The Role of pH in Hair and Scalp Health

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Scalp pH:

Healthy scalp skin has a pH of 5.4-5.9 and a resident bacterial population. The use of hair care products with a high pH increases the pH of the scalp skin, causing irritation, dehydration, and an alteration of the healthy bacterial flora, which can contribute towards the pathogenesis of skin diseases. The use of pH appropriate hair care products is essential to the prevention and treatment of these conditions and has a cosmetic effect on the hair itself.

One of the most significant differences between the composition of the skin on the face and the scalp is the number of sebaceous glands and sweat glands. The scalp has a less efficient barrier function, which means it is more susceptible to water loss from the epidermis. Each location of the skin has a distinct microbiological niche. The environment of the scalp is unique because it contains a large density of sebaceous and sweat glands and as a result, has a high relative humidity. This creates the optimal growing conditions for microbiological colonization. The skin is a nutrient-rich environment due to the continuous secretion of a supply of amino acids, minerals, and sebum.

Hair pH:

With a pH of 3.7, the hair fiber has a more acidic pH than the scalp. Hair is composed of long, parallel chains of amino acids. These chains connect via hydrogen bonding, salt bridges between acid and base groups, and via disulfide bonds. Sebum secreted by the scalp is composed of glyceride, waxes, fatty acids, and squalene. Its function is to coat the cuticle and prevent water loss from within the shaft of the hair. Sebum also contains antimicrobial agents that inhibit the growth of bacteria such as Streptococci and prevent fungal infections of the scalp. Because of its sticky consistency, sebum attracts dirt and impurities. Shampoos are formulated to remove the impurities and excess sebum while leaving a slight coating on the hair shaft that protects against moisture loss.

Hair is extremely sensitive to pH variation by topically applied products. When the hair is exposed to substances more alkaline than the pH of the hair (above 3.7), there is an increase in the negative electrical charge on the surface of the hair fibers. This increases static electricity, friction and, repulsion between the strands of hair and causes cuticle damage and breakages in the hair fiber.

The role of shampoo and conditioner:

Shampoos are formulated with surfactants, substances that act as detergents by lowering surface tension and acting as wetting agents. Surfactants break the forces that bind residues and impurities to the hair and prevent them from attaching to the scalp and hair. There are four distinct classes of surfactants. They are composed of a lipidic chain of hydrocarbons with a polar and nonpolar end. When combined with water, they produce a micelle, a spherical structure with a hydrophobic (water-repelling) interior that binds fats and residues and a hydrophilic (water-loving) exterior that is easily rinsed away with water. The electric charge of the polar molecule determines the class of surfactant as either anionic, atonic, amphoteric, or nonionic. The surfactant sodium lauryl sulfate is an anionic surfactant that leaves an alkaline residue on the hair and scalp and causes calcium deposits to accumulate on hair strands causing them to tangle.

When hair is rinsed with water, which typically has a pH of 6.5-8.5, the negative charge increases and washes out the micelles as it is repelled by the negatively charged strands. The negative net charge causes tangles and frizz to form and causes the hair to absorb more water. Water breaks the hydrogen bonds of the keratin molecule. Once the hair has dried, the hydrogen bonds will reform in the position the hair has dried in. At pH’s higher than 8.5, disulfide bonds begin to break, causing the cuticle to become rough, making hair appear dull. With repeated washing, the accumulation of these broken bonds results in split ends.

Damaged hair with open cuticles is unable to hold onto the pigments in hair dye, and the colour will fade prematurely. Hair treatments like bleaching, dyeing, or the use of heat treatment tools can accelerate the removal of the protective sebum coating from the hair shaft. Conditioner is formulated to replace this coating.

The benefit of using pH balanced hair care:

By using a shampoo with a low pH close to that of the hair, less frizzing occurs due to a reduced negative static electricity on the surface of the hair fiber. A slightly acidic conditioner will shine to the hair as the cuticle is flattened and, therefore, able to reflect more light, appearing shiny and healthy.

Sources:

  • Normal skin surface pH is on average below 5, which is beneficial for its resident flora. International Journal of Cosmetic Science, 2006.
  • Time-dependent variations of the skin barrier function in humans. Transepidermal water loss, SC hydration, skin surface pH and skin temperature. Journal of Investigative Dermatology, 1998.
  • The shampoo pH can affect my hair: myth or reality? International Journal of Trichology, 2014.
  • pH and Hair Shampoo. ChemMatters, 1983
  • Evaluation of pH in bathing soaps and shampoos for the skin and hair care. Indian Journal of Dermatology, 2014.
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