Hydroxy Acids and Factors Affecting Their Performance

The efficacy of a topical hydroxy acid is determined by two major factors – the bioavailable concentration and the vehicle used.

The bioavailability of a hydroxy acid, or the amount that permeates the stratum corneum, is the amount of free acid present in the formulation.

The inherent pKa (or strength) of the hydroxy acid, as well as the pH of the formulation, determines the amount of free acid in a formulation.

If the pH of the formulation is less than the pKa of the acid, the free acid form predominates. If the pH of the formulation is greater than the pKa of the acid, the less effective salt form predominates. It is important to understand that although two products may contain equal percentages of a hydroxy acid, the bioavailability of that hydroxy acid may not be equal if the pH values of the carrier are different.

For example, glycolic acid’s pKa is 3.83. A 10% glycolic acid product formulated at pH 3.0 has a bioavailability of 0.06 and is quite effective on the skin.

On the other hand, a 10% glycolic acid product formulated at pH 5.0 has a bioavailability of 0.06 and is much less effective.

The second major fact or affecting efficacy is the vehicle or carrier, which determines the solubility of a hydroxy acid and the bioavailability though its pH, as discussed above.

For water-soluble hydroxy acids, such as glycolic, lactic, malic, tartaric and citric acid, an oil-in-water emulsion is the vehicle of choice. For more lipid soluble hydroxy acids, such as madelic, benzilic and salicylic acid, a water-in oil formulation is the vehicle of choice.

These are important considerations that will determine the effectiveness of an AHA’s formulation. This is why as a therapist you need to be assured and request evidence from the supplier or manufacturer that the AHA product they are providing you has addressed these issues in their formulation to ensure that they perform to your expectations in terms of results.

The most common side effects of all topical hydroxy acid preparations are irritation and peeling of the skin. If excessive irritation does occur, you will need to decrease the frequency of application, or the strength of the formulation. Because this side effect is very common, you should start with low strength products and increase the strength of the formulation as tolerated.

There are two main misconceptions involving the hydroxy acids. The first involves erroneous concerns for photo-allergic reactions. Because the stratum corneum will thin with hydroxy acid use, clients may notice a tendency to sunburn more quickly. Clients should therefore be educated that this is not an allergic reaction, but rather an expected pharmacological effect. The second misconception involves hydroxy acids being incorrectly included in the group of keratolytic agents. Because keratin proteins are not hydrolysed, it is more accurate to refer to this group of acids as exfoliants.

To find out more about the composition of Chemical Peels, types of acids and treatment precautionary measures, head over to my online shop and get your copy of my e-book "Understaning Chemical Peels".

Yours in skin,

Gay Wardle