Shampoo pH

Key takeaways

• Hair and scalp prefer mildly acidic conditions. The common figure for skin and scalp is a pH of roughly 4.5-5.5, and the main paper on shampoo pH uses 5.5 as its cut-off (Gavazzoni Dias 2014).
• Most shampoos are not that acidic. Of 123 shampoos measured, 38% were at or below pH 5.5 and 62% above it. Of 26 anti-dandruff shampoos, 81% were above 5.5, and so were all the children's shampoos. The overall range was 3.5-9.0. Salon products were most often acidic (75% at or below 5.5). The sample came from the Brazilian market (Gavazzoni Dias 2014).
• Why alkaline pH is thought to matter: it raises the negative charge on the hair surface and so increases friction between fibres, which the author links to cuticle damage and breakage. The paper concludes lower-pH shampoos are "reality and not a myth", but it combines measured pH with mechanism and is not a wear trial.
• We found no direct trial of different shampoo pH on real-world frizz or breakage. The case for acidic products rests on bench and mechanism data.
• The pH of the whole routine counts, not just the shampoo. Conditioners and acidic rinses are usually more acidic; see acidic rinses and pH.
• Soap bars and baking-soda washes are alkaline. Hair-specific clinical data are lacking.
• Skin evidence supports gentle water exposure. In 50 healthy volunteers, both hot and cold water raised skin pH (from 6.33 to 6.65 and 6.62) and water loss, hot being worse. This was hands, not scalp (Herrero-Fernandez 2022).

The science

Hair is keratin with a cuticle of overlapping scales. In water the fibre swells, more so at alkaline pH, which lifts the cuticle. At acidic pH the surface carries less negative charge and fibres slide past each other more easily. Friction is what drives tangling, breakage and frizz when hair is combed wet.

Gavazzoni Dias measured the pH of 123 shampoos (96 popular, 19 dermatological, 15 salon, plus children's products), found most were above 5.5, and concluded that alkaline pH may increase fibre charge, friction, cuticle damage and breakage (Gavazzoni Dias 2014).

Citric acid is the main acidulant in shampoos and conditioners. A 2025 lab study found that citric acid treatment of chemically treated hair reduced calcium and improved mechanical properties, which blends pH and chelation effects (Zhang 2025; see citric acid).

What it means by hair profile

• Coloured, bleached, relaxed or otherwise high-porosity hair: cuticles are already lifted and friction-sensitive, so acidic shampoos and conditioners are a logical choice (see hair porosity).
• Curly and coily hair (3A-4C): prone to tangling and breakage. Low-friction care matters more than exact pH, but acidic conditioning helps in principle.
• Fine or oily hair: pH is rarely the main issue.
• Sensitive or eczema-prone scalp: avoid alkaline soaps and consider mild, pH-balanced shampoos.

Myths and facts

• Myth: shampoo must be exactly pH 5.5. Fact: shampoos range from 3.5 to 9.0, and the clinical effect of small differences is untested.
• Myth: apple cider vinegar rinses restore pH and fix hair. Fact: a 2025 review found essentially no data on them, and it concerned synthetic extensions (Ogah 2025).
• Myth: baking soda is a safe natural shampoo. Fact: it is alkaline, and there are no hair-specific trials.
• Myth: cold water seals the cuticle. Fact: there is no hair study; see water temperature.

Bottom line

If you have damaged, coloured or textured hair, prefer products labelled pH-balanced or acidic where available, and follow shampoo with conditioner. Skip alkaline bars and DIY baking-soda washes unless you already know your hair tolerates them. If a dry or irritated scalp coincides with a new alkaline product, go back to your old one. Do not adjust shampoo pH with kitchen acids on a regular basis.