The chemistry behind AHAs and BHAs in skin care: chemical exfoliation


The world of skin care has developed into an enormous market with lots of different kinds of products. Skin exfoliation, removing small layers of (dead) skin, can be achieved by physically scrubbing the skin with a cosmetic scrub, but it can also be done chemically. By applying certain ingredients on the skin the same effect can be obtained without the harsh rubbing movement. Over the years this method has advanced greatly and has become a regular step in the beauty routines of many. Different brands, varying from low to high price ranges, are now offering products for chemical exfoliation instead of physical scrubbing.


Hydroxy Acids in chemistry

One class of ingredients often used in chemical exfoliation are Hydroxy Acids. You might have heard of AHAs and BHAs, but you might not realise that these are Hydroxy Acids (HA). AHA stands for Alfa-Hydroxy Acid and BHA for Beta-Hydroxy Acid. Looking at the chemistry, Hydroxy Acids are molecules characterised by a Carboxylic Acid group and a Hydroxy group [2]. These are common groups used in chemistry and can be found on many different compounds. To give you an idea: in vinegar, used in our cooking, there is acetic acid which also has a Carboxylic Acid group; and the alcohol we consume is called ethanol which has a Hydroxy group.

When you have both a Carboxylic Acid and a Hydroxy group in one compound, you get a Hydroxy Acid (see below).


AHA and BHA

I mentioned that AHA and BHA stand for Alfa- and Beta-Hydroxy Acid respectively, but what does that mean? It originates back to the chemical structure of the two classes of Hydroxy Acids. Alfa and Beta refer to the so-called Alfa and Beta positions where the Hydroxy group is placed on the molecules with respect to the Carboxylic Acid group.


– In chemistry, molecular structures are often displayed as carbon structures with different groups attached to it, where the unspecified corners are carbon atoms. –

When the Hydroxy group is placed on the carbon directly next to the Carboxylic Acid group this is the Alfa position. When the Hydroxy group is placed on the carbon one position further away from the Carboxylic Acid group – this is the Beta position [2].


Hydroxy Acids in skin care

Now that we know what defines a AHA and a BHA, let’s have a look at some of those Hydroxy Acids and their functions in skin care.


AHA

AHAs are generally quite small molecules that dissolve well in water and provide anti-aging effects on skin [5]. The smallest example of an AHA is Glycolic acid and the slightly larger Lactic acid (see below).

Both acids have been observed to exfoliate the skin and also stimulate cell renewal. The exfoliation allows the newly generated cells to come to the surface. This leads to an increased firmness, thickness and hydration of the skin, which can result in the reduction of fine lines or wrinkles [6].


BHA

Two compounds that are often marketed as BHAs are Salicylic acid (SA) and Lipohydroxy acid (LHA), although researchers claim that these are not actually BHAs due to the ring structure in the molecules (see below) [2,5]. Nevertheless, when cosmetic brands talk about BHAs they still often refer to SA or LHA.

In contrast to AHAs, SA and LHA are oil soluble which makes them useful in the treatment of oily or acne prone skin. They stimulate the exfoliation of the skin and cell renewal in the deeper layer of the skin. Treatment with SA and LHA has shown to lead to a decrease of the top layer and an increase in the deeper layer of the skin [7]. Due to the solubility in oil, they can also decrease the sebum production which can be beneficial in anti-acne treatment [8].


Adding Hydroxy Acids into your routine

As we have seen, AHAs and BHAs could serve different functions in your skin care routine. AHAs are useful in anti-aging care, while BHAs could help in your routine when you have acne prone skin. Hopefully this article has given you a better view on what AHAs and BHAs could do for your skin and why you could incorporate them in your routine.


Sources

[1] Esse Skincare. HYALURONIC SERUM. (2019).

[2] Kornhauser, A. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clin. Cosmet. Investig. Dermatol. 135 (2010) doi:10.2147/ccid.s9042.

[3] Focal Point. White vinegar on wooden table top.

[4] WallpaperTag. Wine background.

[5] Green, B. A., Yu, R. J. & Scott, E. J. Van. Clinical and cosmeceutical uses of hydroxyacids. Clin. Dermatol. 27, 495–501 (2009).

[6] Smith, W. P. Comparative effectiveness of α-hydroxy acids on skin properties. Int. J. Cosmet. Sci. 18, 75–83 (1996).

[7] Saint-léger, D., Lévêque, J. L. & Verschoore, M. The use of hydroxy acids on the skin: Characteristics of C8 -lipohydroxy acid. J. Cosmet. Dermatol. 6, 59–65 (2007).

[8] Arif, T. Salicylic acid as a peeling agent: A comprehensive review. Clin. Cosmet. Investig. Dermatol. 8, 455–461 (2015).

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