amino acids, proteins, melanin pigments, trace elements, and nutrients. Most dye acts on the cortex layer, removing melanin pigment from the hair and replacing it with artificial pigments to achieve the desired color.
Medulla Layer: This innermost layer constitutes about 10% of the hair structure. Fine and soft hair usually lacks a medulla layer.
Melanin: Also known as natural pigment, melanin consists of three primary pigment colors. Asians typically have a higher content of blue-red-yellow pigments in their hair. The more natural melanin a person’s hair contains, the darker the color. Conversely, lighter hair contains less natural pigment. Hair with more natural melanin presents challenges during dyeing, as it interferes with artificial pigments, affecting the desired color outcome and reducing the success rate.
To achieve hair coloring, dye first disrupts the three layers’ structure to allow dye penetration. This process breaks down the longitudinal and transverse connections of proteins, along with significant damage to the cortex’s melanin.
Dyeing Principle: It involves opening and acting on the cuticle and cortex layers through chemical substances. When dye and developer undergo a chemical reaction, ammonia in the dye opens the cuticle scales. Oxygen molecules released by the developer then combine with amino acids, causing artificial pigments to bond with natural pigments, resulting in color. The natural melanin in the hair is replaced during this process, and the hair swells several times its size. The deeper the penetration, the more stable the color. Different dyeing agents yield varying effects.
Relationship between Dye Characteristics and Hair: Different dye types yield different results depending on hair condition and nutritional content. Melanin in hair is sensitive to acidity and alkalinity, being more sensitive to alkalinity. With a pH of 10, proteins dissolve. Since the pH during dyeing is around 10, dyeing is more damaging to the hair than perming. Additionally, seawater and swimming pool water are alkaline, leading to hair dryness and yellowing.
Currently, the market categorizes dyeing products based on their performance into four types: temporary, permanent, oxidative semi-permanent, and oxidative permanent.
- Temporary Dye: Sprays or colored mud, also known as one-time dyes. They adhere temporarily to the cuticle layer, mostly water-soluble and disappear after washing, hence termed temporary dyes.
- Semi-Permanent Dye: Uses different developers mixed with dyes, also known as semi-oxidative dyes. Theoretically, the color remains on the hair for 6 to 8 weeks or possibly shorter. Higher developer levels and semi-permanent dyes damage more cuticle scales and melanin, resulting in brighter colors and faster fading. Conversely, lower developer levels and dyes lead to less cuticle scale and melanin damage, slower fading, and longer retention. Generally, lighter dyes reduce melanin damage in the cortex, thereby reducing hair damage.
- Permanent Dye: Common in salons, with a pH of 9-11, they rely on developers for coloring ability. The stronger the coloring ability, the greater the damage to the cuticle scales. Permanent dye includes various colors such as red, yellow, blue, etc. Different combinations yield different shades, adhering to the cortex and termed as alkaline oxidative dye.
- Acidic Dye: Due to different compositions from the other two, they don’t require developer for oxidation. Hence, the coloring mostly stays on the cuticle layer and doesn’t penetrate the hair as alkaline dyes do. Permanent and semi-permanent dyes are alkaline, requiring developers and undergoing chemical reactions during coloring, while acidic dyes are physical and do not harm the hair.
In summary, their damaging effects on hair are as follows: temporary < semi-permanent < oxidative semi-permanent < oxidative permanent.
