The hair’s pH is very important, as an unbalanced hair and scalp can cause irritation, dryness, dullness, and frizziness. Your hair and scalp hover around 5.5 normally (studies even show that when your hair products are at or below 5.5 pH, it can reduce frizz and damage).
Since water is a neutral pH and most shampoos are alkaline, simply taking a shower changes the pH of your hair. “Water’s pH can vary, but it’s usually around a 7—so when you wash it, you are bringing your hair’s pH up to a mid 6—that’s the same pH as a demi-permanent color,” says colorist and co-founder of the salon Spoke&Weal Christine Thompson. “And that’s every time you wash your hair.”
When the pH of the hair changes, it causes the cuticle to lift, making it frizz-prone and brittle. (On the other hand, when your hair’s cuticle is flat, your hair is healthier, shinier, and smoother.) So the idea is that when you rinse it with something acidic—hello, apple cider vinegar!—it brings the pH back down and seals the cuticle shut, leading to shiny, soft strands.
Other claims say that the vitamins, minerals, and nutritious properties strengthen hair, fight flakes, and even contribute to hair growth—but these don’t have any research to back them up.