The pH of hair has been talked about in general terms, and the value of pH for hair is considered to be between 4.5 to 6.5 and more preferably between 5.5 and 6.0. The measurement of hair and scalp pH has been a challenge from many decades. Only recently, specially designed electrodes were introduced that can measure the pH of wet hair and scalp. There is very little literature dealing with the pH of hair measurements and the impact of pH changes on hair and scalp. Our group has been involved in measuring the pH of the hair and scalp under various product systems applications such as permanent relaxers, permanent waves, permanent colors and hair lighteners/bleaches. The medical community has also put forth their findings that ideal pH of scalp or skin is acidic and around 5 or 5.5. The slightly acidic skin pH helps in creating normal conditions for a pathogenic resident flora[i].
The pH of the hair and scalp changes significantly during contact with cosmetic products such as detergents/soaps, and chemically reactive systems such as hair relaxers, hair bleach, permanent waving and oxidative dyes. The pH of untreated hair is around 5 to 5.5 and it increases to 13 when the hair relaxers are on the hair. Upon rinsing relaxers, the pH drops to about 10 and upon shampooing hair with neutralizing shampoos, the pH drops to around 7.5, which is significantly higher than the starting pH of 5 to 5.5. Similarly, the pH of hair during bleaching increases to 10 and upon rinsing and shampooing the pH of the hair is still around 7 to 7.5, which is higher than the actual pH of the hair, that is 5 to 5.5.
[i] Monika-Hildegard Schmid-Wendtner and HC Korting. (2007). pH and Skin Care. ABW.Wissenschaftsverlag: Berlin, p. 43.