Hair and pH: Weakly Acidic Is the Baseline, Alkali Makes It Swell

pH expresses how acidic or alkaline something is on a scale of 0 to 14, with 7 being neutral. Healthy hair and scalp are weakly acidic at pH 4.5–5.5. At this level the cuticle is closed and the bonds inside the hair (salt bonds) are at their most stable.

When hair becomes alkaline

The cuticle opens and the hair absorbs water and swells. Color, perm and bleach products are alkaline (around pH 8–11) precisely to create this state and deliver the product inside the hair. After a treatment, some alkali from the product remains in the hair for a while (residual alkali), and if the cuticle stays open, color fades and hair dries out more easily.

When hair becomes too acidic

Strong acidity tightens the cuticle too much and makes hair stiff; stronger still, proteins begin to denature. Acidic straightening and acid-heat treatments use this property, and because they combine it with heat, they are considered to require careful handling.

Points to remember for hair care

• For 1–2 weeks after coloring or perming, it is common to use weakly acidic shampoo and treatment to bring the hair's pH back
• Products labeled "weakly acidic" are adjusted to the same pH range as hair
• Soap-based shampoos are alkaline. If you use one, neutralizing with an acidic rinse is assumed

Related: Hair color care / Shampoo