PH: what impact does it have on the health of our hair?

pH is a measurement often used in chemistry and biology. It also plays a fundamental role in hair care. It is a scale measuring the acidity or alkalinity of a solution, ranging from 0, extremely acidic, to 14, highly alkaline, 7 being neutral. The health of our hair and scalp is intrinsically linked to this delicate balance. The pH balance of our hair and our scalp can be disrupted by many factors, whether environmental or linked to the products we use. When this balance is disrupted, the result can be dry, brittle, lacklustre hair or a lack of shine. irritated scalp. Find out everything you need to know about how pH affects the health of your hair and why it's so important to choose the right products to maintain this balance.

What is pH?

The pH (hydrogen potential) is a scale that measures the acidity or alkalinity of an aqueous solution. This scale varies from 0 to 14 :

  • Acid : a pH below 7 indicates an acidic solution. The closer the pH is to 0, the more acidic the solution.
  • Neutral : a pH equal to 7 indicates a neutral solution. Pure water, at a temperature of 25°C, has a pH of 7 and is therefore considered neutral.
  • Basic or alkaline : a pH above 7 indicates a basic (or alkaline) solution. The closer the pH is to 14, the more basic the solution.

The pH of a solution is determined by the concentration of hydrogen ions (H+) it contains. A high concentration of H+ ions makes a solution acidic, while a low concentration makes it basic. Conversely, a high concentration of hydroxide ions (OH-) makes a solution alkaline.

What is the natural pH of the scalp and hair?

The natural pH of the scalp and hair is slightly acidic, generally between 4.5 and 5.5.

An acidic pH on the scalp helps inhibit the growth of bacteria and fungi, helping to prevent infections and scalp conditions such as dandruff or seborrhoeic dermatitis.

The acidity of the pH also helps to keep the hair cuticles (the outer layer of the hair) closed. A closed cuticle protects the hair cortex, preserves moisture and gives hair a smooth, shiny appearance.

A pH balance on the scalp helps regulate the production of sebum, the natural oil produced by the sebaceous glands. An imbalance in pH can disrupt this production, resulting in a scalp that is either too dry or too oily.

Why is pH important for healthy hair?

Hair has a complex structure made up of several layers and components. Each part plays a distinct role in the health, texture, colour and overall strength of the hair. To better understand how product pH and hair care affect your hair, it's essential to understand hair structure. 

Hair structure is broken down into three main components :

  • The cuticle : This is the hair's protective outer layer. It is made up of flattened cells superimposed like roof tiles. Its main role is to protect the cortex, the inner part of the hair, from external aggressions such as UV rays, pollution and chemicals. The health and integrity of the cuticle determine the hair's shine, softness and ability to retain moisture. Hair with a damaged cuticle is often dull, dry and porous.
  • The cortex : Located directly under the cuticle, this is where most of the hair's mass resides. It is made up mainly of hair keratin which is the protein that gives hair its strength and elasticity. The cortex also contains the pigments that define the hair's natural colour. Colouring and bleaching processes mainly target this layer of the hair. The health of the cortex is crucial to the hair's resistance to breakage and mechanical damage.
  • The marrow : This is the centre of the hair, a sort of central core. Its presence is not constant. Some hairs, especially the finest, may not have a clearly defined medulla.

How do alkaline products affect hair?

Alkaline products with a pH above 7 can have significant effects on hair structure and health.

Alkaline products cause thecuticle opening cuticles, which are the protective scales on the hair shaft. Once these cuticles are open, hair can become more porous, losing their natural moisture and becoming more vulnerable to external aggressors such as pollution, heat and chemical treatments.

This cuticle-opening effect is intentionally sought in certain hair procedures, such as straightening treatments or permanent colouring. These treatments require the cuticles to be opened so that the chemicals can penetrate the cortex of the hair and modify its structure.

However, there are a number of reasons for this, prolonged or frequent exposure to alkaline products without appropriate care afterwards can lead to damage cumulative. Hair can become dry, brittle and prone to breakage. What's more, without the protective barrier of tightly closed cuticles, hair can lose its natural shine, resulting in a dull appearance.

Alkaline products can also upset the pH balance of the scalp, potentially causing dryness and irritation.

How do acid products benefit hair health?

Acid products offer a number of advantages for the hair, in particular because of their ability to tighten and close the cuticles of the hair shaft.

An acid pH helps to close and tighten cuticles of the hair. When the cuticles are properly closed and aligned, they act like a protective barrier for the hair cortex, preventing moisture loss and the penetration of harmful environmental factors. What's more, smooth, well-aligned cuticles reflect light better, giving hair a shinier, smoother appearance.

However, just as an excessively alkaline pH can be damaging, an excessively acidic pH can also cause problems.

A pH that is too acidic can irritate the scalpleading to itching or redness or sometimes a burning sensation. Also, although acidity helps to close cuticles, too much can make hair too stiff and prone to breakage. Finally, for those who colour their hair, a pH that is too acidic can alter the colour or accelerate fading.

What pH level should I choose for healthy hair?

To maintain healthy hair, it's essential to choose products with a pH level in harmony with the acidic nature of the hair and scalp. Choose formulations with a pH between 4.5 and 6.5. Although a slightly higher pH can effectively eliminate residues, it is crucial not to disturb the natural balance excessively.

Good to know: the Curly hair Hair is naturally dry, so it's crucial to use products with the right pH. When choosing products for frizzy hair, it's advisable to choose those with a balanced or slightly acidic pH. This includes shampoos, conditioners and hair treatments.

How do you know the pH of a shampoo?

Some manufacturers state the pH of the product directly on the packaging or on the INCI list. If this is not the case, you can use pH test strips. These strips, available online or in specialist shops (pharmacies and scientific supply shops in particular), can be used to test the pH of shampoo. To do this:

  • Put a little shampoo in a container.
  • Add a little water to dilute (this makes the test easier).
  • Dip the strip into the solution.
  • Compare the colour of the strip with the colour guide provided to determine the pH.

pH plays an essential role in the health of your hair. By choosing products that are adapted to your hair's natural pH balance, you can preserve its shine, strength and beauty.

Jean-François Cabos

Jean-François Cabos is the creator of a unique hair care method based on the research he coordinated, which led to the publication of the book "Cheveux, Racines de Vie" with Hélène Clauderer by Robert Laffont (Collection "Réponses/ Santé").

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