Dear Valerie: I’ve noticed more haircare brands promoting the use of acids like glycolic acid and citric acid in their formulations. Glycolic acid is well-known for its exfoliating effects on the skin, but does it serve a similar exfoliating or resurfacing function in hair? 

—Bond Builder

Dear Bond:

When I entered the industry in 2010, I recall Dupont pushing glycolic acid for use in haircare. As a callow chemist, I simply dismissed the effect of acids on hair as working by closing the cuticle, rendering the hair shiny and healthy looking. I didn’t give it much additional thought as acidifying conditioners was a decades-old concept. Dr. Ron DiSalvo once opined that hairdressers added citric acid to conditioners to lower the pH, creating new bonds in hair. Little did the industry know that glycolic acid does so much more, and it’s not exfoliating off loose cuticles!

 Glycolic acid is the lowest molecular weight alpha hydroxy acid and can penetrate hair, reaching the cell membrane complex. It doesn’t need a super low pH to achieve this, around pH 4 is perfectly acceptable, and in fact, desirable. Applying too low a pH to the hair increases hair damage, leaving hair too brittle and stiff.

 What does glycolic acid do once it penetrates hair? It modifies hair’s adsorption isotherm, essentially altering how water binds to hair. When water is displaced, hair is generally stronger and more flexible. This impact is consistent in the effect of glycolic acid on hair. Just 5% glycolic acid increases hair’s elasticity and improves dry combing. It also increases the denaturation temperature of keratin, imparting thermal protective effects. This is regardless of hair ethnicity and hair health.

 Trefor Evans, a brilliant hair scientist retired from TRI Princeton, conducted extensive research in this area. He published much literature, albeit on citric acid and its impact on hair.¹ The same would apply to glycolic acid. I’ve had the pleasure of collaborating with Trefor on acids in hair. It’s really quite an interesting area with real effects on hair, not marketing baloney.

 So, no! glycolic acid isn’t exfoliating hair, like it would skin. Its mechanism is far more sophisticated.