Science: hair and scalp pH
SCIENCE · Hair basics and keratin · Scalp, sulfates and what damages hair · Hard water · Hair and scalp pH
2c · Hair and scalp pH, explained: what pH means for hair, and why it matters
In short
- pH measures acidity. It runs from 0 to 14, with 7 neutral. Each whole number is a tenfold change.
- Healthy hair is slightly acidic. The hair fibre is most stable at about pH 3.7,4 scalp skin sits at about pH 5.5,14 and professional guidance quotes 4.5 to 5.5 for hair and scalp.15
- Alkaline products stress hair. They swell the fibre, lift the cuticle and raise friction, which can show up as frizz, tangling and breakage.14, 4, 16
- A middle range is gentle. In one laboratory study, pH 5 to 7 had minimal effect on hair structure and proteins.16
- Damaged hair reacts more. The effect of pH grows with prior damage.17
- pH is not everything. The cleansing system, conditioning agents and rinsing matter too.
What the numbers are
| WHAT | TYPICAL pH | SOURCE |
|---|---|---|
| Neutral (pure water at 25 °C) | 7 | Definition |
| Hair fibre: isoelectric point, where it carries no net charge | about 3.67 | 4 |
| Scalp skin | about 5.5; skin surface averages below 5 in some studies | 14 |
| Range often quoted as healthy for hair and scalp | 4.5 to 5.5 | 15 |
| Shampoos measured in one study | 3.5 to 9.0 | 14 |
| Laboratory study of hair at pH 3 to 12 | pH 5 to 7 minimal effect; stronger effects at pH 3 and above pH 8 | 16 |
| Perm and reducing products | above 9 | 4 |
| Alkaline relaxers | about 12 to 13 or more | 4 |
Why pH matters for hair
It sets the hair's electrical charge. Above pH 3.67 hair carries a net negative charge, and the more alkaline the product, the stronger that charge. A stronger charge means more friction between fibres, so more tangling and frizz.14, 4
It changes the cuticle. Alkaline solutions swell the fibre and lift the cuticle scales, making hair more porous and prone to breakage.4, 16 Slightly acidic care helps scales lie flat, which is why a low-pH conditioner is suggested after a higher-pH shampoo.14
It affects the scalp. The scalp surface is naturally slightly acidic, which suits its resident bacteria. A more alkaline state can upset the scalp's natural oils and bacteria.15
It affects what sticks to hair. Hair's charge decides which ingredients deposit. Positively charged conditioning agents are drawn to negatively charged hair,4 and so are the calcium and magnesium in hard water.6 In a laboratory study, calcium and magnesium uptake rose as water pH rose from 7 to 9.13 A product's pH also affects how much of it deposits and how much stays as residue.15
Damaged and chemically treated hair
Hair proteins are affected by pH, and the effect tends to grow with prior damage.17 Chemically treated hair has a lower isoelectric point, so it holds on to conditioning ingredients more strongly than virgin hair.4 Colour, perms and relaxers are alkaline on purpose: they must open the cuticle to reach the cortex. Permanent dyes use ammonia or ethanolamine to raise the pH and also remove 18-MEA, the fatty acid that makes hair water-repellent.7, 4 Above about pH 10, alkali can break peptide and disulfide bonds and weaken bleached hair further.17 After these services, gentle, slightly acidic care helps hair recover.
How to read a product's pH claim
- Look for the number. There is no standard pH for shampoos,14 and "pH-balanced" does not name a figure.
- Slightly acidic is generally compatible. Most of the evidence points to about 4.5 to 5.5, with 5 to 7 showing little effect on hair structure in one study.15, 16
- Judge the whole formula. Cleansing agents, conditioning agents, chelators and rinse-off all change how hair feels.
- Strips are rough. Home test strips give an approximate reading; laboratories use a calibrated meter on a diluted sample.
pH questions, answered
What is pH?
pH is a scale from 0 to 14 that shows how acidic or alkaline something is. 7 is neutral, below 7 is acidic and above 7 is alkaline. The scale is logarithmic, so each whole number is a tenfold change.
What is the ideal pH for hair?
There is no single official figure. The hair fibre is most stable at about pH 3.7, scalp skin sits at around pH 5.5, and professional guidance often quotes 4.5 to 5.5 for hair and scalp. In one laboratory study, pH 5 to 7 had minimal effect on hair structure and proteins, while pH 3 and anything above pH 8 had stronger effects.
Is hair acidic or alkaline?
Healthy hair is slightly acidic. Its isoelectric point, where the fibre carries no net charge, is about pH 3.67, and the scalp is around pH 5.5.
Does shampoo pH matter?
Yes. A clinical review found that alkaline pH can increase the negative charge on the hair surface and the friction between fibres, which can lead to cuticle damage, frizz, tangling and breakage, and that shampoos above pH 5.5 may increase friction. Formulation still matters as well as pH: the cleansing agents, the conditioning agents and how well the product rinses all change how hair feels.
What happens to hair at high pH?
Alkaline solutions swell the hair fibre and lift the cuticle scales, which makes hair more porous and more prone to friction and breakage. Above about pH 10, alkali can break peptide and disulfide bonds and weaken hair further, especially hair that is already bleached or damaged.
Why are hair colour, perms and relaxers alkaline?
They have to open the cuticle to reach the cortex. Permanent dyes use ammonia or ethanolamine to raise the pH, perm products are above pH 9 and alkaline relaxers are around pH 12 to 13. This is why hair needs gentle, slightly acidic care after these services.
What is the pH of the scalp?
Scalp skin is around pH 5.5, like the rest of the skin, and skin surface pH averages below 5 in some studies. A slightly acidic surface supports the skin's resident bacteria.
Is a pH-balanced shampoo always better?
Not on its own. A slightly acidic pH is generally compatible with hair and scalp, but there is no standard pH for shampoos, and the term pH-balanced does not name a number. Cleansing agents, conditioning agents, chelators and rinse-off all affect results, so look for the stated pH and judge the whole formula.
Can I test the pH of my shampoo at home?
Test strips give a rough reading. Laboratories measure the pH of a diluted sample with a calibrated pH meter, so a lab figure and a strip reading can differ.
How does pH relate to hard water?
Hair carries a negative charge above pH 3.67, and the calcium and magnesium in hard water are positively charged, so they bind to hair. In a laboratory study, hair took up more calcium and magnesium as the water pH rose from 7 to 9, and bleached hair took up far more than virgin hair.
References
4. Gavazzoni Dias MF. Hair cosmetics: an overview. Int J Trichology. 2015;7(1):2–15. doi:10.4103/0974-7753.153450
6. The structural implications of water hardness metal uptake by human hair. Int J Cosmet Sci. 2011;33(5):477–482. doi:10.1111/j.1468-2494.2011.00659.x. PubMed 21923661
7. Gavazzoni Dias MF, Loures AF, Ekelem C. Hair cosmetics for the hair loss patient. Indian J Plast Surg. 2021;54(4):507–513. doi:10.1055/s-0041-1739241
13. Evans AO, Marsh JM, Wickett RR. The uptake of water hardness metals by human hair. J Cosmet Sci. 2011;62(4):383–391.
14. Gavazzoni Dias MF, de Almeida AM, Cecato PM, Adriano AR, Pichler J. The shampoo pH can affect the hair: myth or reality? Int J Trichology. 2014;6(3):95–99. doi:10.4103/0974-7753.139078
15. What is hair pH and why it matters. Redken India (brand blog article, not peer-reviewed; shown as updated 20 July, year not stated; read 4 October 2026). redken.in
16. Adav SS, Wu ARYL, Ng KW. Insights into structural and proteomic alterations related to pH-induced changes and protein deamidation in hair. Int J Cosmet Sci. 2025. doi:10.1111/ics.13029. PubMed
17. Malinauskyte E, et al. Effect of equilibrium pH on the structure and properties of bleach-damaged human hair fibers. Biopolymers. 2020. doi:10.1002/bip.23401. PMC7757171
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