Shampoo pH: the number brands do not print

Every shampoo label lists its ingredients. Almost none lists its pH, which is a single number that has more to do with how the product treats your hair than most of the ingredients do. This article explains what the number means, what the research on it found, and how to measure it yourself for the cost of a strip of paper.

The scalp's acid mantle

The surface of healthy skin is mildly acidic, with a pH around 4.5 to 5.5. The film that maintains this, a mixture of sebum, sweat and the by-products of skin bacteria, is called the acid mantle. It does several jobs: it discourages the growth of harmful microbes, which prefer a neutral environment; it helps the outer skin layer hold together and retain water; and it keeps the enzymes that maintain the skin barrier working at the pH they need.

Raise the pH of the surface and each of these degrades. The skin loses water faster, feels tight, and becomes more permeable to irritants. The change is temporary, and healthy skin recovers its pH within a few hours. But a product that pushes it up at every wash is pushing against the recovery every day.

What pH does to the hair shaft

Hair has no repair mechanism; it is dead protein once it leaves the follicle. Its surface, the cuticle, is a layer of overlapping scales. At low pH the scales lie flat and the surface carries a slight positive charge. As pH rises the scales lift and the surface charge turns negative. Negatively charged hairs repel each other and the lifted scales catch on their neighbours. The result is frizz, tangling, and increased friction, and friction is what breaks hair when it is brushed.

A 2014 study in the International Journal of Trichology measured the pH of 123 shampoos available in Brazil, including international brands. It found that most sat above 5.5, that a large share of the popular consumer brands sat well above it, and that only a minority were within the range the authors considered ideal for the hair surface. Products marketed for children and some salon products were more likely to be in range. Price was not a reliable guide.

Reference points

Soap bars, syndet bars, and the pH problem

This is the distinction that matters most for anyone considering a shampoo bar. A traditional soap bar is made by saponifying oils with an alkali, and the result is chemically incapable of being acidic: soap at pH 5 is not soap. Cold-process shampoo bars therefore sit around pH 9 to 10, which is why many people who try one report rough, waxy, tangled hair. The traditional fix is a vinegar rinse after every wash to bring the surface back down, which works but is a workaround for a formulation problem.

A syndet bar uses synthetic surfactants (sodium cocoyl isethionate is the most common) pressed into solid form, and can be formulated at any pH the maker chooses, including the scalp's. If a bar is described as pH-balanced or lists a pH near 5.5, it is a syndet. If it lists saponified oils or sodium olivate, sodium cocoate and similar, it is a soap and will be alkaline regardless of what the label implies.

How to test a product at home

1. Buy pH test strips with a range of roughly 3 to 9 and half-unit resolution. Aquarium and pool suppliers sell them; so do pharmacies.
2. For a liquid, dilute a small amount 1:10 in distilled water and dip the strip. Undiluted product can read inaccurately.
3. For a bar, wet it, work up a lather in a small amount of distilled water, and test the lather.
4. Read at 30 seconds against the chart. Strips are accurate to about half a unit, which is enough to separate 5 from 7 from 9.

A brand that publishes its pH has usually formulated for it. A brand that will not tell you when asked has usually not measured it, or does not like the answer.

What to do with the number

• Below 4: unnecessary. There is no benefit to going below the hair's own pH, and strong acids damage protein.
• 4.5 to 5.5: the range to aim for, especially for coloured, fine or fragile hair.
• 6 to 7: acceptable for most people with a conditioner afterwards, which is typically acidic and closes the cuticle.
• Above 8: expect roughness and tangling. Follow with an acid rinse, or choose a different cleanser.