The pH level of a substance is a measure of its acidity. The pH of healthy human hair is somewhat acidic, ranging from about 4.0 to 5.0. The pH of most shampoos is weakly acidic; in a study published in September 2014 in the Indian Journal of Dermatology, 24 of 38 shampoos fell in the range of 6.01 to 7.00.
The pH scale measures the hydrogen-ion content of materials. It ranges from 0 at the low, or most acidic, end to 14.0 at the high, or most alkaline, end. Something with a pH of 7.0, being neither acidic nor basic, is neutral. It is a logarithmic scale, meaning that a numerical change of 1.0 actually represents a tenfold difference in acidity.
Shampoos are formulated to be acidic because of the physical nature of hair. Strongly alkaline solutions break the disulfide chemical bonds in hair, and at a pH of 12 hair actually dissolves.
